feat: userspace faults

Cargo.lock

@@ -2,6 +2,13 @@
 # It is not intended for manual editing.
 version = 4
 
+[[package]]
+name = "client"
+version = "0.1.0"
+dependencies = [
+ "dusk-sys",
+]
+
 [[package]]
 name = "dusk"
 version = "0.1.0"
@@ -9,8 +16,26 @@ dependencies = [
  "limine",
 ]
 
+[[package]]
+name = "dusk-sys"
+version = "0.1.0"
+
+[[package]]
+name = "echo"
+version = "0.1.0"
+dependencies = [
+ "dusk-sys",
+]
+
 [[package]]
 name = "limine"
 version = "0.6.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "29363c0f37e66e18575fadf7141c56ee7ea04ae5fecbeb25eff303f77af203a9"
+
+[[package]]
+name = "omega3"
+version = "0.1.0"
+dependencies = [
+ "dusk-sys",
+]

Cargo.toml

@@ -3,5 +3,13 @@ name = "dusk"
 version = "0.1.0"
 edition = "2024"
 
+[workspace]
+members = [".", "userspace/*"]
+
 [dependencies]
 limine = "0.6.5"
+
+[[bin]]
+name = "dusk"
+test = false
+bench = false

Makefile

@@ -1,21 +1,22 @@
 ARTIFACTS_PATH ?= bin
 IMAGE_NAME ?= dusk.iso
-MODE ?= release
+MODE ?= debug
 ARCH ?= x86_64
 MEMORY ?= 512M
 # In MB
 ISO_SIZE ?= 512
 QEMU_OPTS ?= 
-#MKSQUASHFS_OPTS ?= 
 GDB ?= 
 CPUS ?= 1
 # FAT type
 ESP_BITS ?= 32
-EXPORT_SYMBOLS = true
+#EXPORT_SYMBOLS = true
 
 ISO_PATH = ${ARTIFACTS_PATH}/iso_root
-#INITRAMFS_PATH = ${ARTIFACTS_PATH}/initramfs
+INITRAMFS_PATH = ${ARTIFACTS_PATH}/initramfs
 IMAGE_PATH = ${ARTIFACTS_PATH}/${IMAGE_NAME}
+ESP_IMAGE = ${ARTIFACTS_PATH}/esp.img
+USERSPACE_CARGO_OPTS = --target ${ARCH}-unknown-none
 CARGO_OPTS = -Zjson-target-spec --target=src/arch/${ARCH}/${ARCH}-unknown-none.json
 QEMU_OPTS += -m ${MEMORY} -drive id=hd0,format=raw,file=${IMAGE_PATH}
 LIMINE_BOOT_VARIATION = X64
@@ -26,6 +27,7 @@ KERNEL_FILE = target/${ARCH}-unknown-none/${MODE}/dusk.elf
 
 ifeq (${MODE},release)
 	CARGO_OPTS += --release
+	USERSPACE_CARGO_OPTS += --release
 endif
 
 ifneq (${CPUS},1)
@@ -36,112 +38,87 @@ ifneq (${GDB},)
 	QEMU_OPTS += -s -S
 endif
 
-ifeq (${ARCH},aarch64)
+ifneq (${KVM},)
-	LIMINE_BOOT_VARIATION := AA64
+	QEMU_OPTS += -accel kvm -cpu host
-	UEFI := true
 endif
 
 ifneq (${UEFI},)
 	RUN_OPTS := ovmf-${ARCH}
-	ifeq (${ARCH},aarch64)
-		QEMU_OPTS += -M virt -bios ovmf/ovmf-${ARCH}/OVMF.fd
-	else
 	QEMU_OPTS += -bios ovmf/ovmf-${ARCH}/OVMF.fd
-	endif
 endif
 
 .PHONY: all build
 
 all: build
 
-build: prepare-bin-files compile-bootloader compile-binaries run-scripts build-iso
+build: prepare-bin-files compile-bootloader compile-binaries compile-initramfs build-iso
 
 check: 
-		cargo check
+		cargo check -Zjson-target-spec
 
 prepare-bin-files:
 		# Remove ISO and everything in the bin directory
 		rm -f ${IMAGE_PATH}
 		rm -rf ${ARTIFACTS_PATH}/*
 
-		# Make bin/ bin/iso_root and bin/initramfs
+		# Make bin/ and bin/iso_root
 		mkdir -p ${ARTIFACTS_PATH}
 		mkdir -p ${ISO_PATH}
-		# mkdir -p ${INITRAMFS_PATH}
+		mkdir -p ${INITRAMFS_PATH}
-		mkdir -p ${ARTIFACTS_PATH}/mnt
 
-#copy-initramfs-files:
+compile-user:
-#		echo "Hello World from Initramfs" > ${INITRAMFS_PATH}/example.txt
+		RUSTFLAGS="-C relocation-model=static" cargo build --package omega3 ${USERSPACE_CARGO_OPTS}
-#		echo "Second file for testing" > ${INITRAMFS_PATH}/example2.txt
+		RUSTFLAGS="-C relocation-model=static" cargo build --package client ${USERSPACE_CARGO_OPTS}
-#		mkdir -p ${INITRAMFS_PATH}/firstdir/seconddirbutlonger/
+		RUSTFLAGS="-C relocation-model=static" cargo build --package echo ${USERSPACE_CARGO_OPTS}
-#		mkdir ${INITRAMFS_PATH}/mnt/
-#		echo "Nexted file reads!!" > ${INITRAMFS_PATH}/firstdir/seconddirbutlonger/yeah.txt
 
-#compile-initramfs: copy-initramfs-files
+copy-initramfs-files: compile-user
-#		# Make squashfs without compression temporaily so I can get it working before I have to write a gzip driver
+		cp -v target/${ARCH}-unknown-none/${MODE}/omega3 ${INITRAMFS_PATH}/omega3.elf
-#		mksquashfs ${INITRAMFS_PATH} ${ARTIFACTS_PATH}/initramfs.img ${MKSQUASHFS_OPTS}
+		cp -v target/${ARCH}-unknown-none/${MODE}/client ${INITRAMFS_PATH}/client.elf
+		cp -v target/${ARCH}-unknown-none/${MODE}/echo ${INITRAMFS_PATH}/echo.elf
 
-run-scripts:
+compile-initramfs: copy-initramfs-files
-		# Place the build ID into the binary so it can be read at runtime
+		(cd ${INITRAMFS_PATH} && find . -mindepth 1 | cpio -o -H newc) > ${ARTIFACTS_PATH}/initramfs.img
-		@HASH=$$(md5sum ${KERNEL_FILE} | cut -c1-12) && \
-		sed -i "s/__BUILD_ID__/$${HASH}/" ${KERNEL_FILE}
-
-#ifeq (${EXPORT_SYMBOLS},true)
-#		nm ${KERNEL_FILE} > scripts/symbols.table
-#		@if [ ! -d "scripts/rustc_demangle" ]; then \
-#			git clone "https://github.com/juls0730/rustc_demangle.py" "scripts/rustc_demangle"; \
-#		fi
-#		python scripts/demangle-symbols.py
-#		mv scripts/symbols.table ${INITRAMFS_PATH}/
-#endif
-
-#		python scripts/font.py
-#		mv scripts/font.psf ${INITRAMFS_PATH}/
-
-		#python scripts/initramfs-test.py 100 ${INITRAMFS_PATH}/
 
 copy-iso-files:
 		# Limine files
 		mkdir -p ${ISO_PATH}/boot/limine
 		mkdir -p ${ISO_PATH}/EFI/BOOT
 
-		mkdir -p ${ISO_PATH}/mnt
-
 		cp -v limine.conf limine/limine-bios.sys ${ISO_PATH}/boot/limine
 		cp -v limine/BOOT${LIMINE_BOOT_VARIATION}.EFI ${ISO_PATH}/EFI/BOOT/
 
 		# OS files
 		cp -v ${KERNEL_FILE} ${ISO_PATH}/boot
-		#cp -v ${ARTIFACTS_PATH}/initramfs.img ${ISO_PATH}/boot
+		cp -v ${ARTIFACTS_PATH}/initramfs.img ${ISO_PATH}/boot
 
-partition-iso: copy-iso-files
+build-esp: copy-iso-files
-		# Make empty ISO of 64M in size
+		# Create and populate formatted FAT image for ESP partition (130048 1K-blocks = ~127MiB)
+		mkfs.fat -F ${ESP_BITS} -C ${ESP_IMAGE} 130048
+		mcopy -s -i ${ESP_IMAGE} ${ISO_PATH}/* ::
+
+partition-iso:
+		# Make empty disk image
 		dd if=/dev/zero of=${IMAGE_PATH} bs=1M count=0 seek=${ISO_SIZE}
+ifneq (${UEFI},)
 	parted -s ${IMAGE_PATH} mklabel gpt
-		parted -s ${IMAGE_PATH} mkpart BIOSBOOT 1024s 2047s
-		parted -s ${IMAGE_PATH} set 1 bios_grub on
-
 	parted -s ${IMAGE_PATH} mkpart ESP fat${ESP_BITS} 2048s 262144s
-
+	parted -s ${IMAGE_PATH} set 1 esp on
-		# Make ISO with 1 partition starting at sector 2048 that is 32768 sectors, or 16MiB, in size
+else
-		# Then a second partition spanning the rest of the disk
+	parted -s ${IMAGE_PATH} mklabel msdos
-		parted -s ${IMAGE_PATH} mkpart primary 262145s 100%
+	parted -s ${IMAGE_PATH} mkpart primary fat${ESP_BITS} 2048s 262144s
-		parted -s ${IMAGE_PATH} set 2 esp on
+	parted -s ${IMAGE_PATH} set 1 boot on
-
-build-iso: partition-iso 
-ifeq (${ARCH},x86_64)
-		# Install the Limine bootloader for bios installs
-		./limine/limine bios-install ${IMAGE_PATH}
 endif
 
-		sudo losetup -Pf --show ${IMAGE_PATH} > loopback_dev
+		# Make second partition spanning the rest of the disk
-		sudo mkfs.fat -F ${ESP_BITS} `cat loopback_dev`p2
+		parted -s ${IMAGE_PATH} mkpart primary 262145s 100%
-		sudo mount `cat loopback_dev`p2 ${ARTIFACTS_PATH}/mnt
+
-		sudo cp -r ${ISO_PATH}/* ${ARTIFACTS_PATH}/mnt
+build-iso: partition-iso build-esp
-		sync
+		# Splice ESP partition into sector 2048 of the disk image
-		sudo umount ${ARTIFACTS_PATH}/mnt
+		dd if=${ESP_IMAGE} of=${IMAGE_PATH} bs=512 seek=2048 conv=notrunc
-		sudo losetup -d `cat loopback_dev`
+ifeq (${UEFI},)
-		rm loopback_dev
+	# install limine for legacy bios
+	./limine/limine bios-install ${IMAGE_PATH}
+endif
 
 compile-bootloader:
 	@if [ ! -f "limine/.version" ] || [ "$$(cat limine/.version)" != "${LIMINE_VERSION}" ]; then \
@@ -158,26 +135,24 @@ compile-bootloader:
 compile-binaries:
 		cargo build ${CARGO_OPTS}
 
-ovmf-x86_64: ovmf
+ovmf-x86_64:
 	mkdir -p ovmf/ovmf-x86_64
-	@if [ ! -d "ovmf/ovmf-x86_64/OVMF.fd" ]; then \
+	@if [ ! -f "ovmf/ovmf-x86_64/OVMF.fd" ]; then \
 		cd ovmf/ovmf-x86_64 && curl -Lo OVMF.fd https://retrage.github.io/edk2-nightly/bin/RELEASEX64_OVMF.fd; \
 	fi
 
-ovmf-aarch64: ovmf
-	mkdir -p ovmf/ovmf-aarch64
-	@if [ ! -d "ovmf/ovmf-aarch64/OVMF.fd" ]; then \
-		cd ovmf/ovmf-aarch64 && curl -o OVMF.fd https://retrage.github.io/edk2-nightly/bin/RELEASEAARCH64_QEMU_EFI.fd; \
-	fi
-
 # In debug mode, open a terminal and run this command:
 # gdb target/x86_64-unknown-none/debug/CappuccinOS.elf -ex "target remote :1234"
 
 run: build ${RUN_OPTS} run-${ARCH}
+run-serial: build ${RUN_OPTS} run-${ARCH}-serial
 
 run-x86_64:
 	tmux new-session -d -s qemu 'qemu-system-x86_64 ${QEMU_OPTS}'
 
+run-x86_64-serial:
+	qemu-system-x86_64 ${QEMU_OPTS} -boot d -display none -serial stdio -monitor none -no-reboot
+
 line-count:
 		cloc --quiet --exclude-dir=bin --include-lang=Rust --csv src/ | tail -n 1 | awk -F, '{print $$5}'
 clean:

README.md

@@ -1,3 +1,4 @@
 # DuskOS
 
-A simple microkernel and operating system written in Rust for x86_64.
+A simple work-in-progress microkernel and operating system written in Rust for 
+x86_64.

build.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("cargo:rerun-if-changed=src/arch/x86_64/linker.ld");
+}

limine.conf

@@ -1,7 +1,8 @@
-timeout: 3
+timeout: 0
 
 /DuskOS
     protocol: limine
     
     path: boot():/boot/dusk.elf
+    module_path: boot():/boot/initramfs.img
 

src/arch/mod.rs

@@ -3,3 +3,9 @@ mod x86_64;
 
 #[cfg(target_arch = "x86_64")]
 pub use x86_64::*;
+
+#[cfg(target_arch = "x86_64")]
+pub(crate) use x86_64::{
+    PageTableCreateError, PageTableMapError, PageTableUnmapError, ThreadContext, set_kernel_stack,
+    switch_context,
+};

src/arch/x86_64/apic.rs

@@ -0,0 +1,224 @@
+use core::sync::atomic::{AtomicU32, AtomicUsize, Ordering};
+
+use crate::{
+    arch::{
+        port::write_u8,
+        x86_64::{
+            cpu::{read_msr, write_msr},
+            interrupts::apic_vectors::{
+                APIC_ERROR_VECTOR, APIC_SPURIOUS_VECTOR, APIC_TIMER_VECTOR,
+            },
+        },
+    },
+    memory::{
+        AddressSpace, CachePolicy, FrameAllocator, PagePermissions, PhysicalAddr, VirtualAddr,
+    },
+};
+
+const APIC_ID: u32 = 0x20;
+// End Of Interrupt
+const APIC_EOI: u32 = 0xB0;
+// Task Priority Register
+const APIC_TPR: u32 = 0x80;
+// Spurious Interrupt Vector
+const APIC_SVR: u32 = 0xF0;
+// Error Status Register
+const APIC_ESR: u32 = 0x280;
+const APIC_LVT_ERROR: u32 = 0x370;
+const APIC_LVT_MASKED: u64 = 1 << 16;
+
+const APIC_LVT_TIMER_MODE_PERIODIC: u64 = 1 << 17;
+
+pub const LOCAL_APIC_VIRTUAL_ADDRESS: VirtualAddr = VirtualAddr::new(0xFFFF_FFFD_0000_0000);
+
+const APIC_LVT_TIMER: u32 = 0x320;
+const APIC_TIMER_INITIAL_COUNT: u32 = 0x380;
+const APIC_TIMER_CURRENT_COUNT: u32 = 0x390;
+const APIC_TIMER_DIVIDE_CONFIG: u32 = 0x3E0;
+
+enum LocalApicAccess {
+    X2Apic,
+    XApic,
+}
+
+impl LocalApicAccess {
+    fn read(&self, offset: u32) -> u64 {
+        match self {
+            Self::X2Apic => {
+                let msr = 0x800 + offset / 16;
+                unsafe { read_msr(msr) }
+            }
+            Self::XApic => unsafe {
+                LOCAL_APIC_VIRTUAL_ADDRESS
+                    .as_ptr::<u8>()
+                    .add(offset as usize)
+                    .cast::<u32>()
+                    .read_volatile() as u64
+            },
+        }
+    }
+
+    fn write(&self, offset: u32, value: u64) {
+        match self {
+            Self::X2Apic => {
+                let msr = 0x800 + offset / 16;
+                unsafe { write_msr(msr, value) };
+            }
+            Self::XApic => unsafe {
+                LOCAL_APIC_VIRTUAL_ADDRESS
+                    .as_mut_ptr::<u8>()
+                    .add(offset as usize)
+                    .cast::<u32>()
+                    .write_volatile(value as u32);
+            },
+        }
+    }
+
+    fn end_of_interrupt(&self) {
+        self.write(APIC_EOI, 0);
+    }
+}
+
+#[derive(Debug)]
+pub enum LocalApicError {
+    AddressMismatch,
+    ApicDisabled,
+    FailedToMapApic,
+    NotBootSystemProcessor,
+}
+
+pub struct LocalApic {
+    id: u32,
+    access: LocalApicAccess,
+}
+
+const IA32_APIC_BASE: u32 = 0x1B;
+
+const APIC_BASE_BSP: u64 = 1 << 8;
+const APIC_BASE_X2APIC_ENABLE: u64 = 1 << 10;
+const APIC_BASE_GLOBAL_ENABLE: u64 = 1 << 11;
+// TODO: use MAXPHYADDR
+const APIC_BASE_ADDRESS_MASK: u64 = 0x000F_FFFF_FFFF_F000;
+
+impl LocalApic {
+    pub fn init(
+        local_apic_address: PhysicalAddr,
+        allocator: &mut FrameAllocator,
+        address_space: &mut AddressSpace,
+    ) -> Result<Self, LocalApicError> {
+        let apic_base = unsafe { read_msr(IA32_APIC_BASE) };
+        let bsp = (apic_base & APIC_BASE_BSP) != 0;
+        let x2apix = (apic_base & APIC_BASE_X2APIC_ENABLE) != 0;
+        let enabled = (apic_base & APIC_BASE_GLOBAL_ENABLE) != 0;
+        let xapic_physical_addr = PhysicalAddr::new((apic_base & APIC_BASE_ADDRESS_MASK) as usize);
+
+        if local_apic_address != xapic_physical_addr {
+            return Err(LocalApicError::AddressMismatch);
+        }
+
+        if !enabled {
+            return Err(LocalApicError::ApicDisabled);
+        }
+
+        if !bsp {
+            return Err(LocalApicError::NotBootSystemProcessor);
+        }
+
+        let access = match x2apix {
+            true => LocalApicAccess::X2Apic,
+            false => {
+                address_space
+                    .map(
+                        local_apic_address,
+                        LOCAL_APIC_VIRTUAL_ADDRESS,
+                        PagePermissions::new(true, false, false),
+                        allocator,
+                        CachePolicy::Uncacheable,
+                    )
+                    .map_err(|_| LocalApicError::FailedToMapApic)?;
+
+                LocalApicAccess::XApic
+            }
+        };
+
+        let raw_id = access.read(APIC_ID);
+
+        let id = match access {
+            LocalApicAccess::XApic => (raw_id >> 24) as u32,
+            LocalApicAccess::X2Apic => raw_id as u32,
+        };
+
+        // ensure legacy PIC is disabled
+        unsafe {
+            write_u8(0x21, 0xFF);
+            write_u8(0xA1, 0xFF);
+        }
+
+        access.write(APIC_LVT_ERROR, APIC_ERROR_VECTOR as u64 | APIC_LVT_MASKED);
+        access.write(APIC_ESR, 0);
+        let _ = access.read(APIC_ESR);
+        access.write(APIC_TPR, 0);
+        access.write(APIC_SVR, (1 << 8) | APIC_SPURIOUS_VECTOR as u64);
+
+        Ok(Self { id, access })
+    }
+
+    pub fn start_timer(&self) {
+        self.access.write(APIC_LVT_TIMER, APIC_TIMER_VECTOR as u64);
+    }
+
+    pub fn start_calibration_counter(&self) {
+        self.access
+            .write(APIC_LVT_TIMER, APIC_TIMER_VECTOR as u64 | APIC_LVT_MASKED);
+
+        self.access.write(APIC_TIMER_DIVIDE_CONFIG, 0b11);
+        self.access.write(APIC_TIMER_INITIAL_COUNT, u32::MAX as u64);
+    }
+
+    pub fn stop_timer(&self) {
+        self.access
+            .write(APIC_LVT_TIMER, APIC_TIMER_VECTOR as u64 | APIC_LVT_MASKED);
+        self.access.write(APIC_TIMER_INITIAL_COUNT, 0);
+    }
+
+    pub fn id(&self) -> u32 {
+        self.id
+    }
+}
+
+fn current_access() -> LocalApicAccess {
+    let apic_base = unsafe { read_msr(IA32_APIC_BASE) };
+
+    if apic_base & APIC_BASE_X2APIC_ENABLE != 0 {
+        LocalApicAccess::X2Apic
+    } else {
+        LocalApicAccess::XApic
+    }
+}
+
+pub fn current_timer_count() -> u32 {
+    current_access().read(APIC_TIMER_CURRENT_COUNT) as u32
+}
+
+pub(super) fn end_of_interrupt() {
+    current_access().end_of_interrupt();
+}
+
+static APIC_ERROR_COUNT: AtomicUsize = AtomicUsize::new(0);
+static LAST_APIC_ERROR: AtomicU32 = AtomicU32::new(0);
+
+pub(super) fn record_error() {
+    let access = current_access();
+
+    access.write(APIC_ESR, 0);
+    let error = access.read(APIC_ESR) as u32;
+
+    LAST_APIC_ERROR.store(error, Ordering::SeqCst);
+    APIC_ERROR_COUNT.fetch_add(1, Ordering::SeqCst);
+}
+
+static APIC_TIMER_COUNT: AtomicUsize = AtomicUsize::new(0);
+
+pub(super) fn record_timer() {
+    APIC_TIMER_COUNT.fetch_add(1, Ordering::SeqCst);
+}

src/arch/x86_64/cpu.rs

@@ -0,0 +1,250 @@
+use core::arch::{asm, naked_asm};
+
+use crate::memory::VirtualAddr;
+
+#[repr(C, align(64))]
+pub struct CpuLocal {
+    pub kernel_stack_top: usize,
+    pub user_rsp_scratch: usize,
+    pub cpu_id: u32,
+}
+
+pub static mut BOOT_CPU: CpuLocal = CpuLocal {
+    kernel_stack_top: 0,
+    user_rsp_scratch: 0,
+    cpu_id: 0,
+};
+
+#[derive(Debug)]
+pub struct ThreadContext {
+    rsp: usize,
+}
+
+impl ThreadContext {
+    pub fn new(
+        user_entry: VirtualAddr,
+        user_stack: VirtualAddr,
+        kernel_stack_top: VirtualAddr,
+    ) -> Self {
+        // Stack layout (grows downwards from kernel_stack_top):
+        // [top - 8]  = user_thread_entry (popped by `ret`)
+        // [top - 16] = rbp (0)
+        // [top - 24] = rbx (user_stack)
+        // [top - 32] = r12 (user_entry)
+        // [top - 40] = r13 (0)
+        // [top - 48] = r14 (0)
+        // [top - 56] = r15 (0) <- initial rsp
+
+        let stack_ptr = (kernel_stack_top.as_usize() - 56) as *mut usize;
+
+        unsafe {
+            stack_ptr.add(0).write(0); // r15
+            stack_ptr.add(1).write(0); // r14
+            stack_ptr.add(2).write(0); // r13
+            stack_ptr.add(3).write(user_entry.as_usize()); // r12 (user_entry)
+            stack_ptr.add(4).write(user_stack.as_usize()); // rbx (user_stack)
+            stack_ptr.add(5).write(0); // rbp (0)
+            stack_ptr
+                .add(6)
+                .write(user_thread_entry as *const () as usize); // return address
+        }
+
+        Self {
+            rsp: kernel_stack_top.as_usize() - 56,
+        }
+    }
+
+    pub fn empty() -> Self {
+        Self { rsp: 0 }
+    }
+}
+
+#[unsafe(naked)]
+unsafe extern "C" fn user_thread_entry() -> ! {
+    naked_asm!(
+        // r12 = user_entry, rbx = user_stack
+        "mov rdi, r12",
+        "mov rsi, rbx",
+        "call {enter_user}",
+        enter_user = sym crate::arch::enter_user,
+    );
+}
+
+#[unsafe(naked)]
+pub unsafe extern "C" fn switch_context(prev: *mut ThreadContext, next: *const ThreadContext) {
+    naked_asm!(
+        "push rbp",
+        "push rbx",
+        "push r12",
+        "push r13",
+        "push r14",
+        "push r15",
+        "",
+        // save current rsp into prev.rsp
+        "mov [rdi], rsp",
+        // load next rsp into rsp
+        "mov rsp, [rsi]",
+        "",
+        "pop r15",
+        "pop r14",
+        "pop r13",
+        "pop r12",
+        "pop rbx",
+        "pop rbp",
+        "ret",
+    )
+}
+
+#[derive(Debug)]
+pub enum CpuFeaturesError {
+    CpuidFeaturesNotSupported,
+    SyscallNotSupported,
+    InvalidPhysicalAddressWidth,
+    InvalidVirtualAddressWidth,
+}
+
+#[derive(Clone, Copy)]
+pub(crate) struct CpuFeatures {
+    pub nx_supported: bool,
+    pub nx_enabled: bool,
+    pub global_pages: bool,
+    pub physical_address_bits: u8,
+    pub virtual_address_bits: u8,
+    pub five_level_paging_active: bool,
+}
+
+// Extended features
+const IA32_EFER: u32 = 0xC0000080;
+
+pub fn detect_features_and_enable() -> Result<CpuFeatures, CpuFeaturesError> {
+    let mut features = CpuFeatures {
+        nx_supported: false,
+        nx_enabled: false,
+        global_pages: false,
+        physical_address_bits: 0,
+        virtual_address_bits: 0,
+        five_level_paging_active: false,
+    };
+
+    let cpuid_result = core::arch::x86_64::__cpuid(0x80000000);
+
+    if cpuid_result.eax < 0x80000008 {
+        return Err(CpuFeaturesError::CpuidFeaturesNotSupported);
+    }
+
+    let cpuid_result = core::arch::x86_64::__cpuid(0x80000001);
+
+    features.nx_supported = cpuid_result.edx & (1 << 20) != 0;
+
+    // TODO: on AMD K6 *only*, this bit is bit 10, should we consider that edge case?
+    let syscall_supported = cpuid_result.edx & (1 << 11) != 0;
+    if !syscall_supported {
+        return Err(CpuFeaturesError::SyscallNotSupported);
+    }
+
+    unsafe {
+        let efer = read_msr(IA32_EFER);
+        let value = efer | 1;
+        write_msr(IA32_EFER, value);
+    };
+
+    let cpuid_result = core::arch::x86_64::__cpuid(0x80000008);
+
+    features.physical_address_bits = (cpuid_result.eax & 0xFF) as u8;
+    if !(12..=52).contains(&features.physical_address_bits) {
+        return Err(CpuFeaturesError::InvalidPhysicalAddressWidth);
+    }
+
+    features.virtual_address_bits = (cpuid_result.eax >> 8 & 0xFF) as u8;
+    features.five_level_paging_active = read_cr4() & (1 << 12) != 0;
+
+    let required_virtual_address_bits = if features.five_level_paging_active {
+        57
+    } else {
+        48
+    };
+    if features.virtual_address_bits < required_virtual_address_bits {
+        return Err(CpuFeaturesError::InvalidVirtualAddressWidth);
+    }
+
+    let cpuid_result = core::arch::x86_64::__cpuid(0x1);
+
+    features.global_pages = cpuid_result.edx & (1 << 13) != 0;
+
+    if features.global_pages {
+        let cr4 = read_cr4();
+        write_cr4(cr4 | 1 << 7);
+    }
+
+    if features.nx_supported {
+        let msr_supported = cpuid_result.edx & (1 << 5) != 0;
+
+        if !msr_supported {
+            return Err(CpuFeaturesError::CpuidFeaturesNotSupported);
+        }
+
+        // mother efer
+        let efer = unsafe { read_msr(IA32_EFER) };
+
+        unsafe {
+            write_msr(IA32_EFER, efer | (1 << 11));
+        }
+
+        features.nx_enabled = unsafe { read_msr(IA32_EFER) } & (1 << 11) != 0;
+    }
+
+    Ok(features)
+}
+
+fn write_cr4(value: usize) {
+    unsafe {
+        asm!(
+            "mov cr4, {}",
+            in(reg) value,
+            options(nostack, preserves_flags)
+        );
+    }
+}
+
+fn read_cr4() -> usize {
+    let value: usize;
+
+    unsafe {
+        asm!(
+            "mov {}, cr4",
+            out(reg) value,
+            options(nomem, nostack, preserves_flags),
+        );
+    }
+
+    value
+}
+
+pub(super) unsafe fn read_msr(msr: u32) -> u64 {
+    let low: u32;
+    let high: u32;
+
+    unsafe {
+        asm!(
+            "rdmsr",
+            in("ecx") msr,
+            out("eax") low,
+            out("edx") high,
+            options(nomem, nostack, preserves_flags),
+        );
+    }
+
+    ((high as u64) << 32) | low as u64
+}
+
+pub(super) unsafe fn write_msr(msr: u32, value: u64) {
+    unsafe {
+        asm!(
+            "wrmsr",
+            in("ecx") msr,
+            in("eax") value as u32,
+            in("edx") (value >> 32) as u32,
+            options(nomem, nostack, preserves_flags),
+        );
+    }
+}

src/arch/x86_64/gdt.rs

@@ -1,13 +1,15 @@
 use core::arch::asm;
 
+use crate::memory::VirtualAddr;
+
 #[repr(C, align(8))]
-struct Gdt {
+pub(super) struct Gdt {
-    entries: [u64; 5],
+    pub entries: [u64; 7],
 }
 
 impl Gdt {
     pub const fn new() -> Self {
-        Self { entries: [0; 5] }
+        Self { entries: [0; 7] }
     }
 }
 
@@ -47,19 +49,22 @@ const DOUBLE_FAULT_STACK_SIZE: usize = 16 * 1024;
 
 pub(super) const KERNEL_CODE_SELECTOR: u16 = 1 * 8;
 pub(super) const KERNEL_DATA_SELECTOR: u16 = 2 * 8;
-pub(super) const TSS_SELECTOR: u16 = 3 * 8;
+pub(super) const USER_DATA_SELECTOR: u16 = (3 * 8) | 3;
+pub(super) const USER_CODE_SELECTOR: u16 = (4 * 8) | 3;
+pub(super) const TSS_SELECTOR: u16 = 5 * 8;
 
 #[repr(align(16))]
+#[allow(unused)] // field 0 is read, rust just cant tell
 struct ExceptionStack([u8; DOUBLE_FAULT_STACK_SIZE]);
 
 static mut GDT: Gdt = Gdt::new();
 static mut TSS: TaskStateSegment = TaskStateSegment::new();
 static mut DOUBLE_FAULT_STACK: ExceptionStack = ExceptionStack([0; DOUBLE_FAULT_STACK_SIZE]);
 
-pub fn init() {
+const _: () = assert!(core::mem::size_of::<TaskStateSegment>() == 104);
-    const _: () = assert!(core::mem::size_of::<TaskStateSegment>() == 104);
+const _: () = assert!(core::mem::size_of::<GdtPointer>() == 10);
-    const _: () = assert!(core::mem::size_of::<GdtPointer>() == 10);
 
+pub fn init() {
     unsafe {
         let stack_bottom = core::ptr::addr_of_mut!(DOUBLE_FAULT_STACK) as u64;
         let stack_top = stack_bottom + DOUBLE_FAULT_STACK_SIZE as u64;
@@ -73,6 +78,10 @@ pub fn init() {
                 0,
                 kernel_code_descriptor(),
                 kernel_data_descriptor(),
+                // In Long Mode, userland CS will be loaded from STAR 63:48 + 16
+                // and userland SS from STAR 63:48 + 8 on SYSRET.
+                user_data_descriptor(),
+                user_code_descriptor(),
                 tss_low,
                 tss_high,
             ],
@@ -83,6 +92,12 @@ pub fn init() {
     }
 }
 
+pub fn set_kernel_stack(stack_top: VirtualAddr) {
+    unsafe {
+        TSS.privilege_stacks[0] = stack_top.as_usize() as u64;
+    }
+}
+
 fn gdt_reload() {
     unsafe {
         asm!(
@@ -118,17 +133,34 @@ fn gdt_reload() {
 }
 
 const PRESENT: u64 = 1 << 47;
-const USER_DESCRIPTOR: u64 = 1 << 44;
+const CODE_DATA_DESCRIPTOR: u64 = 1 << 44;
+const USER_PRIVILEGE: u64 = 3 << 45;
 const EXECUTABLE: u64 = 1 << 43;
 const READ_WRITE: u64 = 1 << 41;
+const GRANULARITY: u64 = 1 << 55;
+const SIZE: u64 = 1 << 54;
 const LONG_MODE: u64 = 1 << 53;
 
 fn kernel_code_descriptor() -> u64 {
-    PRESENT | USER_DESCRIPTOR | EXECUTABLE | READ_WRITE | LONG_MODE
+    PRESENT | CODE_DATA_DESCRIPTOR | EXECUTABLE | READ_WRITE | LONG_MODE | GRANULARITY
 }
 
 fn kernel_data_descriptor() -> u64 {
-    PRESENT | USER_DESCRIPTOR | READ_WRITE
+    PRESENT | CODE_DATA_DESCRIPTOR | READ_WRITE | SIZE | GRANULARITY
+}
+
+fn user_code_descriptor() -> u64 {
+    PRESENT
+        | CODE_DATA_DESCRIPTOR
+        | USER_PRIVILEGE
+        | EXECUTABLE
+        | READ_WRITE
+        | LONG_MODE
+        | GRANULARITY
+}
+
+fn user_data_descriptor() -> u64 {
+    PRESENT | CODE_DATA_DESCRIPTOR | USER_PRIVILEGE | READ_WRITE | SIZE | GRANULARITY
 }
 
 fn tss_descriptor(tss: *const TaskStateSegment) -> [u64; 2] {

src/arch/x86_64/interrupts/apic_vectors.rs

@@ -0,0 +1,40 @@
+use super::idt::{self, InterruptFrame, stub_no_err};
+use crate::arch::{apic, timer};
+
+pub const PIT_CALIBRATION_VECTOR: u8 = 0xF1;
+pub const APIC_TIMER_VECTOR: u8 = 0xFD;
+pub const APIC_ERROR_VECTOR: u8 = 0xFE;
+pub const APIC_SPURIOUS_VECTOR: u8 = 0xFF;
+
+stub_no_err!(stub_pit_calibration, 0xF1);
+stub_no_err!(stub_apic_timer, 0xFD);
+stub_no_err!(stub_apic_error, 0xFE);
+stub_no_err!(stub_apic_spurious, 0xFF);
+
+pub(super) fn handle(frame: &mut InterruptFrame) {
+    match frame.vector as u8 {
+        PIT_CALIBRATION_VECTOR => {
+            timer::record_pit_calibration();
+            apic::end_of_interrupt();
+        }
+        APIC_TIMER_VECTOR => {
+            apic::record_timer();
+            apic::end_of_interrupt();
+        }
+        APIC_ERROR_VECTOR => {
+            apic::record_error();
+            apic::end_of_interrupt();
+        }
+        APIC_SPURIOUS_VECTOR => {
+            // No EOI
+        }
+        _ => {}
+    }
+}
+
+pub(super) fn install(idt: &mut idt::Idt) {
+    idt.set_handler(PIT_CALIBRATION_VECTOR, stub_pit_calibration, 0);
+    idt.set_handler(APIC_ERROR_VECTOR, stub_apic_error, 0);
+    idt.set_handler(APIC_TIMER_VECTOR, stub_apic_timer, 0);
+    idt.set_handler(APIC_SPURIOUS_VECTOR, stub_apic_spurious, 0);
+}

src/arch/x86_64/interrupts/exceptions.rs

@@ -1,77 +1,115 @@
 use core::arch::asm;
 
-use super::idt::{self, InterruptStackFrame};
+use super::idt::{self, InterruptFrame, InterruptStackFrame, stub_err, stub_no_err};
-use crate::{hcf, println};
+use crate::{
+    hcf, println,
+    task::tcb::{ExitReason, Fault},
+};
 
-macro_rules! fatal_without_error_code {
+stub_no_err!(stub_divide_error, 0);
-    ($handler:ident, $name:literal) => {
+stub_no_err!(stub_debug, 1);
-        extern "x86-interrupt" fn $handler(frame: InterruptStackFrame) {
+stub_no_err!(stub_non_maskable_interrupt, 2);
-            fatal_exception($name, &frame, None);
+stub_no_err!(stub_breakpoint, 3);
-        }
+stub_no_err!(stub_invalid_opcode, 6);
-    };
+stub_no_err!(stub_device_not_available, 7);
-}
+stub_err!(stub_double_fault, 8);
+stub_err!(stub_invalid_tss, 10);
+stub_err!(stub_segment_not_present, 11);
+stub_err!(stub_stack_segment_fault, 12);
+stub_err!(stub_general_protection, 13);
+stub_err!(stub_page_fault, 14);
+stub_no_err!(stub_x87_floating_point, 16);
+stub_err!(stub_alignment_check, 17);
+stub_no_err!(stub_machine_check, 18);
+stub_no_err!(stub_simd_floating_point, 19);
+stub_no_err!(stub_user_test_exit, 0x80);
 
-macro_rules! fatal_with_error_code {
+const EXCEPTION_NAMES: [&str; 32] = [
-    ($handler:ident, $name:literal) => {
+    "DIVIDE ERROR",
-        extern "x86-interrupt" fn $handler(frame: InterruptStackFrame, error_code: u64) {
+    "DEBUG",
-            fatal_exception($name, &frame, Some(error_code));
+    "NON-MASKABLE INTERRUPT",
-        }
+    "BREAKPOINT",
-    };
+    "OVERFLOW",
-}
+    "BOUND RANGE EXCEEDED",
+    "INVALID OPCODE",
+    "DEVICE NOT AVAILABLE",
+    "DOUBLE FAULT",
+    "COPROCESSOR SEGMENT OVERRUN",
+    "INVALID TSS",
+    "SEGMENT NOT PRESENT",
+    "STACK-SEGMENT FAULT",
+    "GENERAL PROTECTION FAULT",
+    "PAGE FAULT",
+    "RESERVED",
+    "x87 FLOATING-POINT EXCEPTION",
+    "ALIGNMENT CHECK",
+    "MACHINE CHECK",
+    "SIMD FLOATING-POINT EXCEPTION",
+    "VIRTUALIZATION EXCEPTION",
+    "CONTROL PROTECTION EXCEPTION",
+    "RESERVED",
+    "RESERVED",
+    "RESERVED",
+    "RESERVED",
+    "RESERVED",
+    "RESERVED",
+    "HYPERVISOR INJECTION EXCEPTION",
+    "VMM COMMUNICATION EXCEPTION",
+    "SECURITY EXCEPTION",
+    "RESERVED",
+];
 
-extern "x86-interrupt" fn debug_handler(frame: InterruptStackFrame) {
+pub(super) fn handle(frame: &mut InterruptFrame) {
-    fatal_exception("DEBUG EXCEPTION", &frame, None);
+    let is_user = frame.stack_frame.code_segment & 0b11 == 3;
-}
+    let vector = frame.vector as u8;
+    let name = EXCEPTION_NAMES
+        .get(vector as usize)
+        .copied()
+        .unwrap_or("UNKNOWN EXCEPTION");
 
-extern "x86-interrupt" fn non_maskable_interrupt_handler(frame: InterruptStackFrame) {
+    if !is_user {
-    fatal_exception("NON-MASKABLE INTERRUPT", &frame, None);
+        if vector == 14 {
-}
+            report_exception(name, &frame.stack_frame, Some(frame.error_code));
-
-extern "x86-interrupt" fn breakpoint_handler(frame: InterruptStackFrame) {
-    report_exception("BREAKPOINT", &frame, None);
-}
-
-extern "x86-interrupt" fn double_fault_handler(frame: InterruptStackFrame, error_code: u64) {
-    fatal_exception("DOUBLE FAULT", &frame, Some(error_code));
-}
-
-extern "x86-interrupt" fn page_fault_handler(frame: InterruptStackFrame, error_code: u64) {
-    report_exception("PAGE FAULT", &frame, Some(error_code));
             println!("Faulting address: {:#X}", read_cr2());
-    print_page_fault_error(error_code);
+            print_page_fault_error(frame.error_code);
             hcf();
+        }
+
+        fatal_exception(name, &frame.stack_frame, Some(frame.error_code));
+    }
+
+    let fault = match vector {
+        // Page fault, GPF, Stack/Segment faults -> SegmentationFault
+        11 | 12 | 13 | 14 => Fault::SegmentationFault,
+        // Invalid Opcode -> IllegalInstruction
+        6 => Fault::IllegalInstruction,
+        // Divide by zero, Alignment check, SIMD/x87 -> Abort
+        0 | 16 | 17 | 19 => Fault::Abort,
+        _ => Fault::Abort,
+    };
+
+    crate::task::scheduler::exit_current(ExitReason::Fault(fault));
 }
 
-fatal_without_error_code!(divide_error_handler, "DIVIDE ERROR");
-fatal_without_error_code!(invalid_opcode_handler, "INVALID OPCODE");
-fatal_without_error_code!(device_not_available_handler, "DEVICE NOT AVAILABLE");
-fatal_without_error_code!(x87_floating_point_handler, "X87 FLOATING-POINT EXCEPTION");
-fatal_without_error_code!(machine_check_handler, "MACHINE CHECK");
-fatal_without_error_code!(simd_floating_point_handler, "SIMD FLOATING-POINT EXCEPTION");
-
-fatal_with_error_code!(invalid_tss_handler, "INVALID TSS");
-fatal_with_error_code!(segment_not_present_handler, "SEGMENT NOT PRESENT");
-fatal_with_error_code!(stack_segment_fault_handler, "STACK-SEGMENT FAULT");
-fatal_with_error_code!(general_protection_handler, "GENERAL PROTECTION FAULT");
-fatal_with_error_code!(alignment_check_handler, "ALIGNMENT CHECK");
-
 pub(super) fn install(idt: &mut idt::Idt) {
-    idt.set_handler(0, divide_error_handler, 0);
+    idt.set_handler(0, stub_divide_error, 0);
-    idt.set_handler(1, debug_handler, 0);
+    idt.set_handler(1, stub_debug, 0);
-    idt.set_handler(2, non_maskable_interrupt_handler, 0);
+    idt.set_handler(2, stub_non_maskable_interrupt, 0);
-    idt.set_handler(3, breakpoint_handler, 0);
+    idt.set_user_handler(3, stub_breakpoint, 0);
-    idt.set_handler(6, invalid_opcode_handler, 0);
+    idt.set_handler(6, stub_invalid_opcode, 0);
-    idt.set_handler(7, device_not_available_handler, 0);
+    idt.set_handler(7, stub_device_not_available, 0);
-    idt.set_error_code_handler(8, double_fault_handler, 1);
+    idt.set_handler(8, stub_double_fault, 1);
-    idt.set_error_code_handler(10, invalid_tss_handler, 0);
+    idt.set_handler(10, stub_invalid_tss, 0);
-    idt.set_error_code_handler(11, segment_not_present_handler, 0);
+    idt.set_handler(11, stub_segment_not_present, 0);
-    idt.set_error_code_handler(12, stack_segment_fault_handler, 0);
+    idt.set_handler(12, stub_stack_segment_fault, 0);
-    idt.set_error_code_handler(13, general_protection_handler, 0);
+    idt.set_handler(13, stub_general_protection, 0);
-    idt.set_error_code_handler(14, page_fault_handler, 0);
+    idt.set_handler(14, stub_page_fault, 0);
-    idt.set_handler(16, x87_floating_point_handler, 0);
+    idt.set_handler(16, stub_x87_floating_point, 0);
-    idt.set_error_code_handler(17, alignment_check_handler, 0);
+    idt.set_handler(17, stub_alignment_check, 0);
-    idt.set_handler(18, machine_check_handler, 0);
+    idt.set_handler(18, stub_machine_check, 0);
-    idt.set_handler(19, simd_floating_point_handler, 0);
+    idt.set_handler(19, stub_simd_floating_point, 0);
+
+    idt.set_user_handler(0x80, stub_user_test_exit, 0);
 }
 
 fn read_cr2() -> u64 {
@@ -134,10 +172,10 @@ fn report_exception(name: &'static str, frame: &InterruptStackFrame, error_code:
             "kernel"
         }
     );
-    println!("RIP: {:#X}", frame.instruction_pointer.as_u64());
+    println!("RIP: {:#X}", frame.instruction_pointer.as_usize());
     println!("CS: {:#X}", frame.code_segment);
     println!("FLAGS: {:#X}", frame.cpu_flags);
-    println!("RSP: {:#X}", frame.stack_pointer.as_u64());
+    println!("RSP: {:#X}", frame.stack_pointer.as_usize());
     println!("SS: {:#X}", frame.stack_segment);
 
     if let Some(error_code) = error_code {

src/arch/x86_64/interrupts/idt.rs

@@ -1,5 +1,8 @@
 use super::exceptions;
-use crate::{arch::x86_64::gdt::KERNEL_CODE_SELECTOR, memory::VirtualAddr};
+use crate::{
+    arch::x86_64::{gdt::KERNEL_CODE_SELECTOR, interrupts::apic_vectors},
+    memory::VirtualAddr,
+};
 
 #[repr(C, packed)]
 #[derive(Clone, Copy)]
@@ -15,15 +18,15 @@ struct IdtEntry {
 
 impl IdtEntry {
     const fn missing() -> Self {
-        return Self {
+        Self {
             offset_low: 0,
-            code_selector: 0x08,
+            code_selector: 0,
             ist: 0,
             attributes: 0,
             offset_middle: 0,
             offset_high: 0,
             reserved: 0,
-        };
+        }
     }
 }
 
@@ -35,7 +38,7 @@ struct IdtPointer {
 
 #[repr(C)]
 #[derive(Clone, Copy, Debug)]
-pub(super) struct InterruptStackFrame {
+pub struct InterruptStackFrame {
     pub instruction_pointer: VirtualAddr,
     pub code_segment: u64,
     pub cpu_flags: u64,
@@ -43,12 +46,36 @@ pub(super) struct InterruptStackFrame {
     pub stack_segment: u64,
 }
 
+#[repr(C)]
+#[derive(Debug)]
+pub struct InterruptFrame {
+    pub rax: u64,
+    pub rcx: u64,
+    pub rdx: u64,
+    pub rsi: u64,
+    pub rdi: u64,
+    pub r8: u64,
+    pub r9: u64,
+    pub r10: u64,
+    pub r11: u64,
+    pub rbx: u64,
+    pub rbp: u64,
+    pub r12: u64,
+    pub r13: u64,
+    pub r14: u64,
+    pub r15: u64,
+    pub vector: u64,
+    pub error_code: u64,
+    pub stack_frame: InterruptStackFrame,
+}
+
 const INTERRUPT_GATE: u8 = 0b1110;
 const PRESENT: u8 = 1 << 7;
 const KERNEL_INTERRUPT_GATE: u8 = PRESENT | INTERRUPT_GATE;
+const USER_DPL: u8 = 3 << 5;
+const USER_INTERRUPT_GATE: u8 = PRESENT | USER_DPL | INTERRUPT_GATE;
 
-pub(super) type Handler = extern "x86-interrupt" fn(InterruptStackFrame);
+pub(super) type RawHandler = unsafe extern "C" fn();
-pub(super) type ErrorCodeHandler = extern "x86-interrupt" fn(InterruptStackFrame, u64);
 
 pub(super) struct Idt {
     entries: [IdtEntry; 256],
@@ -61,27 +88,22 @@ impl Idt {
         }
     }
 
-    pub(super) fn set_handler(&mut self, vector: u8, handler: Handler, ist: u8) {
+    pub(super) fn set_handler(&mut self, vector: u8, handler: RawHandler, ist: u8) {
-        self.set_handler_address(vector, handler as usize, ist);
+        self.set_handler_address(vector, handler as usize, ist, KERNEL_INTERRUPT_GATE);
     }
 
-    pub(super) fn set_error_code_handler(
+    pub(super) fn set_user_handler(&mut self, vector: u8, handler: RawHandler, ist: u8) {
-        &mut self,
+        self.set_handler_address(vector, handler as usize, ist, USER_INTERRUPT_GATE);
-        vector: u8,
-        handler: ErrorCodeHandler,
-        ist: u8,
-    ) {
-        self.set_handler_address(vector, handler as usize, ist);
     }
 
-    fn set_handler_address(&mut self, vector: u8, address: usize, ist: u8) {
+    fn set_handler_address(&mut self, vector: u8, address: usize, ist: u8, attributes: u8) {
         self.entries[vector as usize] = IdtEntry {
             offset_low: address as u16,
             offset_middle: (address >> 16) as u16,
             offset_high: (address >> 32) as u32,
             code_selector: KERNEL_CODE_SELECTOR,
             ist: ist & 0b111,
-            attributes: KERNEL_INTERRUPT_GATE,
+            attributes,
             reserved: 0,
         };
     }
@@ -89,14 +111,119 @@ impl Idt {
 
 static mut IDT: Idt = Idt::new();
 
-pub fn idt_init() {
+const _: () = assert!(core::mem::size_of::<IdtEntry>() == 16);
-    const _: () = assert!(core::mem::size_of::<IdtEntry>() == 16);
+const _: () = assert!(core::mem::size_of::<IdtPointer>() == 10);
-    const _: () = assert!(core::mem::size_of::<IdtPointer>() == 10);
+const _: () = assert!(core::mem::size_of::<InterruptStackFrame>() == 40);
-    const _: () = assert!(core::mem::size_of::<InterruptStackFrame>() == 40);
+const _: () = assert!(core::mem::size_of::<InterruptFrame>() == 176);
+const _: () = assert!(core::mem::offset_of!(InterruptFrame, stack_frame) == 136);
 
+#[unsafe(naked)]
+pub(super) unsafe extern "C" fn interrupt_common() {
+    core::arch::naked_asm!(
+        "push r15",
+        "push r14",
+        "push r13",
+        "push r12",
+        "push rbp",
+        "push rbx",
+        "push r11",
+        "push r10",
+        "push r9",
+        "push r8",
+        "push rdi",
+        "push rsi",
+        "push rdx",
+        "push rcx",
+        "push rax",
+
+        // Check CS: bit 0 and 1 are CPL. If CPL != 0 (user mode), swapgs
+        "test byte ptr [rsp + 144], 3",
+        "jz 1f",
+        "swapgs",
+        "1:",
+
+        "mov rdi, rsp",
+        "cld",
+        "call {dispatch}",
+
+        // Check CS: if returning to user mode, swapgs
+        "test byte ptr [rsp + 144], 3",
+        "jz 2f",
+        "swapgs",
+        "2:",
+
+        "pop rax",
+        "pop rcx",
+        "pop rdx",
+        "pop rsi",
+        "pop rdi",
+        "pop r8",
+        "pop r9",
+        "pop r10",
+        "pop r11",
+        "pop rbx",
+        "pop rbp",
+        "pop r12",
+        "pop r13",
+        "pop r14",
+        "pop r15",
+
+        "add rsp, 16",
+        "iretq",
+
+        dispatch = sym interrupt_dispatch,
+    );
+}
+
+extern "C" fn interrupt_dispatch(frame: &mut InterruptFrame) {
+    let vector = frame.vector as u8;
+    match vector {
+        0..=31 | 0x80 => exceptions::handle(frame),
+        apic_vectors::PIT_CALIBRATION_VECTOR
+        | apic_vectors::APIC_TIMER_VECTOR
+        | apic_vectors::APIC_ERROR_VECTOR
+        | apic_vectors::APIC_SPURIOUS_VECTOR => apic_vectors::handle(frame),
+        _ => {
+            crate::println!("Unhandled interrupt vector: {:#X}", vector);
+        }
+    }
+}
+
+macro_rules! stub_no_err {
+    ($name:ident, $vec:literal) => {
+        #[unsafe(naked)]
+        pub(super) unsafe extern "C" fn $name() {
+            core::arch::naked_asm!(
+                "push 0",
+                concat!("push ", stringify!($vec)),
+                "jmp {common}",
+                common = sym $crate::arch::x86_64::interrupts::idt::interrupt_common,
+            );
+        }
+    };
+}
+
+macro_rules! stub_err {
+    ($name:ident, $vec:literal) => {
+        #[unsafe(naked)]
+        pub(super) unsafe extern "C" fn $name() {
+            core::arch::naked_asm!(
+                concat!("push ", stringify!($vec)),
+                "jmp {common}",
+                common = sym $crate::arch::x86_64::interrupts::idt::interrupt_common,
+            );
+        }
+    };
+}
+
+pub(super) use stub_err;
+pub(super) use stub_no_err;
+
+pub fn idt_init() {
     let mut idt = Idt::new();
 
     exceptions::install(&mut idt);
+    apic_vectors::install(&mut idt);
 
     unsafe {
         core::ptr::addr_of_mut!(IDT).write(idt);

src/arch/x86_64/interrupts/mod.rs

@@ -1,12 +1,48 @@
 use core::arch::asm;
 
+pub(super) mod apic_vectors;
 mod exceptions;
 mod idt;
 
 pub use idt::idt_init as init;
 
-pub fn disable_interrupts() {
+#[inline(always)]
+pub fn disable_interrupts_and_save() -> u64 {
+    let flags: u64;
+
     unsafe {
-        asm!("cli");
+        asm!("
+            pushfq",
+            "pop {flags}",
+            "cli",
+            flags = out(reg) flags,
+        );
+    }
+
+    flags
+}
+
+#[inline(always)]
+pub fn restore_interrupts(flags: u64) {
+    unsafe {
+        asm!(
+            "push {flags}",
+            "popfq",
+            flags = in(reg) flags,
+        );
+    }
+}
+
+#[inline(always)]
+pub fn disable_interrupts() {
+    unsafe {
+        asm!("cli", options(nostack));
+    }
+}
+
+#[inline(always)]
+pub fn enable_interrupts() {
+    unsafe {
+        asm!("sti", options(nostack));
     }
 }

src/arch/x86_64/io_apic.rs

@@ -0,0 +1,193 @@
+use crate::{
+    memory::{
+        AddressSpace, CachePolicy, FrameAllocator, PagePermissions, PhysicalAddr, VirtualAddr,
+    },
+    platform::acpi::{InterruptPolarity, TriggerMode},
+};
+
+const IOWIN: usize = 0x10;
+
+const IOAPIC_ID: u8 = 0x00;
+const IOAPIC_VERSION: u8 = 0x01;
+const IOAPIC_REDIRECTION_BASE: u8 = 0x10;
+
+const MASKED: u32 = 1 << 16;
+
+pub const IOAPIC_VIRTUAL_ADDRESS: VirtualAddr = VirtualAddr::new(0xFFFF_FFFD_1000_0000);
+
+#[derive(Debug)]
+#[allow(unused)]
+pub enum IoApicError {
+    IdMismatch { expected: u8, actual: u8 },
+    GsiOutsideRange,
+    FailedToMapIoApic,
+    InvalidRedirectionIndex,
+}
+
+pub struct RedirectionConfig {
+    pub vector: u8,
+    pub destination: u8,
+    pub polarity: InterruptPolarity,
+    pub trigger: TriggerMode,
+}
+
+pub struct IoApic {
+    base: VirtualAddr,
+    global_interrupt_base: u32,
+    redirection_entry_count: u32,
+}
+
+impl IoApic {
+    pub fn new(
+        expected_id: u8,
+        physical_address: PhysicalAddr,
+        global_interrupt_base: u32,
+        virtual_address: VirtualAddr,
+        allocator: &mut FrameAllocator,
+        address_space: &mut AddressSpace,
+    ) -> Result<Self, IoApicError> {
+        address_space
+            .map(
+                physical_address,
+                virtual_address,
+                PagePermissions::new(true, false, false),
+                allocator,
+                CachePolicy::Uncacheable,
+            )
+            .map_err(|_| IoApicError::FailedToMapIoApic)?;
+
+        let mut io_apic = Self {
+            base: virtual_address,
+            global_interrupt_base: global_interrupt_base,
+            redirection_entry_count: 0,
+        };
+
+        let version = io_apic.read(IOAPIC_VERSION);
+        io_apic.redirection_entry_count = ((version >> 16) & 0xFF) + 1;
+
+        let id = ((io_apic.read(IOAPIC_ID) >> 24) & 0xF) as u8;
+        if id != expected_id {
+            return Err(IoApicError::IdMismatch {
+                expected: expected_id,
+                actual: id,
+            });
+        }
+
+        Ok(io_apic)
+    }
+
+    fn read(&mut self, register: u8) -> u32 {
+        unsafe {
+            self.base
+                .as_mut_ptr::<u32>()
+                .write_volatile(register as u32);
+
+            self.base
+                .as_ptr::<u8>()
+                .add(IOWIN)
+                .cast::<u32>()
+                .read_volatile()
+        }
+    }
+
+    fn write(&mut self, register: u8, value: u32) {
+        unsafe {
+            self.base
+                .as_mut_ptr::<u32>()
+                .write_volatile(register as u32);
+
+            self.base
+                .as_mut_ptr::<u8>()
+                .add(IOWIN)
+                .cast::<u32>()
+                .write_volatile(value);
+        }
+    }
+
+    fn redirection_index(&self, gsi: u32) -> Result<u32, IoApicError> {
+        let index = gsi
+            .checked_sub(self.global_interrupt_base)
+            .ok_or(IoApicError::GsiOutsideRange)?;
+
+        if index >= self.redirection_entry_count {
+            return Err(IoApicError::GsiOutsideRange);
+        }
+
+        Ok(index)
+    }
+
+    pub fn handles_gsi(&mut self, gsi: u32) -> bool {
+        match gsi.checked_sub(self.global_interrupt_base) {
+            Some(index) => index < self.redirection_entry_count,
+            None => false,
+        }
+    }
+
+    fn redirection_registers(&mut self, gsi: u32) -> Result<(u8, u8), IoApicError> {
+        let index = self.redirection_index(gsi)?;
+        let low_register = u8::try_from(IOAPIC_REDIRECTION_BASE as u32 + index * 2)
+            .map_err(|_| IoApicError::InvalidRedirectionIndex)?;
+
+        let high_register = u8::try_from(IOAPIC_REDIRECTION_BASE as u32 + index * 2 + 1)
+            .map_err(|_| IoApicError::InvalidRedirectionIndex)?;
+
+        Ok((low_register, high_register))
+    }
+
+    pub fn configure_masked(
+        &mut self,
+        gsi: u32,
+        config: RedirectionConfig,
+    ) -> Result<(), IoApicError> {
+        let mut entry = config.vector as u64;
+
+        match config.polarity {
+            InterruptPolarity::ActiveHigh => {
+                entry |= 0 << 13;
+            }
+            InterruptPolarity::ActiveLow => {
+                entry |= 1 << 13;
+            }
+        }
+
+        match config.trigger {
+            TriggerMode::Edge => {
+                entry |= 0 << 15;
+            }
+            TriggerMode::Level => {
+                entry |= 1 << 15;
+            }
+        }
+
+        entry |= 1 << 16;
+        entry |= (config.destination as u64) << 56;
+
+        let (low_register, high_register) = self.redirection_registers(gsi)?;
+
+        let old_low = self.read(low_register);
+        self.write(low_register, old_low | MASKED);
+
+        self.write(high_register, (entry >> 32) as u32);
+        self.write(low_register, entry as u32 | MASKED);
+
+        Ok(())
+    }
+
+    pub fn unmask(&mut self, gsi: u32) -> Result<(), IoApicError> {
+        let (low_register, _) = self.redirection_registers(gsi)?;
+        let low = self.read(low_register);
+
+        self.write(low_register, low & !MASKED);
+
+        Ok(())
+    }
+
+    pub fn mask(&mut self, gsi: u32) -> Result<(), IoApicError> {
+        let (low_register, _) = self.redirection_registers(gsi)?;
+        let low = self.read(low_register);
+
+        self.write(low_register, low | MASKED);
+
+        Ok(())
+    }
+}

src/arch/x86_64/linker.ld

@@ -25,7 +25,10 @@ SECTIONS
     /* that is the beginning of the region. */
     /* Additionally, leave space for the ELF headers by adding SIZEOF_HEADERS to the */
     /* base load address. */
-    . = 0xffffffff80000000 + SIZEOF_HEADERS;
+    . = 0xffffffff80000000;
+
+    __text_start = .;
+    . += SIZEOF_HEADERS;
 
     .text : {
         *(.text .text.*)
@@ -33,6 +36,8 @@ SECTIONS
 
     /* Move to the next memory page for .rodata */
     . = ALIGN(CONSTANT(MAXPAGESIZE));
+    __text_end = .;
+    __rodata_start = .;
 
     .rodata : {
         *(.rodata .rodata.*)
@@ -40,6 +45,8 @@ SECTIONS
 
     /* Move to the next memory page for .data */
     . = ALIGN(CONSTANT(MAXPAGESIZE));
+    __rodata_end = .;
+    __data_start = .;
     
     .data : {
         *(.data .data.*)
@@ -56,6 +63,11 @@ SECTIONS
         *(.dynamic)
     } :data :dynamic
 
+    .got : {
+        *(.got .got.*)
+        *(.got.plt .got.plt.*)
+    } :data
+
     /* NOTE: .bss needs to be the last thing mapped to :data, otherwise lots of */
     /* unnecessary zeros will be written to the binary. */
     /* If you need, for example, .init_array and .fini_array, those should be placed */
@@ -65,6 +77,9 @@ SECTIONS
         *(COMMON)
     } :data
 
+    . = ALIGN(CONSTANT(MAXPAGESIZE));
+    __data_end = .;
+
     /* Discard .note.* and .eh_frame* since they may cause issues on some hosts. */
     /* Also discard the program interpreter section since we do not need one. This is */
     /* more or less equivalent to the --no-dynamic-linker linker flag, except that it */

src/arch/x86_64/mod.rs

@@ -1,19 +1,209 @@
+pub(super) mod apic;
+mod cpu;
 mod gdt;
 mod interrupts;
+pub(super) mod io_apic;
+mod paging;
+mod pit;
 pub mod port;
+mod syscall;
+pub mod timer;
 
 use core::arch::asm;
 
-pub use interrupts::disable_interrupts;
+pub use cpu::{ThreadContext, switch_context};
+pub use interrupts::{disable_interrupts, disable_interrupts_and_save, restore_interrupts};
+pub(crate) use paging::{
+    MapError as PageTableMapError, PageTableCreateError, UnmapError as PageTableUnmapError,
+};
+pub use paging::{PageTable, PagingConfig};
 
-use crate::println;
+pub struct ArchState {
+    pub paging: PagingConfig,
+}
 
-pub fn init() {
+use crate::{
+    KernelHandoff,
+    arch::x86_64::cpu::BOOT_CPU,
+    memory::{AddressSpace, FrameAllocator, VirtualAddr},
+    platform::acpi::Madt,
+    println,
+};
+
+pub fn init() -> ArchState {
     disable_interrupts();
     println!("Loading GDT...");
     gdt::init();
     println!("Loading IDT...");
     interrupts::init();
+    println!("Detecting CPU features...");
+    let cpu_features = cpu::detect_features_and_enable();
+    let paging =
+        PagingConfig::from_features(cpu_features.expect("required CPU features are not supported"));
+    ArchState { paging }
+}
+
+pub fn set_kernel_stack(stack_top: VirtualAddr) {
+    gdt::set_kernel_stack(stack_top);
+
+    unsafe {
+        BOOT_CPU.kernel_stack_top = stack_top.as_usize();
+    }
+}
+
+#[derive(Debug)]
+#[allow(unused)]
+pub enum InterruptInitError {
+    InvalidLocalApicId,
+    InvalidLocalApicAddress,
+    FailedToGetIoApic,
+    IoApicError(io_apic::IoApicError),
+    LocalApicError(apic::LocalApicError),
+    TimerCalibrationError(timer::TimerCalibrationError),
+    MalformedMadt,
+    PitNotHandled,
+}
+
+pub struct InterruptController {
+    local_apic: apic::LocalApic,
+    io_apic: io_apic::IoApic,
+    local_timer_frequency: u64,
+}
+
+pub fn init_interrupt_controller(
+    madt: &Madt<'_>,
+    allocator: &mut FrameAllocator,
+    address_space: &mut AddressSpace,
+) -> Result<InterruptController, InterruptInitError> {
+    let local_apic_address = madt
+        .effective_local_apic_address()
+        .map_err(|_| InterruptInitError::InvalidLocalApicAddress)?;
+
+    let mut local_apic = apic::LocalApic::init(local_apic_address, allocator, address_space)
+        .map_err(|err| InterruptInitError::LocalApicError(err))?;
+
+    let io_apic_info = madt
+        .sole_io_apic()
+        .map_err(|_| InterruptInitError::FailedToGetIoApic)?;
+
+    let pit_route = madt
+        .isa_irq_route(0x0)
+        .map_err(|_| InterruptInitError::MalformedMadt)?;
+
+    let mut io_apic = io_apic::IoApic::new(
+        io_apic_info.id,
+        io_apic_info.apic_address,
+        io_apic_info.global_system_interrupt_base,
+        io_apic::IOAPIC_VIRTUAL_ADDRESS,
+        allocator,
+        address_space,
+    )
+    .map_err(|err| InterruptInitError::IoApicError(err))?;
+
+    let pit_handled = io_apic.handles_gsi(pit_route.gsi);
+    if !pit_handled {
+        return Err(InterruptInitError::PitNotHandled);
+    }
+
+    let destination =
+        u8::try_from(local_apic.id()).map_err(|_| InterruptInitError::InvalidLocalApicId)?;
+
+    io_apic
+        .configure_masked(
+            pit_route.gsi,
+            io_apic::RedirectionConfig {
+                vector: interrupts::apic_vectors::PIT_CALIBRATION_VECTOR,
+                destination,
+                polarity: pit_route.polarity,
+                trigger: pit_route.trigger,
+            },
+        )
+        .map_err(|err| InterruptInitError::IoApicError(err))?;
+
+    let local_timer_frequency =
+        timer::calibrate_local_apic(&mut local_apic, &mut io_apic, pit_route)
+            .map_err(|err| InterruptInitError::TimerCalibrationError(err))?;
+
+    Ok(InterruptController {
+        local_apic,
+        io_apic,
+        local_timer_frequency,
+    })
+}
+
+/// # Safety
+///
+/// The caller must ensure:
+/// - The stack is currently mapped, writable, and 16-byte aligned
+pub unsafe fn enter_kernel(stack_top: VirtualAddr, handoff: *mut KernelHandoff) -> ! {
+    unsafe {
+        gdt::set_kernel_stack(stack_top);
+
+        BOOT_CPU.kernel_stack_top = stack_top.as_usize();
+        syscall::init(&raw const BOOT_CPU);
+
+        asm!(
+            "mov rsp, {stack_top}",
+            "xor rbp, rbp",
+            "mov rdi, {handoff}",
+            "call {kernel_main}",
+            stack_top = in(reg) stack_top.as_usize(),
+            handoff = in(reg) handoff,
+            kernel_main = sym crate::kernel_main,
+            options(noreturn)
+        );
+    };
+}
+
+/// # Safety
+///
+/// - `user_instruction_pointer` and `user_stack_pointer` must be valid user mappings.
+/// - The active address space must contain the kernel and supplied user mappings.
+pub unsafe fn enter_user(
+    user_instruction_pointer: VirtualAddr,
+    user_stack_pointer: VirtualAddr,
+) -> ! {
+    unsafe {
+        asm!(
+            "mov ds, {user_data_selector:x}",
+            "mov es, {user_data_selector:x}",
+            "mov fs, {user_data_selector:x}",
+
+            "push {user_data_selector}",
+            "push {user_stack_pointer}",
+            "push 0x202", // RFLAGS (IF=1, bit 1 reserved=1)
+            "push {user_code_selector}",
+            "push {user_instruction_pointer}",
+
+            // clear GPRs
+            "xor rax, rax",
+            "xor rbx, rbx",
+            "xor rcx, rcx",
+            "xor rdx, rdx",
+            "xor rsi, rsi",
+            "xor rdi, rdi",
+            "xor rbp, rbp",
+            "xor r8, r8",
+            "xor r9, r9",
+            "xor r10, r10",
+            "xor r11, r11",
+            "xor r12, r12",
+            "xor r13, r13",
+            "xor r14, r14",
+            "xor r15, r15",
+
+            // Kernel GS is CpuLocal; leave it in IA32_KERNEL_GS_BASE so
+            // syscall_entry can recover it with SWAPGS.
+            "swapgs",
+            "mov gs, {user_data_selector:x}",
+            "iretq",
+            user_data_selector = in(reg) gdt::USER_DATA_SELECTOR as usize,
+            user_code_selector = in(reg) gdt::USER_CODE_SELECTOR as usize,
+            user_instruction_pointer = in(reg) user_instruction_pointer.as_usize(),
+            user_stack_pointer = in(reg) user_stack_pointer.as_usize(),
+            options(noreturn)
+        );
+    }
 }
 
 pub fn halt() {

src/arch/x86_64/paging.rs

@@ -0,0 +1,834 @@
+use core::arch::asm;
+
+use crate::{
+    arch::x86_64::cpu::CpuFeatures,
+    memory::{
+        CachePolicy, DirectMap, FrameAddr, FrameAllocator, OwnedFrame, PagePermissions,
+        PageTableMapping, PhysicalAddr, VirtualAddr,
+    },
+};
+
+pub const PAGE_SIZE: usize = 4096;
+pub const PAGE_TABLE_ENTRIES: usize = 512;
+
+#[derive(Clone, Copy)]
+pub struct PagingConfig {
+    physical_address_bits: u8,
+    global_pages: bool,
+    nx_enabled: bool,
+    mode: PagingMode,
+}
+
+impl PagingConfig {
+    pub fn from_features(features: CpuFeatures) -> Self {
+        Self {
+            physical_address_bits: features.physical_address_bits,
+            nx_enabled: features.nx_enabled,
+            global_pages: features.global_pages,
+            mode: if features.five_level_paging_active {
+                PagingMode::FiveLevel
+            } else {
+                PagingMode::FourLevel
+            },
+        }
+    }
+
+    pub const fn physical_address_mask(&self) -> usize {
+        ((1 << self.physical_address_bits) - 1) & !0xFFF
+    }
+
+    pub const fn physical_address_limit(&self) -> usize {
+        1 << self.physical_address_bits
+    }
+}
+
+#[derive(Clone, Copy)]
+enum PagingMode {
+    FourLevel,
+    FiveLevel,
+}
+
+const MAX_INTERMEDIATE_LEVELS: usize = 4;
+const FOUR_LEVEL_INTERMEDIATES: [PageTableLevel; 3] = [
+    PageTableLevel::Pml4,
+    PageTableLevel::Pdpt,
+    PageTableLevel::PageDirectory,
+];
+const FIVE_LEVEL_INTERMEDIATES: [PageTableLevel; 4] = [
+    PageTableLevel::Pml5,
+    PageTableLevel::Pml4,
+    PageTableLevel::Pdpt,
+    PageTableLevel::PageDirectory,
+];
+
+impl PagingMode {
+    const fn virtual_address_bits(&self) -> u32 {
+        match self {
+            PagingMode::FourLevel => 48,
+            PagingMode::FiveLevel => 57,
+        }
+    }
+
+    fn intermediate_levels(&self) -> &'static [PageTableLevel] {
+        match self {
+            PagingMode::FourLevel => &FOUR_LEVEL_INTERMEDIATES,
+            PagingMode::FiveLevel => &FIVE_LEVEL_INTERMEDIATES,
+        }
+    }
+}
+
+#[derive(Clone, Copy, PartialEq, Eq)]
+enum PageTableLevel {
+    Pml5,
+    Pml4,
+    Pdpt,
+    PageDirectory,
+}
+
+impl PageTableLevel {
+    const fn index(self, address: usize) -> usize {
+        let shift = match self {
+            Self::Pml5 => 48,
+            Self::Pml4 => 39,
+            Self::Pdpt => 30,
+            Self::PageDirectory => 21,
+        };
+
+        address >> shift & 0x1FF
+    }
+
+    const fn large_page_size(self) -> Option<usize> {
+        match self {
+            Self::Pdpt => Some(1 << 30),
+            Self::PageDirectory => Some(1 << 21),
+            Self::Pml5 | Self::Pml4 => None,
+        }
+    }
+}
+
+enum PageTableEntryError {
+    PhysicalAddressTooLarge,
+    NoExecuteUnsupported,
+}
+
+#[repr(transparent)]
+#[derive(Clone, Copy, PartialEq, Eq)]
+struct PageTableEntry(u64);
+
+impl PageTableEntry {
+    const PRESENT: u64 = 1 << 0;
+    const WRITABLE: u64 = 1 << 1;
+    const USER_ACCESSIBLE: u64 = 1 << 2;
+    const HUGE_PAGE: u64 = 1 << 7;
+    const GLOBAL: u64 = 1 << 8;
+    const NX: u64 = 1 << 63;
+
+    const WRITE_THROUGH: u64 = 1 << 3;
+    const CACHE_DISABLED: u64 = 1 << 4;
+    // const PAT: u64 = 1 << 7;
+
+    const fn new(
+        physical_address: PhysicalAddr,
+        permissions: PagePermissions,
+        cache_policy: CachePolicy,
+        config: PagingConfig,
+        global: bool,
+    ) -> Result<Self, PageTableEntryError> {
+        if physical_address.as_usize() >= config.physical_address_limit() {
+            return Err(PageTableEntryError::PhysicalAddressTooLarge);
+        }
+
+        let mut value = physical_address.as_usize() as u64 | Self::PRESENT;
+
+        if permissions.writable {
+            value |= Self::WRITABLE;
+        }
+        if permissions.user_accessible {
+            value |= Self::USER_ACCESSIBLE;
+        }
+        if !permissions.executable {
+            if !config.nx_enabled {
+                return Err(PageTableEntryError::NoExecuteUnsupported);
+            }
+
+            value |= Self::NX;
+        }
+
+        if global && config.global_pages {
+            value |= Self::GLOBAL;
+        }
+
+        // TODO: these bit positions very by page size
+        // set PAT
+        match cache_policy {
+            CachePolicy::WriteBack => {}
+            CachePolicy::Uncacheable => {
+                value |= Self::CACHE_DISABLED | Self::WRITE_THROUGH;
+            }
+        };
+
+        Ok(Self(value))
+    }
+
+    fn new_table(
+        frame: FrameAddr,
+        user_accessible: bool,
+        config: PagingConfig,
+    ) -> Result<Self, PageTableEntryError> {
+        if frame.start_address().as_usize() >= config.physical_address_limit() {
+            return Err(PageTableEntryError::PhysicalAddressTooLarge);
+        }
+
+        let mut value = frame.start_address().as_usize() as u64 | Self::PRESENT | Self::WRITABLE;
+        if user_accessible {
+            value |= Self::USER_ACCESSIBLE;
+        }
+
+        Ok(Self(value))
+    }
+
+    const fn null() -> Self {
+        Self(0)
+    }
+
+    fn physical_address(&self, config: PagingConfig) -> PhysicalAddr {
+        PhysicalAddr::new(self.0 as usize & config.physical_address_mask())
+    }
+
+    fn is_present(&self) -> bool {
+        self.0 & Self::PRESENT != 0
+    }
+
+    fn writable(&self) -> bool {
+        self.0 & Self::WRITABLE != 0
+    }
+
+    fn executable(&self) -> bool {
+        self.0 & Self::NX == 0
+    }
+
+    fn is_user_accessible(&self) -> bool {
+        self.0 & Self::USER_ACCESSIBLE != 0
+    }
+
+    fn is_huge(&self) -> bool {
+        self.0 & Self::HUGE_PAGE != 0
+    }
+
+    fn is_global(&self) -> bool {
+        self.0 & Self::GLOBAL != 0
+    }
+
+    fn table_frame(&self, config: PagingConfig) -> Option<FrameAddr> {
+        if !self.is_present() || self.is_huge() {
+            return None;
+        }
+
+        FrameAddr::from_start_address(self.physical_address(config))
+    }
+
+    fn leaf_frame(&self, config: PagingConfig) -> Option<FrameAddr> {
+        if !self.is_present() {
+            return None;
+        }
+
+        FrameAddr::from_start_address(self.physical_address(config))
+    }
+
+    fn permissions(&self) -> PagePermissions {
+        PagePermissions::new(
+            self.writable(),
+            self.executable(),
+            self.is_user_accessible(),
+        )
+    }
+}
+
+pub(crate) enum MapError {
+    InvalidVirtualAddress,
+    VirtualAddressUnaligned,
+    PhysicalAddressTooLarge,
+    PageAlreadyMapped,
+    HugePageConflict,
+    NoExecuteUnsupported,
+    OutOfFrames,
+    PageTableOutsideDirectMap,
+    InvalidPageTableEntry,
+}
+
+#[derive(Debug)]
+pub(crate) enum UnmapError {
+    InvalidVirtualAddress,
+    VirtualAddressUnaligned,
+    PageNotMapped,
+    HugePageConflict,
+    PageTableOutsideDirectMap,
+    InvalidPageTableEntry,
+}
+
+#[derive(Clone, Copy)]
+struct EntryLocation {
+    table: FrameAddr,
+    index: usize,
+}
+
+#[derive(Debug)]
+pub(crate) enum PageTableCreateError {
+    PhysicalAddressTooLarge,
+    OutOfFrames,
+}
+
+pub struct PageTable {
+    pub direct_map: DirectMap,
+    config: PagingConfig,
+    frame: OwnedFrame,
+}
+
+impl PartialEq for PageTable {
+    fn eq(&self, other: &Self) -> bool {
+        self.frame.frame_address() == other.frame.frame_address()
+    }
+}
+
+impl Eq for PageTable {}
+
+impl PageTable {
+    pub fn new(
+        direct_map: DirectMap,
+        config: PagingConfig,
+        allocator: &mut FrameAllocator,
+    ) -> Result<Self, PageTableCreateError> {
+        let frame = allocator.alloc().ok_or(PageTableCreateError::OutOfFrames)?;
+
+        if frame.frame_address().start_address().as_usize() >= config.physical_address_limit() {
+            unsafe { allocator.dealloc(frame) };
+
+            return Err(PageTableCreateError::PhysicalAddressTooLarge);
+        }
+
+        Ok(Self {
+            frame,
+            direct_map,
+            config,
+        })
+    }
+
+    pub const fn config(&self) -> PagingConfig {
+        self.config
+    }
+
+    fn is_active(&self) -> bool {
+        let cr3 = unsafe { read_cr3(self.config) };
+
+        cr3.start_address() == self.frame.frame_address().start_address()
+    }
+
+    fn table(&self, frame: FrameAddr) -> Option<&[PageTableEntry; PAGE_TABLE_ENTRIES]> {
+        let virtual_addr = self.direct_map.translate(frame.start_address())?;
+
+        Some(unsafe { &*(virtual_addr.as_ptr::<[PageTableEntry; PAGE_TABLE_ENTRIES]>()) })
+    }
+
+    fn table_mut(&mut self, frame: FrameAddr) -> Option<&mut [PageTableEntry; PAGE_TABLE_ENTRIES]> {
+        let virtual_addr = self.direct_map.translate(frame.start_address())?;
+
+        Some(unsafe { &mut *(virtual_addr.as_mut_ptr::<[PageTableEntry; PAGE_TABLE_ENTRIES]>()) })
+    }
+
+    fn is_canonical(&self, addr: usize) -> bool {
+        let bits = self.config.mode.virtual_address_bits();
+        let shift = usize::BITS - bits;
+
+        (((addr << shift) as isize >> shift) as usize) == addr
+    }
+
+    pub fn to_physical(&self, addr: VirtualAddr) -> Option<PhysicalAddr> {
+        let address = addr.as_usize();
+
+        if !self.is_canonical(address) {
+            return None;
+        }
+
+        let mut table_frame = self.frame.frame_address();
+
+        for &level in self.config.mode.intermediate_levels() {
+            let table = self.table(table_frame)?;
+            let entry = table[level.index(address)];
+
+            if !entry.is_present() {
+                return None;
+            }
+
+            if entry.is_huge() {
+                return translate_huge_page(entry, address, level.large_page_size()?, self.config);
+            }
+
+            table_frame = entry.table_frame(self.config)?;
+        }
+
+        let page_table = self.table(table_frame)?;
+        let entry = page_table[p1_index(address)];
+        let physical_base = entry.leaf_frame(self.config)?.start_address().as_usize();
+
+        physical_base
+            .checked_add(page_offset(address))
+            .map(PhysicalAddr::new)
+    }
+
+    pub fn to_virtual(&self, addr: PhysicalAddr) -> Option<VirtualAddr> {
+        self.direct_map.translate(addr)
+    }
+
+    pub fn mapping(&self, virtual_addr: VirtualAddr) -> Option<PageTableMapping> {
+        let address = virtual_addr.as_usize();
+
+        if !self.is_canonical(address) {
+            return None;
+        }
+
+        let mut table_frame = self.frame.frame_address();
+        let mut permissions = PagePermissions::new(true, true, true);
+
+        for &level in self.config.mode.intermediate_levels() {
+            let table = self.table(table_frame)?;
+            let entry = table[level.index(address)];
+
+            if !entry.is_present() {
+                return None;
+            }
+
+            let entry_permissions = entry.permissions();
+            permissions.writable &= entry_permissions.writable;
+            permissions.user_accessible &= entry_permissions.user_accessible;
+            permissions.executable &= entry_permissions.executable;
+
+            if entry.is_huge() {
+                level.large_page_size()?;
+                return Some(PageTableMapping { permissions });
+            }
+
+            table_frame = entry.table_frame(self.config)?;
+        }
+
+        let page_table = self.table(table_frame)?;
+        let entry = page_table[p1_index(address)];
+
+        if !entry.is_present() {
+            return None;
+        }
+
+        let entry_permissions = entry.permissions();
+        permissions.writable &= entry_permissions.writable;
+        permissions.user_accessible &= entry_permissions.user_accessible;
+        permissions.executable &= entry_permissions.executable;
+
+        Some(PageTableMapping { permissions })
+    }
+
+    fn get_next_level(
+        &self,
+        parent: FrameAddr,
+        index: usize,
+        level: PageTableLevel,
+    ) -> Result<FrameAddr, UnmapError> {
+        let parent_table = self
+            .table(parent)
+            .ok_or(UnmapError::PageTableOutsideDirectMap)?;
+        let entry = parent_table[index];
+
+        if entry.is_huge() {
+            return if level.large_page_size().is_some() {
+                Err(UnmapError::HugePageConflict)
+            } else {
+                Err(UnmapError::InvalidPageTableEntry)
+            };
+        }
+
+        entry
+            .table_frame(self.config)
+            .ok_or(UnmapError::PageNotMapped)
+    }
+
+    fn apply_user_upgrades(
+        &mut self,
+        upgrades: &[Option<EntryLocation>; MAX_INTERMEDIATE_LEVELS],
+        count: usize,
+    ) {
+        for location in upgrades[..count].iter().flatten() {
+            let entry = &mut self
+                .table_mut(location.table)
+                .expect("validated page table left the direct map")[location.index];
+            entry.0 |= PageTableEntry::USER_ACCESSIBLE;
+        }
+    }
+
+    pub fn map(
+        &mut self,
+        mapped_addr: VirtualAddr,
+        frame: FrameAddr,
+        permissions: PagePermissions,
+        allocator: &mut FrameAllocator,
+        cache_policy: CachePolicy,
+        global: bool,
+    ) -> Result<(), MapError> {
+        let address = mapped_addr.as_usize();
+
+        if !self.is_canonical(address) {
+            return Err(MapError::InvalidVirtualAddress);
+        }
+
+        if address % PAGE_SIZE != 0 {
+            return Err(MapError::VirtualAddressUnaligned);
+        }
+
+        let leaf_entry = PageTableEntry::new(
+            frame.start_address(),
+            permissions,
+            cache_policy,
+            self.config,
+            global,
+        )
+        .map_err(|error| match error {
+            PageTableEntryError::PhysicalAddressTooLarge => MapError::PhysicalAddressTooLarge,
+            PageTableEntryError::NoExecuteUnsupported => MapError::NoExecuteUnsupported,
+        })?;
+
+        let levels = self.config.mode.intermediate_levels();
+        let mut current_table_frame_addr = self.frame.frame_address();
+        let mut first_missing = None;
+        let mut user_upgrades: [Option<EntryLocation>; MAX_INTERMEDIATE_LEVELS] =
+            [None; MAX_INTERMEDIATE_LEVELS];
+        let mut user_upgrade_count = 0;
+
+        for (depth, &level) in levels.iter().enumerate() {
+            let index = level.index(address);
+            let table = self
+                .table(current_table_frame_addr)
+                .ok_or(MapError::PageTableOutsideDirectMap)?;
+            let entry = table[index];
+
+            if !entry.is_present() {
+                first_missing = Some((
+                    depth,
+                    EntryLocation {
+                        table: current_table_frame_addr,
+                        index,
+                    },
+                ));
+                break;
+            }
+
+            if entry.is_huge() {
+                return if level.large_page_size().is_some() {
+                    Err(MapError::HugePageConflict)
+                } else {
+                    Err(MapError::InvalidPageTableEntry)
+                };
+            }
+
+            if permissions.user_accessible && !entry.is_user_accessible() {
+                user_upgrades[user_upgrade_count] = Some(EntryLocation {
+                    table: current_table_frame_addr,
+                    index,
+                });
+                user_upgrade_count += 1;
+            }
+
+            current_table_frame_addr = entry
+                .table_frame(self.config)
+                .ok_or(MapError::InvalidPageTableEntry)?;
+        }
+
+        if first_missing.is_none() {
+            let pt = self
+                .table(current_table_frame_addr)
+                .ok_or(MapError::PageTableOutsideDirectMap)?;
+            if pt[p1_index(address)].is_present() {
+                return Err(MapError::PageAlreadyMapped);
+            }
+
+            self.apply_user_upgrades(&user_upgrades, user_upgrade_count);
+            self.table_mut(current_table_frame_addr)
+                .expect("validated page table left the direct map")[p1_index(address)] = leaf_entry;
+            self.flush_tlb_if_active(mapped_addr);
+            return Ok(());
+        }
+
+        let (missing_depth, publication_location) = first_missing.unwrap();
+        let private_table_count = levels.len() - missing_depth;
+        let mut private_tables: [Option<OwnedFrame>; MAX_INTERMEDIATE_LEVELS] =
+            core::array::from_fn(|_| None);
+
+        let prepare_result = (|| {
+            for slot in &mut private_tables[..private_table_count] {
+                let frame = allocator.alloc().ok_or(MapError::OutOfFrames)?;
+                let address = frame.frame_address();
+                // Track ownership before validation so every error uses the same cleanup.
+                *slot = Some(frame);
+
+                PageTableEntry::new_table(address, permissions.user_accessible, self.config)
+                    .map_err(|_| MapError::PhysicalAddressTooLarge)?;
+            }
+
+            for private_index in 0..private_table_count {
+                let private_frame = private_tables[private_index]
+                    .as_ref()
+                    .unwrap()
+                    .frame_address();
+
+                if private_index + 1 < private_table_count {
+                    let child = private_tables[private_index + 1]
+                        .as_ref()
+                        .unwrap()
+                        .frame_address();
+                    let child_entry =
+                        PageTableEntry::new_table(child, permissions.user_accessible, self.config)
+                            .map_err(|_| MapError::PhysicalAddressTooLarge)?;
+                    let child_index = levels[missing_depth + private_index + 1].index(address);
+                    self.table_mut(private_frame)
+                        .ok_or(MapError::PageTableOutsideDirectMap)?[child_index] = child_entry;
+                } else {
+                    self.table_mut(private_frame)
+                        .ok_or(MapError::PageTableOutsideDirectMap)?[p1_index(address)] =
+                        leaf_entry;
+                }
+            }
+
+            PageTableEntry::new_table(
+                private_tables[0].as_ref().unwrap().frame_address(),
+                permissions.user_accessible,
+                self.config,
+            )
+            .map_err(|_| MapError::PhysicalAddressTooLarge)
+        })();
+
+        let publication_entry = match prepare_result {
+            Ok(entry) => entry,
+            Err(error) => {
+                for frame in private_tables.into_iter().rev().flatten() {
+                    unsafe { allocator.dealloc(frame) };
+                }
+                return Err(error);
+            }
+        };
+
+        self.apply_user_upgrades(&user_upgrades, user_upgrade_count);
+        self.table_mut(publication_location.table)
+            .expect("validated publication table left the direct map")
+            [publication_location.index] = publication_entry;
+
+        // The published page table now owns these frames.
+        for frame in private_tables.into_iter().flatten() {
+            let _ = frame.into_raw();
+        }
+
+        self.flush_tlb_if_active(mapped_addr);
+
+        Ok(())
+    }
+
+    /// # Safety
+    ///
+    /// The caller must ensure:
+    /// - The page being unmapped does not unmap the HHDM, kernel image, or stack
+    /// - The caller must ensure that the page is not currently in use
+    pub unsafe fn unmap(
+        &mut self,
+        mapped_addr: VirtualAddr,
+        allocator: &mut FrameAllocator,
+    ) -> Result<FrameAddr, UnmapError> {
+        if !self.is_canonical(mapped_addr.as_usize()) {
+            return Err(UnmapError::InvalidVirtualAddress);
+        }
+
+        if mapped_addr.as_usize() % PAGE_SIZE != 0 {
+            return Err(UnmapError::VirtualAddressUnaligned);
+        }
+
+        let address = mapped_addr.as_usize();
+        let levels = self.config.mode.intermediate_levels();
+        let mut table_frames: [Option<FrameAddr>; MAX_INTERMEDIATE_LEVELS + 1] =
+            core::array::from_fn(|_| None);
+        table_frames[0] = Some(self.frame.frame_address());
+
+        let mut current_table = self.frame.frame_address();
+        for (depth, &level) in levels.iter().enumerate() {
+            current_table = self.get_next_level(current_table, level.index(address), level)?;
+            table_frames[depth + 1] = Some(current_table);
+        }
+
+        let config = self.config;
+        let page_table = self
+            .table_mut(current_table)
+            .ok_or(UnmapError::PageTableOutsideDirectMap)?;
+        let entry_index = p1_index(address);
+        let entry = page_table[entry_index];
+        let frame = entry.leaf_frame(config).ok_or(UnmapError::PageNotMapped)?;
+        page_table[entry_index] = PageTableEntry::null();
+
+        let mut child_is_empty = page_table.iter().all(|entry| !entry.is_present());
+        let mut deallocatable_frames: [Option<FrameAddr>; MAX_INTERMEDIATE_LEVELS] =
+            core::array::from_fn(|_| None);
+        let mut deallocatable_count = 0;
+
+        for depth in (0..levels.len()).rev() {
+            if !child_is_empty {
+                break;
+            }
+
+            let parent_frame_addr = table_frames[depth].unwrap();
+            let child_frame = table_frames[depth + 1].unwrap();
+            self.table_mut(parent_frame_addr)
+                .expect("validated page table left the direct map")[levels[depth].index(address)] =
+                PageTableEntry::null();
+
+            deallocatable_frames[deallocatable_count] = Some(child_frame);
+            deallocatable_count += 1;
+
+            if depth > 0 {
+                child_is_empty = self
+                    .table(parent_frame_addr)
+                    .expect("validated page table left the direct map")
+                    .iter()
+                    .all(|entry| !entry.is_present());
+            }
+        }
+
+        self.flush_tlb_if_active(mapped_addr);
+
+        for frame in deallocatable_frames[..deallocatable_count].iter().flatten() {
+            unsafe { allocator.dealloc(OwnedFrame::from_raw(*frame)) };
+        }
+
+        Ok(frame)
+    }
+
+    pub fn copy_kernel_mappings_to(&self, destination: &mut PageTable) {
+        let src = self
+            .table(self.frame.frame_address())
+            .expect("source page table outside direct map");
+        let dest = destination
+            .table_mut(destination.frame.frame_address())
+            .expect("destination page table outside direct map");
+
+        dest[256..512].copy_from_slice(&src[256..512]);
+    }
+
+    fn flush_tlb_if_active(&self, page: VirtualAddr) {
+        debug_assert!(self.is_canonical(page.as_usize()));
+
+        if self.is_active() {
+            unsafe {
+                asm!("invlpg [{}]", in(reg) page.as_usize(), options(nostack, preserves_flags));
+            }
+        }
+    }
+
+    /// # Safety
+    ///
+    /// The caller must ensure:
+    /// - The new space must map the kernel image, stack, and the code being executed
+    /// - The new space must map the HHDM
+    pub unsafe fn activate(&self) {
+        unsafe {
+            asm!(
+                "mov cr3, {}",
+                in(reg) self.frame.frame_address().start_address().as_usize(),
+                options(nostack, preserves_flags)
+            );
+        };
+    }
+
+    fn destroy_children(&mut self, table: FrameAddr, depth: usize, allocator: &mut FrameAllocator) {
+        let levels = self.config.mode.intermediate_levels();
+
+        if depth == levels.len() {
+            return;
+        }
+
+        for idx in 0..PAGE_TABLE_ENTRIES {
+            let entry = self.table(table).expect("page table outside direct map")[idx];
+
+            if !entry.is_present() || entry.is_huge() {
+                continue;
+            }
+
+            let child = entry
+                .table_frame(self.config)
+                .expect("invalid page table entry");
+            self.destroy_children(child, depth + 1, allocator);
+
+            self.table_mut(table)
+                .expect("page table outside direct map")[idx] = PageTableEntry::null();
+
+            unsafe { allocator.dealloc(OwnedFrame::from_raw(child)) };
+        }
+    }
+
+    pub unsafe fn destroy(mut self, allocator: &mut FrameAllocator) {
+        assert!(!self.is_active(), "attempted to destroy active page table");
+
+        self.destroy_children(self.frame.frame_address(), 0, allocator);
+
+        unsafe { allocator.dealloc(self.frame) };
+    }
+
+    pub unsafe fn destroy_user(mut self, allocator: &mut FrameAllocator) {
+        assert!(!self.is_active(), "attempted to destroy active page table");
+
+        let root_frame = self.frame.frame_address();
+
+        for idx in 0..256 {
+            let entry = self.table(root_frame).expect("table outside direct map")[idx];
+            if !entry.is_present() || entry.is_huge() {
+                continue;
+            }
+
+            let child = entry
+                .table_frame(self.config)
+                .expect("invalid page table entry");
+            self.destroy_children(child, 1, allocator);
+
+            self.table_mut(root_frame)
+                .expect("table outside direct map")[idx] = PageTableEntry::null();
+            unsafe { allocator.dealloc(OwnedFrame::from_raw(child)) };
+        }
+
+        unsafe { allocator.dealloc(self.frame) };
+    }
+}
+
+fn translate_huge_page(
+    entry: PageTableEntry,
+    virtual_addr: usize,
+    page_size: usize,
+    config: PagingConfig,
+) -> Option<PhysicalAddr> {
+    let physical_base = entry.physical_address(config).as_usize() & !(page_size - 1);
+
+    let offset = virtual_addr & (page_size - 1);
+
+    physical_base.checked_add(offset).map(PhysicalAddr::new)
+}
+
+fn p1_index(addr: usize) -> usize {
+    (addr >> 12) & 0x1FF
+}
+
+fn page_offset(addr: usize) -> usize {
+    addr & 0xFFF
+}
+
+unsafe fn read_cr3(config: PagingConfig) -> FrameAddr {
+    let value: usize;
+    unsafe {
+        asm!(
+            "mov {}, cr3",
+            out(reg) value,
+            options(nomem, nostack, preserves_flags),
+        );
+    }
+
+    FrameAddr::from_start_address(PhysicalAddr::new(value & config.physical_address_mask()))
+        .expect("CR3 contains an unaligned page-table address")
+}

src/arch/x86_64/pit.rs

@@ -0,0 +1,23 @@
+use crate::arch::port::write_u8;
+
+const PIT_CHANNEL_0: u16 = 0x40;
+const PIT_COMMAND: u16 = 0x43;
+
+const CHANNEL_0: u8 = 0b00 << 6;
+// access mode = lobyte/hibyte
+const LOW_HIGH: u8 = 0b11 << 4;
+// interrupt on terminal count
+const MODE_0: u8 = 0b000 << 1;
+const BINARY: u8 = 0;
+
+pub const PIT_FREQUENCY: u64 = 1_193_182;
+pub const PIT_CALIBRATION_COUNT: u16 = u16::MAX;
+
+pub fn start_pit_one_shot(count: u16) {
+    unsafe {
+        write_u8(PIT_COMMAND, CHANNEL_0 | LOW_HIGH | MODE_0 | BINARY);
+
+        write_u8(PIT_CHANNEL_0, count as u8);
+        write_u8(PIT_CHANNEL_0, (count >> 8) as u8);
+    }
+}

src/arch/x86_64/port.rs

@@ -8,52 +8,52 @@ pub unsafe fn read_u8(port: u16) -> u8 {
             "in al, dx",
             in("dx") port,
             out("al") value,
-            options(nomem, nostack, preserves_flags),
-        );
-    }
-    value
-}
-
-#[inline(always)]
-pub unsafe fn read_u8_slice(port: u16, slice: &mut [u8]) {
-    unsafe {
-        asm!(
-            "rep insb",
-            in("dx") port,
-            inout("rdi") slice.as_mut_ptr() => _,
-            inout("rcx") slice.len() => _,
             options(nostack, preserves_flags),
         );
     }
-}
-
-#[inline(always)]
-pub unsafe fn read_u16(port: u16) -> u16 {
-    let value: u16;
-    unsafe {
-        asm!(
-            "in ax, dx",
-            in("dx") port,
-            out("ax") value,
-            options(nomem, nostack, preserves_flags),
-        );
-    }
     value
 }
 
-#[inline(always)]
+// #[inline(always)]
-pub unsafe fn read_u32(port: u16) -> u32 {
+// pub unsafe fn read_u8_slice(port: u16, slice: &mut [u8]) {
-    let value: u32;
+//     unsafe {
-    unsafe {
+//         asm!(
-        asm!(
+//             "rep insb",
-            "in eax, dx",
+//             in("dx") port,
-            in("dx") port,
+//             inout("rdi") slice.as_mut_ptr() => _,
-            out("eax") value,
+//             inout("rcx") slice.len() => _,
-            options(nomem, nostack, preserves_flags),
+//             options(nostack, preserves_flags),
-        );
+//         );
-    }
+//     }
-    value
+// }
-}
+
+// #[inline(always)]
+// pub unsafe fn read_u16(port: u16) -> u16 {
+//     let value: u16;
+//     unsafe {
+//         asm!(
+//             "in ax, dx",
+//             in("dx") port,
+//             out("ax") value,
+//             options(nomem, nostack, preserves_flags),
+//         );
+//     }
+//     value
+// }
+
+// #[inline(always)]
+// pub unsafe fn read_u32(port: u16) -> u32 {
+//     let value: u32;
+//     unsafe {
+//         asm!(
+//             "in eax, dx",
+//             in("dx") port,
+//             out("eax") value,
+//             options(nomem, nostack, preserves_flags),
+//         );
+//     }
+//     value
+// }
 
 #[inline(always)]
 pub unsafe fn write_u8(port: u16, value: u8) {
@@ -62,44 +62,44 @@ pub unsafe fn write_u8(port: u16, value: u8) {
             "out dx, al",
             in("dx") port,
             in("al") value,
-            options(nomem, nostack, preserves_flags),
-        );
-    }
-}
-
-#[inline(always)]
-pub unsafe fn write_u8_slice(port: u16, slice: &[u8]) {
-    unsafe {
-        asm!(
-            "rep outsb",
-            in("dx") port,
-            inout("rsi") slice.as_ptr() => _,
-            inout("rcx") slice.len() => _,
             options(nostack, preserves_flags),
         );
     }
 }
 
-#[inline(always)]
+// #[inline(always)]
-pub unsafe fn write_u16(port: u16, value: u16) {
+// pub unsafe fn write_u8_slice(port: u16, slice: &[u8]) {
-    unsafe {
+//     unsafe {
-        asm!(
+//         asm!(
-            "out dx, ax",
+//             "rep outsb",
-            in("dx") port,
+//             in("dx") port,
-            in("ax") value,
+//             inout("rsi") slice.as_ptr() => _,
-            options(nomem, nostack, preserves_flags),
+//             inout("rcx") slice.len() => _,
-        );
+//             options(nostack, preserves_flags),
-    }
+//         );
-}
+//     }
+// }
 
-#[inline(always)]
+// #[inline(always)]
-pub unsafe fn write_u32(port: u16, value: u32) {
+// pub unsafe fn write_u16(port: u16, value: u16) {
-    unsafe {
+//     unsafe {
-        asm!(
+//         asm!(
-            "out dx, eax",
+//             "out dx, ax",
-            in("dx") port,
+//             in("dx") port,
-            in("eax") value,
+//             in("ax") value,
-            options(nomem, nostack, preserves_flags),
+//             options(nomem, nostack, preserves_flags),
-        );
+//         );
-    }
+//     }
-}
+// }
+
+// #[inline(always)]
+// pub unsafe fn write_u32(port: u16, value: u32) {
+//     unsafe {
+//         asm!(
+//             "out dx, eax",
+//             in("dx") port,
+//             in("eax") value,
+//             options(nomem, nostack, preserves_flags),
+//         );
+//     }
+// }

src/arch/x86_64/syscall.rs

@@ -0,0 +1,114 @@
+use core::arch::{asm, naked_asm};
+
+use crate::arch::x86_64::{
+    cpu::{CpuLocal, write_msr},
+    gdt::{KERNEL_CODE_SELECTOR, KERNEL_DATA_SELECTOR},
+};
+
+const IA32_STAR: u32 = 0xC000_0081;
+const IA32_LSTAR: u32 = 0xC000_0082;
+const IA32_CSTAR: u32 = 0xC000_0083;
+const IA32_FMASK: u32 = 0xC000_0084;
+const IA32_GS_BASE: u32 = 0xC000_0101;
+const IA32_KERNEL_GS_BASE: u32 = 0xC000_0102;
+
+const RFLAGS_MASK: u64 = 0x257FD5; // Clear IF, TF, DF, IOPL, NT, AC
+
+#[repr(C)]
+struct SyscallFrame {
+    pub r15: u64,
+    pub r14: u64,
+    pub r13: u64,
+    pub r12: u64,
+    pub rbp: u64,
+    pub rbx: u64,
+    pub r9: u64,          // arg5
+    pub r8: u64,          // arg4
+    pub r10: u64,         // arg3
+    pub rdx: u64,         // arg2
+    pub rsi: u64,         // arg1
+    pub rdi: u64,         // arg0
+    pub rax: u64,         // syscall number on entry / return value on exit
+    pub user_rip: u64,    // rcx
+    pub user_rflags: u64, // r11
+    pub user_rsp: u64,
+}
+
+pub fn init(cpu_local: *const CpuLocal) {
+    unsafe {
+        let star = ((KERNEL_DATA_SELECTOR as u64) << 48) | ((KERNEL_CODE_SELECTOR as u64) << 32);
+        write_msr(IA32_STAR, star);
+        write_msr(IA32_LSTAR, syscall_entry as *const () as u64);
+        write_msr(IA32_CSTAR, 0);
+        write_msr(IA32_FMASK, RFLAGS_MASK);
+        write_msr(IA32_GS_BASE, 0);
+        write_msr(IA32_KERNEL_GS_BASE, cpu_local as u64);
+
+        // Kernel code always runs with GS pointing at CpuLocal. User entry
+        // swaps this into IA32_KERNEL_GS_BASE before transitioning to ring 3.
+        asm!("swapgs", options(nostack, preserves_flags));
+    }
+}
+
+#[unsafe(naked)]
+unsafe extern "C" fn syscall_entry() {
+    naked_asm!(
+        "swapgs",
+        "mov gs:[8], rsp", // user_rsp_scratch
+        "mov rsp, gs:[0]", // kernel_stack_top
+        "",
+        // build the syscall frame
+        "push qword ptr gs:[8]", // user_rsp
+        "push r11",              // user_rflags
+        "push rcx",              // user_rip
+        "push rax",
+        "push rdi",
+        "push rsi",
+        "push rdx",
+        "push r10",
+        "push r8",
+        "push r9",
+        "push rbx",
+        "push rbp",
+        "push r12",
+        "push r13",
+        "push r14",
+        "push r15",
+        "",
+        // Syscall calling convention:
+        // Syscall number in rax, args in rdi, rsi, rdx, r10, r8, r9
+        "mov rdi, rsp",
+        "call {dispatch}",
+        "",
+        // restore the syscall frame
+        "pop r15",
+        "pop r14",
+        "pop r13",
+        "pop r12",
+        "pop rbp",
+        "pop rbx",
+        "pop r9",
+        "pop r8",
+        "pop r10",
+        "pop rdx",
+        "pop rsi",
+        "pop rdi",
+        "pop rax",              // return value
+        "pop rcx",              // user_rip for sysret
+        "pop r11",              // user_rflags for sysret
+        "pop qword ptr gs:[8]", // user_rsp
+        "",
+        "mov rsp, gs:[8]", // switch to user stack
+        "swapgs",
+        "sysretq",
+        dispatch = sym syscall_dispatch,
+    );
+}
+
+extern "C" fn syscall_dispatch(frame: &mut SyscallFrame) {
+    let ret = crate::syscall::handle(
+        frame.rax, frame.rdi, frame.rsi, frame.rdx, frame.r10, frame.r8, frame.r9,
+    );
+
+    frame.rax = ret;
+}

src/arch/x86_64/timer.rs

@@ -0,0 +1,82 @@
+use core::sync::atomic::{AtomicBool, AtomicU32, Ordering};
+
+use crate::{
+    arch::{
+        apic::{self, LocalApic},
+        disable_interrupts,
+        io_apic::IoApic,
+        x86_64::{
+            interrupts::enable_interrupts,
+            pit::{PIT_CALIBRATION_COUNT, PIT_FREQUENCY, start_pit_one_shot},
+        },
+    },
+    platform::acpi::IsaIrqRoute,
+};
+
+static PIT_FIRED: AtomicBool = AtomicBool::new(false);
+static LAPIC_COUNT_AT_PIT: AtomicU32 = AtomicU32::new(0);
+
+#[derive(Debug)]
+pub enum TimerCalibrationError {
+    PitTimeout,
+    IoApicNotHandled,
+    FrequencyOverflow,
+    InvalidTimerCount,
+}
+
+pub fn calibrate_local_apic(
+    local_apic: &mut LocalApic,
+    io_apic: &mut IoApic,
+    pit_route: IsaIrqRoute,
+) -> Result<u64, TimerCalibrationError> {
+    PIT_FIRED.store(false, Ordering::SeqCst);
+    LAPIC_COUNT_AT_PIT.store(0, Ordering::SeqCst);
+
+    io_apic
+        .unmask(pit_route.gsi)
+        .map_err(|_| TimerCalibrationError::IoApicNotHandled)?;
+    local_apic.start_calibration_counter();
+
+    start_pit_one_shot(PIT_CALIBRATION_COUNT);
+
+    enable_interrupts();
+
+    while !PIT_FIRED.load(Ordering::SeqCst) {
+        if apic::current_timer_count() == 0 {
+            disable_interrupts();
+            let _ = io_apic.mask(pit_route.gsi);
+            local_apic.stop_timer();
+
+            return Err(TimerCalibrationError::PitTimeout);
+        }
+
+        core::hint::spin_loop();
+    }
+
+    disable_interrupts();
+
+    io_apic
+        .mask(pit_route.gsi)
+        .map_err(|_| TimerCalibrationError::IoApicNotHandled)?;
+    local_apic.stop_timer();
+
+    let elapsed = u32::MAX - LAPIC_COUNT_AT_PIT.load(Ordering::SeqCst);
+
+    if elapsed == 0 {
+        return Err(TimerCalibrationError::InvalidTimerCount);
+    }
+
+    let ticks_per_second = (elapsed as u64)
+        .checked_mul(PIT_FREQUENCY)
+        .ok_or(TimerCalibrationError::FrequencyOverflow)?
+        / PIT_CALIBRATION_COUNT as u64;
+
+    Ok(ticks_per_second)
+}
+
+pub fn record_pit_calibration() {
+    let current = apic::current_timer_count();
+
+    LAPIC_COUNT_AT_PIT.store(current, Ordering::SeqCst);
+    PIT_FIRED.store(true, Ordering::SeqCst);
+}

src/arch/x86_64/x86_64-unknown-none.json

@@ -8,11 +8,13 @@
     "target-c-int-width": 32,
     "features": "-mmx,-sse,+soft-float",
     "rustc-abi": "softfloat",
-  "os": "DawnOS",
     "linker": "rust-lld",
     "linker-flavor": "ld.lld",
     "pre-link-args": {
-    "ld.lld": ["-melf_x86_64", "--script=./src/arch/x86_64/linker.ld"]
+        "ld.lld": [
+            "-melf_x86_64",
+            "--script=./src/arch/x86_64/linker.ld"
+        ]
     },
     "panic-strategy": "abort",
     "exe-suffix": ".elf",

src/boot/limine.rs

@@ -1,8 +1,15 @@
 use ::limine as limine_api;
+use limine::paging::PagingMode;
+use limine::request::{ExecutableCmdlineRequest, ModulesRequest, PagingModeRequest, RsdpRequest};
 
 use limine_api::request::{ExecutableAddressRequest, HhdmRequest, MemmapRequest};
 use limine_api::{BaseRevision, RequestsEndMarker, RequestsStartMarker};
 
+use crate::memory::{
+    BootString, InitramfsImage, KernelMemoryLayout, KernelSegment, MemoryMap, MemoryRegion,
+    MemoryRegionKind, PagePermissions, PhysicalAddr, VirtualAddr,
+};
+
 /// Sets the base revision to the latest revision supported by the crate.
 /// See specification for further info.
 /// Be sure to mark all limine requests with #[used], otherwise they may be removed by the compiler.
@@ -15,6 +22,14 @@ static BASE_REVISION: BaseRevision = BaseRevision::new();
 #[unsafe(link_section = ".requests")]
 static KERNEL_ADDRESS_REQUEST: ExecutableAddressRequest = ExecutableAddressRequest::new();
 
+#[used]
+#[unsafe(link_section = ".requests")]
+static KERNEL_CMDLINE_REQUEST: ExecutableCmdlineRequest = ExecutableCmdlineRequest::new();
+
+#[used]
+#[unsafe(link_section = ".requests")]
+static KERNEL_MODULE_REQUEST: ModulesRequest = ModulesRequest::new();
+
 #[used]
 #[unsafe(link_section = ".requests")]
 static HHDM_REQUEST: HhdmRequest = HhdmRequest::new();
@@ -23,6 +38,15 @@ static HHDM_REQUEST: HhdmRequest = HhdmRequest::new();
 #[unsafe(link_section = ".requests")]
 static MEMMAP_REQUEST: MemmapRequest = MemmapRequest::new();
 
+#[used]
+#[unsafe(link_section = ".requests")]
+static PAGING_REQUEST: PagingModeRequest =
+    PagingModeRequest::new(PagingMode::MAX, PagingMode::MAX, PagingMode::MIN);
+
+#[used]
+#[unsafe(link_section = ".requests")]
+static RSDP_REQUEST: RsdpRequest = RsdpRequest::new();
+
 /// Define the stand and end markers for Limine requests.
 #[used]
 #[unsafe(link_section = ".requests_start_marker")]
@@ -31,22 +55,98 @@ static _START_MARKER: RequestsStartMarker = RequestsStartMarker::new();
 #[unsafe(link_section = ".requests_end_marker")]
 static _END_MARKER: RequestsEndMarker = RequestsEndMarker::new();
 
+unsafe extern "C" {
+    static __text_start: u64;
+    static __text_end: u64;
+
+    static __rodata_start: u64;
+    static __rodata_end: u64;
+
+    static __data_start: u64;
+    static __data_end: u64;
+}
+
+const MAX_COMMAND_LINE_LENGTH: usize = 512;
+
 pub struct BootInfo {
-    pub kernel_address: crate::memory::PhysicalAddr,
+    pub kernel_layout: KernelMemoryLayout,
-    pub hhdm_offset: u64,
+    pub command_line: BootString<MAX_COMMAND_LINE_LENGTH>,
-    entries: &'static [&'static limine_api::memmap::Entry],
+    pub initramfs: InitramfsImage,
+    pub hhdm_offset: usize,
+    pub memory_map: MemoryMap,
+    pub rsdp: VirtualAddr,
 }
 
 impl BootInfo {
-    pub fn memory_regions(&self) -> impl Iterator<Item = crate::memory::MemoryRegion> + '_ {
+    pub fn memory_regions(&self) -> impl Iterator<Item = MemoryRegion> + Clone + '_ {
-        use crate::memory::{MemoryRegion, MemoryRegionKind};
+        self.memory_map.iter()
-        self.entries.iter().map(|&entry| MemoryRegion {
+    }
-            start: crate::memory::PhysicalAddr::new(entry.base),
+}
-            length: entry.length,
+
+#[derive(Debug)]
+pub enum BootError {
+    UnsupportedBaseRevision,
+    FailedToGetKernelAddress,
+    FailedToGetKernelCmdline,
+    FailedToGetModules,
+    FailedToGetInitramfs,
+    FailedToGetHHDMAddress,
+    FailedToGetMemmap,
+    TooManyMemoryRegions,
+    FailedToLocateKernel,
+    FailedToGetRsdp,
+}
+
+pub fn load_boot_info() -> Result<BootInfo, BootError> {
+    if !BASE_REVISION.is_supported() {
+        return Err(BootError::UnsupportedBaseRevision);
+    }
+
+    let kernel_address = KERNEL_ADDRESS_REQUEST
+        .response()
+        .ok_or(BootError::FailedToGetKernelAddress)?;
+    let kernel_cmdline = KERNEL_CMDLINE_REQUEST
+        .response()
+        .ok_or(BootError::FailedToGetKernelCmdline)?;
+    let command_line = BootString::from_bytes(kernel_cmdline.cmdline().as_bytes());
+
+    let modules = KERNEL_MODULE_REQUEST
+        .response()
+        .ok_or(BootError::FailedToGetModules)?;
+    let initramfs = modules
+        .modules()
+        .get(0)
+        .map(|module| {
+            let start = VirtualAddr::new(module.data().as_ptr() as usize);
+            let length = module.data().len();
+            InitramfsImage { start, length }
+        })
+        .ok_or(BootError::FailedToGetInitramfs)?;
+
+    let hhdm_offset = HHDM_REQUEST
+        .response()
+        .ok_or(BootError::FailedToGetHHDMAddress)?
+        .offset as usize;
+
+    let rsdp = RSDP_REQUEST.response().ok_or(BootError::FailedToGetRsdp)?;
+
+    let memmap = MEMMAP_REQUEST
+        .response()
+        .ok_or(BootError::FailedToGetMemmap)?
+        .entries();
+
+    let mut memory_map = MemoryMap::new();
+    for entry in memmap.iter() {
+        memory_map
+            .push(MemoryRegion {
+                start: PhysicalAddr::new(entry.base as usize),
+                length: entry.length as usize,
                 kind: match entry.type_ {
                     limine_api::memmap::MEMMAP_USABLE => MemoryRegionKind::Usable,
                     limine_api::memmap::MEMMAP_RESERVED => MemoryRegionKind::Reserved,
-                limine_api::memmap::MEMMAP_ACPI_RECLAIMABLE => MemoryRegionKind::AcpiReclaimable,
+                    limine_api::memmap::MEMMAP_ACPI_RECLAIMABLE => {
+                        MemoryRegionKind::AcpiReclaimable
+                    }
                     limine_api::memmap::MEMMAP_ACPI_NVS => MemoryRegionKind::AcpiNvs,
                     limine_api::memmap::MEMMAP_BAD_MEMORY => MemoryRegionKind::BadMemory,
                     limine_api::memmap::MEMMAP_BOOTLOADER_RECLAIMABLE => {
@@ -60,39 +160,87 @@ impl BootInfo {
                     _ => MemoryRegionKind::Reserved,
                 },
             })
-    }
+            .map_err(|_| BootError::TooManyMemoryRegions)?;
-}
-
-#[derive(Debug)]
-pub enum BootError {
-    UnsupportedBaseRevision,
-    FailedToGetKernelAddress,
-    FailedToGetHHDMAddress,
-    FailedToGetMemmap,
-}
-
-pub fn load_boot_info() -> Result<BootInfo, BootError> {
-    if !BASE_REVISION.is_supported() {
-        return Err(BootError::UnsupportedBaseRevision);
     }
 
-    let kernel_address = KERNEL_ADDRESS_REQUEST
+    let mut segment_physical = kernel_address.physical_base as usize;
-        .response()
+    let mut segment_virtual = core::ptr::addr_of!(__text_start) as usize;
-        .ok_or(BootError::FailedToGetKernelAddress)?
-        .physical_base;
-    let hhdm_offset = HHDM_REQUEST
-        .response()
-        .ok_or(BootError::FailedToGetHHDMAddress)?
-        .offset;
 
-    let memmap = MEMMAP_REQUEST
+    let mut segment_length = 0;
-        .response()
+
-        .ok_or(BootError::FailedToGetMemmap)?
+    let kernel_text_segment = KernelSegment {
-        .entries();
+        physical_base: PhysicalAddr::new(segment_physical),
+        virtual_base: VirtualAddr::new(segment_virtual),
+        length: (core::ptr::addr_of!(__text_end) as usize)
+            - (core::ptr::addr_of!(__text_start) as usize),
+        permissions: PagePermissions::new(false, true, false),
+    };
+
+    segment_length += kernel_text_segment.length;
+
+    segment_virtual = core::ptr::addr_of!(__rodata_start) as usize;
+    segment_physical = kernel_address.physical_base as usize
+        + (segment_virtual - kernel_address.virtual_base as usize);
+
+    let kernel_rodata_segment = KernelSegment {
+        physical_base: PhysicalAddr::new(segment_physical),
+        virtual_base: VirtualAddr::new(segment_virtual),
+        length: (core::ptr::addr_of!(__rodata_end) as usize)
+            - (core::ptr::addr_of!(__rodata_start) as usize),
+        permissions: PagePermissions::new(false, false, false),
+    };
+
+    segment_length += kernel_rodata_segment.length;
+
+    segment_virtual = core::ptr::addr_of!(__data_start) as usize;
+    segment_physical = kernel_address.physical_base as usize
+        + (segment_virtual - kernel_address.virtual_base as usize);
+
+    let kernel_data_segment = KernelSegment {
+        physical_base: PhysicalAddr::new(segment_physical),
+        virtual_base: VirtualAddr::new(segment_virtual),
+        length: (core::ptr::addr_of!(__data_end) as usize)
+            - (core::ptr::addr_of!(__data_start) as usize),
+        permissions: PagePermissions::new(true, false, false),
+    };
+
+    segment_length += kernel_data_segment.length;
+
+    #[cfg(debug_assertions)]
+    {
+        let mut kernel_length = None;
+        for &entry in memmap.iter() {
+            if entry.type_ != limine_api::memmap::MEMMAP_EXECUTABLE_AND_MODULES {
+                continue;
+            }
+
+            if entry.base != kernel_address.physical_base {
+                continue;
+            }
+
+            kernel_length = Some(entry.length as usize);
+            break;
+        }
+
+        if kernel_length.is_none() {
+            return Err(BootError::FailedToLocateKernel);
+        }
+
+        debug_assert_eq!(segment_length, kernel_length.unwrap());
+    }
 
     Ok(BootInfo {
-        kernel_address: crate::memory::PhysicalAddr::new(kernel_address),
+        kernel_layout: KernelMemoryLayout {
+            segments: [
+                kernel_text_segment,
+                kernel_rodata_segment,
+                kernel_data_segment,
+            ],
+        },
+        command_line,
+        initramfs,
         hhdm_offset,
-        entries: memmap,
+        memory_map,
+        rsdp: VirtualAddr::new(rsdp.address as usize),
     })
 }

src/boot/mod.rs

@@ -1,3 +1,3 @@
 mod limine;
 
-pub use limine::{BootError, BootInfo, load_boot_info};
+pub use limine::{BootInfo, load_boot_info};

src/debug/serial.rs

@@ -9,13 +9,13 @@ mod register {
     pub const INTERRUPT_ENABLE: u16 = 1;
     pub const DIVISOR_HIGH: u16 = 1;
 
-    pub const INTERRUPT_IDENTIFICATION: u16 = 2;
+    // pub const INTERRUPT_IDENTIFICATION: u16 = 2;
     pub const FIFO_CONTROL: u16 = 2;
 
     pub const LINE_CONTROL: u16 = 3;
     pub const MODEM_CONTROL: u16 = 4;
     pub const LINE_STATUS: u16 = 5;
-    pub const MODEM_STATUS: u16 = 6;
+    // pub const MODEM_STATUS: u16 = 6;
     pub const SCRATCH: u16 = 7;
 }
 
@@ -160,6 +160,12 @@ pub fn init() -> Result<(), SerialPortError> {
     com1().init()
 }
 
+pub fn write_bytes(bytes: &[u8]) {
+    for byte in bytes {
+        com1().write_byte(*byte);
+    }
+}
+
 pub fn print(args: core::fmt::Arguments) {
     use core::fmt::Write;
 

src/format/cpio.rs

@@ -0,0 +1,80 @@
+// CPIO newc
+
+#[repr(C)]
+struct Header {
+    pub c_magic: [u8; 6],
+    pub c_ino: [u8; 8],
+    pub c_mode: [u8; 8],
+    pub c_uid: [u8; 8],
+    pub c_gid: [u8; 8],
+    pub c_nlink: [u8; 8],
+    pub c_mtime: [u8; 8],
+    pub c_filesize: [u8; 8],
+    pub c_devmajor: [u8; 8],
+    pub c_devminor: [u8; 8],
+    pub c_rdevmajor: [u8; 8],
+    pub c_rdevminor: [u8; 8],
+    pub c_namesize: [u8; 8],
+    pub c_check: [u8; 8],
+}
+
+impl Header {
+    pub fn from_bytes(bytes: &[u8]) -> Option<Self> {
+        if bytes.len() < core::mem::size_of::<Header>() {
+            return None;
+        }
+
+        let header: Header = unsafe { core::ptr::read(bytes.as_ptr() as *const Header) };
+
+        if header.c_magic != *b"070701" {
+            return None;
+        }
+
+        Some(header)
+    }
+}
+
+pub fn find_file<'a>(archive: &'a [u8], target: &str) -> Option<&'a [u8]> {
+    let mut offset = 0;
+
+    while offset + core::mem::size_of::<Header>() <= archive.len() {
+        if offset + core::mem::size_of::<Header>() > archive.len() {
+            return None;
+        }
+
+        let header = Header::from_bytes(&archive[offset..])?;
+        let header_start = offset;
+        offset += core::mem::size_of::<Header>();
+
+        let file_len =
+            usize::from_str_radix(core::str::from_utf8(&header.c_filesize).ok()?, 16).ok()?;
+        let name_len =
+            usize::from_str_radix(core::str::from_utf8(&header.c_namesize).ok()?, 16).ok()?;
+
+        if offset + name_len > archive.len() {
+            return None;
+        }
+
+        let name_bytes = &archive[offset..offset + name_len];
+        let name = core::str::from_utf8(name_bytes)
+            .ok()?
+            .trim_end_matches('\0');
+
+        if name == "TRAILER!!!" {
+            return None;
+        }
+
+        let data_start = header_start + ((core::mem::size_of::<Header>() + name_len + 3) & !3);
+        if data_start + file_len > archive.len() {
+            return None;
+        }
+
+        if name == target {
+            return Some(&archive[data_start..data_start + file_len]);
+        }
+
+        offset = data_start + ((file_len + 3) & !3);
+    }
+
+    None
+}

src/format/elf.rs

@@ -0,0 +1,105 @@
+pub struct ElfError;
+
+#[cfg(target_arch = "x86_64")]
+const MACHINE: u16 = 62;
+#[cfg(target_arch = "aarch64")]
+const MACHINE: u16 = 183;
+#[cfg(target_arch = "riscv64")]
+const MACHINE: u16 = 243;
+
+pub struct Elf<'a> {
+    bytes: &'a [u8],
+    headers: &'a [u8],
+    pub entry: usize,
+}
+
+pub struct Segment<'a> {
+    pub data: &'a [u8],
+    pub address: usize,
+    pub memory_size: usize,
+    pub writable: bool,
+    pub executable: bool,
+}
+
+impl<'a> Elf<'a> {
+    pub fn parse(bytes: &'a [u8]) -> Result<Self, ElfError> {
+        let header = bytes.get(..64).ok_or(ElfError)?;
+        // Bootstrap images are static ELF64 executables in the native ISA, always LE.
+        if &header[..7] != b"\x7fELF\x02\x01\x01"
+            || u16_at(header, 16) != 2
+            || u16_at(header, 18) != MACHINE
+            || u32_at(header, 20) != 1
+            || u16_at(header, 52) != 64
+            || u16_at(header, 54) != 56
+        {
+            return Err(ElfError);
+        }
+
+        let offset = usize_at(header, 32);
+        let count = usize::from(u16_at(header, 56));
+        let end = offset.checked_add(count * 56).ok_or(ElfError)?;
+        let headers = bytes.get(offset..end).ok_or(ElfError)?;
+        if headers
+            .chunks_exact(56)
+            .any(|h| matches!(u32_at(h, 0), 2 | 3))
+        {
+            // There is no dynamic linker or relocation processing during bootstrap.
+            return Err(ElfError);
+        }
+
+        Ok(Self {
+            bytes,
+            headers,
+            entry: usize_at(header, 24),
+        })
+    }
+
+    pub fn segments(&self) -> impl Iterator<Item = Result<Segment<'a>, ElfError>> + '_ {
+        self.headers
+            .chunks_exact(56)
+            .filter(|h| u32_at(h, 0) == 1)
+            .map(|h| {
+                let offset = usize_at(h, 8);
+                let address = usize_at(h, 16);
+                let file_size = usize_at(h, 32);
+                let memory_size = usize_at(h, 40);
+                let alignment = usize_at(h, 48);
+                if file_size > memory_size
+                    || (alignment > 1
+                        && (!alignment.is_power_of_two()
+                            || address % alignment != offset % alignment))
+                {
+                    return Err(ElfError);
+                }
+                let end = offset.checked_add(file_size).ok_or(ElfError)?;
+                let data = self.bytes.get(offset..end).ok_or(ElfError)?;
+                let flags = u32_at(h, 4);
+                Ok(Segment {
+                    data,
+                    address,
+                    memory_size,
+                    writable: flags & 2 != 0,
+                    executable: flags & 1 != 0,
+                })
+            })
+    }
+}
+
+// Callers only read fixed offsets within already bounds-checked headers.
+fn u16_at(bytes: &[u8], offset: usize) -> u16 {
+    let mut value = [0; 2];
+    value.copy_from_slice(&bytes[offset..offset + 2]);
+    u16::from_le_bytes(value)
+}
+
+fn u32_at(bytes: &[u8], offset: usize) -> u32 {
+    let mut value = [0; 4];
+    value.copy_from_slice(&bytes[offset..offset + 4]);
+    u32::from_le_bytes(value)
+}
+
+fn usize_at(bytes: &[u8], offset: usize) -> usize {
+    let mut value = [0; 8];
+    value.copy_from_slice(&bytes[offset..offset + 8]);
+    u64::from_le_bytes(value) as usize
+}

src/format/mod.rs

@@ -0,0 +1,2 @@
+pub mod cpio;
+pub mod elf;

src/main.rs

@@ -1,4 +1,3 @@
-#![feature(abi_x86_interrupt)]
 #![allow(clippy::needless_return)]
 #![no_std]
 #![no_main]
@@ -6,23 +5,133 @@
 mod arch;
 mod boot;
 mod debug;
+mod format;
 mod memory;
+mod platform;
+mod syscall;
+mod task;
 
-use crate::debug::serial;
+use crate::{
+    debug::serial,
+    memory::{AddressSpace, MemoryRegionKind, init_frame_allocator, init_kernel_address_space},
+};
+
+pub struct KernelHandoff {
+    allocator: memory::FrameAllocator,
+    address_space: AddressSpace,
+    direct_map: memory::DirectMap,
+    boot_info: boot::BootInfo,
+    handoff_frame: memory::OwnedFrame,
+}
+
+const _: () = assert!(core::mem::size_of::<KernelHandoff>() <= memory::FRAME_SIZE);
 
 #[unsafe(no_mangle)]
 pub extern "C" fn _start() -> ! {
     serial::init().unwrap();
 
-    arch::init();
+    let arch_state = arch::init();
-    let _boot_info = boot::load_boot_info().unwrap();
+    let boot_info = boot::load_boot_info().unwrap();
+    let direct_map = memory::DirectMap::new(boot_info.hhdm_offset);
+
+    let mut allocator = memory::FrameAllocator::new(boot_info.memory_regions(), direct_map)
+        .expect("failed to create frame allocator");
+
+    println!("Initializing page table...");
+    let mut address_space = AddressSpace::new_kernel(
+        direct_map,
+        boot_info.memory_regions(),
+        &boot_info.kernel_layout,
+        arch_state.paging,
+        &mut allocator,
+    )
+    .expect("failed to create page table");
+
+    println!("Entering kernel main...");
+
+    let kernel_stack =
+        crate::task::scheduler::allocate_kernel_stack(&mut address_space, &mut allocator)
+            .expect("failed to allocate bootstrap stack");
+
+    let handoff_frame = allocator
+        .alloc()
+        .expect("failed to allocate frame for kernel handoff");
+    let handoff_addr = address_space
+        .to_virtual(handoff_frame.frame_address().start_address())
+        .expect("failed to map kernel handoff");
+
+    let bootstrap_stack_top = kernel_stack.top();
+    let handoff = KernelHandoff {
+        allocator,
+        address_space,
+        direct_map,
+        boot_info,
+        handoff_frame,
+    };
 
-    let addr = 0xDEADBEEF as *mut u32;
     unsafe {
-        *addr = 0xDEADBEEF;
+        handoff_addr.as_mut_ptr::<KernelHandoff>().write(handoff);
+        (*handoff_addr.as_mut_ptr::<KernelHandoff>())
+            .address_space
+            .activate();
+
+        arch::enter_kernel(
+            bootstrap_stack_top,
+            handoff_addr.as_mut_ptr::<KernelHandoff>(),
+        );
+    }
+}
+
+pub unsafe extern "C" fn kernel_main(handoff: *mut KernelHandoff) -> ! {
+    let (mut allocator, mut address_space, direct_map, boot_info, handoff_frame) = unsafe {
+        let handoff = handoff.read();
+        (
+            handoff.allocator,
+            handoff.address_space,
+            handoff.direct_map,
+            handoff.boot_info,
+            handoff.handoff_frame,
+        )
+    };
+
+    unsafe {
+        allocator.dealloc(handoff_frame);
     }
 
-    hcf();
+    allocator.reclaim_regions(
+        boot_info.memory_regions(),
+        MemoryRegionKind::BootloaderReclaimable,
+    );
+
+    println!("Initializing local ACPI...",);
+
+    let acpi = platform::acpi::init(&boot_info, direct_map).expect("failed to initialize ACPI");
+
+    println!("Parsing MADT...");
+
+    let madt = acpi
+        .madt()
+        .expect("failed to parse ACPI")
+        .expect("MADT not found");
+
+    println!("Initializing interrupt controller...");
+
+    let _interrupt_controller =
+        arch::init_interrupt_controller(&madt, &mut allocator, &mut address_space)
+            .expect("failed to initialize interrupt controller");
+
+    task::bootstrap::spawn(
+        "omega3.elf",
+        &boot_info.initramfs,
+        &mut address_space,
+        &mut allocator,
+        direct_map,
+    );
+
+    init_frame_allocator(allocator);
+    init_kernel_address_space(address_space);
+
+    task::scheduler::start();
 }
 
 #[panic_handler]

src/memory/address_space.rs

@@ -0,0 +1,460 @@
+use core::cell::UnsafeCell;
+
+use crate::{
+    arch::{PageTable, PageTableCreateError, PageTableMapError, PageTableUnmapError, PagingConfig},
+    memory::{
+        CachePolicy, DirectMap, FRAME_SIZE, FrameAddr, FrameAllocator, KernelMemoryLayout,
+        MemoryRegion, MemoryRegionKind, PagePermissions, PhysicalAddr, USER_SPACE_END, VirtualAddr,
+    },
+};
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[repr(transparent)]
+pub struct AddressSpaceId(usize);
+
+const MAX_ADDRESS_SPACES: usize = 32;
+struct AddressSpaceTable {
+    entries: [Option<AddressSpace>; MAX_ADDRESS_SPACES],
+}
+
+impl AddressSpaceTable {
+    const fn new() -> Self {
+        Self {
+            entries: [const { None }; MAX_ADDRESS_SPACES],
+        }
+    }
+
+    fn insert(&mut self, address_space: AddressSpace) -> Result<AddressSpaceId, AddressSpace> {
+        for (i, slot) in self.entries.iter_mut().enumerate() {
+            if slot.is_none() {
+                *slot = Some(address_space);
+                return Ok(AddressSpaceId(i));
+            }
+        }
+
+        Err(address_space)
+    }
+
+    fn get(&self, id: AddressSpaceId) -> Option<&AddressSpace> {
+        self.entries.get(id.0).and_then(Option::as_ref)
+    }
+
+    fn get_mut(&mut self, id: AddressSpaceId) -> Option<&mut AddressSpace> {
+        self.entries.get_mut(id.0).and_then(Option::as_mut)
+    }
+
+    fn remove(&mut self, id: AddressSpaceId) -> Option<AddressSpace> {
+        self.entries.get_mut(id.0).and_then(Option::take)
+    }
+}
+
+struct GlobalAddressSpaceTable(UnsafeCell<AddressSpaceTable>);
+
+unsafe impl Sync for GlobalAddressSpaceTable {}
+
+static ADDRESS_SPACE_TABLE: GlobalAddressSpaceTable =
+    GlobalAddressSpaceTable(UnsafeCell::new(AddressSpaceTable::new()));
+
+pub fn insert_address_space(address_space: AddressSpace) -> Result<AddressSpaceId, AddressSpace> {
+    let table = unsafe { &mut *ADDRESS_SPACE_TABLE.0.get() };
+
+    table.insert(address_space)
+}
+
+pub fn remove_address_space(id: AddressSpaceId) -> Option<AddressSpace> {
+    let table = unsafe { &mut *ADDRESS_SPACE_TABLE.0.get() };
+
+    table.remove(id)
+}
+
+pub fn with_address_space<R>(id: AddressSpaceId, f: impl FnOnce(&AddressSpace) -> R) -> Option<R> {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+    let table = unsafe { &*ADDRESS_SPACE_TABLE.0.get() };
+    let res = table.get(id).map(f);
+    crate::arch::restore_interrupts(interrupt_state);
+    res
+}
+
+pub fn with_address_space_mut<R>(
+    id: AddressSpaceId,
+    f: impl FnOnce(&mut AddressSpace) -> R,
+) -> Option<R> {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+    let table = unsafe { &mut *ADDRESS_SPACE_TABLE.0.get() };
+    let res = table.get_mut(id).map(f);
+    crate::arch::restore_interrupts(interrupt_state);
+    res
+}
+
+struct GlobalKernelAddressSpace(UnsafeCell<Option<AddressSpace>>);
+
+unsafe impl Sync for GlobalKernelAddressSpace {}
+
+static KERNEL_ADDRESS_SPACE: GlobalKernelAddressSpace =
+    GlobalKernelAddressSpace(UnsafeCell::new(None));
+
+pub fn init_kernel_address_space(address_space: AddressSpace) {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+    unsafe {
+        *KERNEL_ADDRESS_SPACE.0.get() = Some(address_space);
+    }
+    crate::arch::restore_interrupts(interrupt_state);
+}
+
+pub fn with_kernel_address_space<R>(f: impl FnOnce(&mut AddressSpace) -> R) -> R {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+    let space = unsafe {
+        (&mut *KERNEL_ADDRESS_SPACE.0.get())
+            .as_mut()
+            .expect("kernel address space not initialized")
+    };
+    let res = f(space);
+    crate::arch::restore_interrupts(interrupt_state);
+    res
+}
+
+#[derive(Debug)]
+pub enum MapError {
+    InvalidVirtualAddress,
+    VirtualAddressUnaligned,
+    PhysicalAddressTooLarge,
+    PhysicalAddressUnaligned,
+    RangeLengthUnaligned,
+    AddressOverflow,
+    AlreadyMapped,
+    MappingConflict,
+    UnsupportedPermissions,
+    OutOfMemory,
+    PageTableUnavailable,
+    CorruptedPageTable,
+    InvalidUserAddress,
+    InvalidUserMap,
+}
+
+impl From<PageTableMapError> for MapError {
+    fn from(error: PageTableMapError) -> Self {
+        match error {
+            PageTableMapError::InvalidVirtualAddress => Self::InvalidVirtualAddress,
+            PageTableMapError::VirtualAddressUnaligned => Self::VirtualAddressUnaligned,
+            PageTableMapError::PhysicalAddressTooLarge => Self::PhysicalAddressTooLarge,
+            PageTableMapError::PageAlreadyMapped => Self::AlreadyMapped,
+            PageTableMapError::HugePageConflict => Self::MappingConflict,
+            PageTableMapError::NoExecuteUnsupported => Self::UnsupportedPermissions,
+            PageTableMapError::OutOfFrames => Self::OutOfMemory,
+            PageTableMapError::PageTableOutsideDirectMap => Self::PageTableUnavailable,
+            PageTableMapError::InvalidPageTableEntry => Self::CorruptedPageTable,
+        }
+    }
+}
+
+#[derive(Debug)]
+pub enum UnmapError {
+    InvalidVirtualAddress,
+    VirtualAddressUnaligned,
+    NotMapped,
+    MappingConflict,
+    PageTableUnavailable,
+    CorruptedPageTable,
+    InvalidUserAddress,
+}
+
+impl From<PageTableUnmapError> for UnmapError {
+    fn from(error: PageTableUnmapError) -> Self {
+        match error {
+            PageTableUnmapError::InvalidVirtualAddress => Self::InvalidVirtualAddress,
+            PageTableUnmapError::VirtualAddressUnaligned => Self::VirtualAddressUnaligned,
+            PageTableUnmapError::PageNotMapped => Self::NotMapped,
+            PageTableUnmapError::HugePageConflict => Self::MappingConflict,
+            PageTableUnmapError::PageTableOutsideDirectMap => Self::PageTableUnavailable,
+            PageTableUnmapError::InvalidPageTableEntry => Self::CorruptedPageTable,
+        }
+    }
+}
+
+#[derive(Debug)]
+#[allow(unused)]
+pub enum AddressSpaceCreateError {
+    AddressOutsideDirectMap,
+    PhysicalAddressTooLarge,
+    OutOfMemory,
+    Map(MapError),
+}
+
+impl From<PageTableCreateError> for AddressSpaceCreateError {
+    fn from(error: PageTableCreateError) -> Self {
+        match error {
+            PageTableCreateError::PhysicalAddressTooLarge => Self::PhysicalAddressTooLarge,
+            PageTableCreateError::OutOfFrames => Self::OutOfMemory,
+        }
+    }
+}
+
+pub struct PageTableMapping {
+    pub permissions: PagePermissions,
+}
+
+#[derive(PartialEq, Eq)]
+enum AddressSpaceKind {
+    Kernel,
+    User,
+}
+
+#[derive(PartialEq, Eq)]
+pub struct AddressSpace {
+    root: PageTable,
+    kind: AddressSpaceKind,
+}
+
+impl AddressSpace {
+    pub fn new_kernel<I: Iterator<Item = MemoryRegion> + Clone>(
+        direct_map: DirectMap,
+        memory_regions: I,
+        layout: &KernelMemoryLayout,
+        paging_config: PagingConfig,
+        allocator: &mut FrameAllocator,
+    ) -> Result<Self, AddressSpaceCreateError> {
+        let mut space = AddressSpace {
+            root: PageTable::new(direct_map, paging_config, allocator)?,
+            kind: AddressSpaceKind::Kernel,
+        };
+
+        // map hhdm
+        for region in memory_regions.clone() {
+            // we exlcude KernelAndModules from the hhdm because if it were mapped, it would
+            // undermind the permissions of the explicitly mapped kernel image
+            if matches!(
+                region.kind,
+                MemoryRegionKind::Reserved | MemoryRegionKind::BadMemory
+            ) {
+                continue;
+            }
+
+            // we shouldnt HHDM the kernel image, but we should map modules
+            if region.kind == MemoryRegionKind::KernelAndModules
+                && region.start == layout.segments[0].physical_base
+            {
+                continue;
+            }
+
+            let cache_policy = if matches!(
+                region.kind,
+                MemoryRegionKind::MappedReserved | MemoryRegionKind::Framebuffer
+            ) {
+                CachePolicy::Uncacheable
+            } else {
+                CachePolicy::WriteBack
+            };
+
+            let res = space.map_range(
+                region.start,
+                direct_map
+                    .translate(region.start)
+                    .ok_or(AddressSpaceCreateError::AddressOutsideDirectMap)?,
+                region.length,
+                PagePermissions::new(true, false, false),
+                allocator,
+                cache_policy,
+            );
+
+            if let Err(err) = res {
+                unsafe { space.destroy(allocator) };
+                return Err(AddressSpaceCreateError::Map(err));
+            }
+        }
+
+        let mut old_segment_stop: Option<usize> = None;
+
+        for segment in layout.segments.iter() {
+            if let Some(stop) = old_segment_stop {
+                debug_assert_eq!(segment.physical_base.as_usize(), stop);
+            }
+
+            old_segment_stop = Some(segment.physical_base.as_usize() + segment.length);
+
+            let res = space.map_range(
+                segment.physical_base,
+                segment.virtual_base,
+                segment.length,
+                segment.permissions,
+                allocator,
+                CachePolicy::WriteBack,
+            );
+
+            if let Err(err) = res {
+                unsafe { space.destroy(allocator) };
+                return Err(AddressSpaceCreateError::Map(err));
+            }
+        }
+
+        Ok(space)
+    }
+
+    pub fn new_user(&self, allocator: &mut FrameAllocator) -> Result<Self, PageTableCreateError> {
+        let mut user_root = PageTable::new(self.root.direct_map, self.root.config(), allocator)?;
+
+        self.root.copy_kernel_mappings_to(&mut user_root);
+
+        Ok(Self {
+            root: user_root,
+            kind: AddressSpaceKind::User,
+        })
+    }
+
+    pub fn map(
+        &mut self,
+        physical_addr: PhysicalAddr,
+        virtual_addr: VirtualAddr,
+        permissions: PagePermissions,
+        allocator: &mut FrameAllocator,
+        cache_policy: CachePolicy,
+    ) -> Result<(), MapError> {
+        let global = self.kind == AddressSpaceKind::Kernel;
+
+        if self.kind == AddressSpaceKind::User {
+            if virtual_addr.as_usize() >= USER_SPACE_END.as_usize() {
+                return Err(MapError::InvalidUserAddress);
+            }
+
+            if !permissions.user_accessible {
+                return Err(MapError::InvalidUserMap);
+            }
+
+            // TODO: a user address space should not be able to map kernel memory
+            // or ACPI memory, or anything like that
+        }
+
+        let frame = FrameAddr::from_start_address(physical_addr)
+            .ok_or(MapError::PhysicalAddressUnaligned)?;
+
+        self.root
+            .map(
+                virtual_addr,
+                frame,
+                permissions,
+                allocator,
+                cache_policy,
+                global,
+            )
+            .map_err(MapError::from)
+    }
+
+    pub fn map_range(
+        &mut self,
+        physical_start: PhysicalAddr,
+        virtual_start: VirtualAddr,
+        length: usize,
+        permissions: PagePermissions,
+        allocator: &mut FrameAllocator,
+        cache_policy: CachePolicy,
+    ) -> Result<(), MapError> {
+        if length == 0 {
+            return Ok(());
+        }
+
+        if physical_start.as_usize() % FRAME_SIZE != 0 {
+            return Err(MapError::PhysicalAddressUnaligned);
+        }
+
+        if virtual_start.as_usize() % FRAME_SIZE != 0 {
+            return Err(MapError::VirtualAddressUnaligned);
+        }
+
+        if length % FRAME_SIZE != 0 {
+            return Err(MapError::RangeLengthUnaligned);
+        }
+
+        let last_offset = length - FRAME_SIZE;
+
+        physical_start
+            .as_usize()
+            .checked_add(last_offset)
+            .ok_or(MapError::AddressOverflow)?;
+        virtual_start
+            .as_usize()
+            .checked_add(last_offset)
+            .ok_or(MapError::AddressOverflow)?;
+
+        let page_count = length / FRAME_SIZE;
+        let mut mapped_pages = 0;
+
+        while mapped_pages < page_count {
+            let offset = mapped_pages * FRAME_SIZE;
+            let physical_addr = PhysicalAddr::new(physical_start.as_usize() + offset);
+            let virtual_addr = VirtualAddr::new(virtual_start.as_usize() + offset);
+
+            if let Err(err) = self.map(
+                physical_addr,
+                virtual_addr,
+                permissions,
+                allocator,
+                cache_policy,
+            ) {
+                for rollback_idx in (0..mapped_pages).rev() {
+                    let rollback_offset = rollback_idx * FRAME_SIZE;
+
+                    unsafe {
+                        // make sure we use the root page tableq directl since the public AddressSpace API
+                        // might *at some point* reject kernel mappings
+                        self.root
+                            .unmap(
+                                VirtualAddr::new(virtual_start.as_usize() + rollback_offset),
+                                allocator,
+                            )
+                            .expect("failed to roll back a mapped page");
+                    }
+                }
+
+                return Err(err);
+            }
+
+            mapped_pages += 1;
+        }
+
+        Ok(())
+    }
+
+    /// # Safety
+    ///
+    /// The caller must ensure:
+    /// - The page is not currently in use
+    pub unsafe fn unmap(
+        &mut self,
+        virtual_addr: VirtualAddr,
+        allocator: &mut FrameAllocator,
+    ) -> Result<FrameAddr, UnmapError> {
+        if self.kind == AddressSpaceKind::User && virtual_addr.as_usize() >= 0x0000_8000_0000_0000 {
+            return Err(UnmapError::InvalidUserAddress);
+        }
+
+        unsafe { self.root.unmap(virtual_addr, allocator) }.map_err(UnmapError::from)
+    }
+
+    pub fn to_physical(&self, virtual_addr: VirtualAddr) -> Option<PhysicalAddr> {
+        self.root.to_physical(virtual_addr)
+    }
+
+    pub fn to_virtual(&self, physical_addr: PhysicalAddr) -> Option<VirtualAddr> {
+        self.root.to_virtual(physical_addr)
+    }
+
+    pub fn mapping(&self, virtual_addr: VirtualAddr) -> Option<PageTableMapping> {
+        self.root.mapping(virtual_addr)
+    }
+
+    pub unsafe fn activate(&self) {
+        unsafe { self.root.activate() }
+    }
+
+    /// # Safety
+    ///
+    /// The caller must ensure this apge table is not active on any CPU and
+    /// no CPU or kernel operation can access its paging structures.
+    pub unsafe fn destroy(self, allocator: &mut FrameAllocator) {
+        unsafe {
+            match self.kind {
+                AddressSpaceKind::Kernel => self.root.destroy(allocator),
+                AddressSpaceKind::User => self.root.destroy_user(allocator),
+            }
+        }
+    }
+}

src/memory/frame.rs

@@ -0,0 +1,397 @@
+use core::cell::UnsafeCell;
+
+use crate::memory::{DirectMap, MemoryRegion, MemoryRegionKind, PhysicalAddr, VirtualAddr};
+
+pub const FRAME_SIZE: usize = 4096;
+
+pub fn align_up_to_frame(addr: usize) -> Option<usize> {
+    addr.checked_add(FRAME_SIZE as usize - 1)
+        .map(|addr| addr & !(FRAME_SIZE - 1))
+}
+
+pub fn align_down_to_frame(addr: usize) -> usize {
+    addr & !(FRAME_SIZE - 1)
+}
+
+#[repr(u8)]
+#[derive(Clone, Copy, PartialEq, Eq)]
+enum FrameState {
+    Reserved = 0b00,
+    Free = 0b01,
+    Allocated = 0b10,
+}
+
+struct GlobalFrameAllocator(UnsafeCell<Option<FrameAllocator>>);
+unsafe impl Sync for GlobalFrameAllocator {}
+
+static FRAME_ALLOCATOR: GlobalFrameAllocator = GlobalFrameAllocator(UnsafeCell::new(None));
+
+pub fn init_global(allocator: FrameAllocator) {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+    unsafe {
+        *FRAME_ALLOCATOR.0.get() = Some(allocator);
+    }
+    crate::arch::restore_interrupts(interrupt_state);
+}
+
+pub fn alloc_frame() -> Option<OwnedFrame> {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+    let allocator = unsafe { &mut *FRAME_ALLOCATOR.0.get() };
+    let frame = allocator.as_mut().and_then(|a| a.alloc());
+    crate::arch::restore_interrupts(interrupt_state);
+    frame
+}
+
+pub unsafe fn dealloc_frame(frame: OwnedFrame) {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+    let allocator = unsafe { &mut *FRAME_ALLOCATOR.0.get() };
+    if let Some(a) = allocator.as_mut() {
+        unsafe { a.dealloc(frame) };
+    }
+    crate::arch::restore_interrupts(interrupt_state);
+}
+
+#[allow(unused)]
+pub fn with_allocator<R>(f: impl FnOnce(&mut FrameAllocator) -> R) -> R {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+    let allocator = unsafe {
+        (&mut *FRAME_ALLOCATOR.0.get())
+            .as_mut()
+            .expect("frame allocator not initialized")
+    };
+    let result = f(allocator);
+    crate::arch::restore_interrupts(interrupt_state);
+    result
+}
+
+// 64 KiB per GiB
+struct Bitmap {
+    start: VirtualAddr,
+    frame_count: usize,
+}
+
+impl Bitmap {
+    fn new(start: VirtualAddr, frame_count: usize) -> Self {
+        Self { start, frame_count }
+    }
+
+    fn state(&self, frame_idx: usize) -> FrameState {
+        assert!(frame_idx < self.frame_count, "frame index out of bounds");
+
+        let byte_idx = frame_idx / 4;
+        let shift = (frame_idx % 4) * 2;
+        let byte = unsafe { self.start.as_ptr::<u8>().add(byte_idx).read() };
+
+        match (byte >> shift) & 0b11 {
+            0b00 => FrameState::Reserved,
+            0b01 => FrameState::Free,
+            0b10 => FrameState::Allocated,
+            _ => panic!("invalid frame state"),
+        }
+    }
+
+    fn set_state(&mut self, frame_idx: usize, state: FrameState) {
+        assert!(frame_idx < self.frame_count, "frame index out of bounds");
+
+        let byte_idx = frame_idx / 4;
+        let shift = (frame_idx % 4) * 2;
+        let ptr = unsafe { self.start.as_mut_ptr::<u8>().add(byte_idx) };
+        let byte = unsafe { ptr.read() };
+        let mask = 0b11 << shift;
+
+        unsafe {
+            ptr.write((byte & !mask) | ((state as u8) << shift));
+        }
+    }
+}
+
+#[derive(Debug)]
+pub enum FrameAllocatorInitError {
+    AddressOverflow,
+    NoUsableFrames,
+    NoBitmapStorage,
+    BitmapOutsideDirectMap,
+}
+
+// very very simple bitmap frame/page allocator
+pub struct FrameAllocator {
+    bitmap: Bitmap,
+    next_search: usize,
+    allocatable_frames: usize,
+    free_frames: usize,
+    direct_map: DirectMap,
+}
+
+impl FrameAllocator {
+    pub fn new<I>(regions: I, direct_map: DirectMap) -> Result<Self, FrameAllocatorInitError>
+    where
+        I: Iterator<Item = MemoryRegion> + Clone,
+    {
+        let mut highest_frame: Option<usize> = None;
+
+        for region in regions.clone() {
+            if !Self::should_track(region.kind) {
+                continue;
+            }
+
+            let range = Self::usable_frame_range(region)?;
+            if range.is_empty() {
+                continue;
+            }
+
+            highest_frame = Some(highest_frame.map_or(range.end, |current| current.max(range.end)));
+        }
+
+        let highest_frame = highest_frame.ok_or(FrameAllocatorInitError::NoUsableFrames)?;
+
+        let bitmap_bytes = highest_frame.div_ceil(4);
+        let bitmap_frame_count = bitmap_bytes.div_ceil(FRAME_SIZE);
+        let bitmap_storage_bytes = bitmap_frame_count
+            .checked_mul(FRAME_SIZE)
+            .ok_or(FrameAllocatorInitError::AddressOverflow)?;
+
+        let mut bitmap_start_frame: Option<usize> = None;
+
+        for region in regions.clone() {
+            if !Self::can_store_bitmap(region.kind) {
+                continue;
+            }
+
+            let range = Self::usable_frame_range(region)?;
+            if range.is_empty() {
+                continue;
+            }
+
+            if range.len() < bitmap_frame_count {
+                continue;
+            }
+
+            bitmap_start_frame = Some(range.start);
+            break;
+        }
+
+        if bitmap_start_frame.is_none() {
+            return Err(FrameAllocatorInitError::NoBitmapStorage);
+        }
+
+        let bitmap_start_frame = bitmap_start_frame.unwrap();
+
+        let bitmap_physical_addr = PhysicalAddr::new(bitmap_start_frame * FRAME_SIZE);
+        let bitmap_virtual = direct_map
+            .translate(bitmap_physical_addr)
+            .ok_or(FrameAllocatorInitError::BitmapOutsideDirectMap)?;
+
+        unsafe {
+            // set everyting to unavailable
+            core::ptr::write_bytes(bitmap_virtual.as_mut_ptr::<u8>(), 0, bitmap_storage_bytes);
+        }
+
+        let mut allocatable_frames = 0;
+        let mut free_frames = 0;
+        let mut bitmap = Bitmap::new(bitmap_virtual, highest_frame);
+
+        let bitmap_end_frame = bitmap_start_frame
+            .checked_add(bitmap_frame_count)
+            .ok_or(FrameAllocatorInitError::AddressOverflow)?;
+
+        for region in regions {
+            if !Self::is_initially_free(region.kind) {
+                continue;
+            }
+
+            for frame_idx in Self::usable_frame_range(region)? {
+                let is_bitmap_storage =
+                    frame_idx >= bitmap_start_frame && frame_idx < bitmap_end_frame;
+                if is_bitmap_storage {
+                    continue;
+                }
+
+                bitmap.set_state(frame_idx, FrameState::Free);
+                allocatable_frames += 1;
+                free_frames += 1;
+            }
+        }
+
+        Ok(Self {
+            bitmap,
+            next_search: bitmap_start_frame + bitmap_frame_count,
+            allocatable_frames,
+            free_frames,
+            direct_map,
+        })
+    }
+
+    fn should_track(kind: MemoryRegionKind) -> bool {
+        matches!(
+            kind,
+            MemoryRegionKind::Usable
+                | MemoryRegionKind::BootloaderReclaimable
+                | MemoryRegionKind::AcpiReclaimable
+        )
+    }
+
+    fn can_store_bitmap(kind: MemoryRegionKind) -> bool {
+        kind == MemoryRegionKind::Usable
+    }
+
+    fn is_initially_free(kind: MemoryRegionKind) -> bool {
+        kind == MemoryRegionKind::Usable
+    }
+
+    fn find_free_in(&self, start: usize, end: usize) -> Option<usize> {
+        (start..end).find(|&index| self.bitmap.state(index) == FrameState::Free)
+    }
+
+    fn find_free_frame(&self) -> Option<usize> {
+        self.find_free_in(self.next_search, self.bitmap.frame_count)
+            .or_else(|| self.find_free_in(0, self.next_search))
+    }
+
+    pub fn reclaim_regions<I: Iterator<Item = MemoryRegion> + Clone>(
+        &mut self,
+        memory_map: I,
+        region_kind: MemoryRegionKind,
+    ) {
+        if !matches!(
+            region_kind,
+            MemoryRegionKind::BootloaderReclaimable | MemoryRegionKind::AcpiReclaimable
+        ) {
+            return;
+        }
+
+        for region in memory_map {
+            if region.kind == region_kind {
+                for frame_idx in Self::usable_frame_range(region).expect("invalid memory region") {
+                    if self.bitmap.state(frame_idx) != FrameState::Reserved {
+                        continue;
+                    }
+
+                    self.bitmap.set_state(frame_idx, FrameState::Free);
+                    self.allocatable_frames += 1;
+                    self.free_frames += 1;
+                    self.next_search = self.next_search.min(frame_idx);
+                }
+            }
+        }
+    }
+
+    pub fn alloc_nozero(&mut self) -> Option<OwnedFrame> {
+        if self.free_frames == 0 {
+            return None;
+        }
+
+        let frame_idx = self.find_free_frame()?;
+
+        self.bitmap.set_state(frame_idx, FrameState::Allocated);
+        self.free_frames -= 1;
+        self.next_search = frame_idx.saturating_add(1);
+
+        Some(OwnedFrame::new(FrameAddr::from_index(frame_idx)))
+    }
+
+    pub fn alloc(&mut self) -> Option<OwnedFrame> {
+        let frame = self.alloc_nozero()?;
+
+        let start = self
+            .direct_map
+            .translate(frame.frame_address().start_address())
+            .expect("frame is outside the direct map");
+
+        unsafe {
+            core::ptr::write_bytes(start.as_mut_ptr::<u8>(), 0, FRAME_SIZE);
+        }
+
+        Some(frame)
+    }
+
+    /// # Safety
+    ///
+    /// The caller must ensure:
+    /// - The frame is currently owned by the caller
+    /// - it is not currently in use
+    /// - it has not been freed
+    pub unsafe fn dealloc(&mut self, frame: OwnedFrame) {
+        let frame_idx = frame.index();
+
+        match self.bitmap.state(frame_idx) {
+            FrameState::Allocated => {
+                self.bitmap.set_state(frame_idx, FrameState::Free);
+                self.free_frames += 1;
+                self.next_search = self.next_search.min(frame_idx);
+            }
+            FrameState::Free => panic!("attempted to free free frame"),
+            FrameState::Reserved => {
+                panic!("attempted to free reserved frame");
+            }
+        };
+    }
+
+    fn usable_frame_range(
+        region: MemoryRegion,
+    ) -> Result<core::ops::Range<usize>, FrameAllocatorInitError> {
+        let region_end = region
+            .start
+            .as_usize()
+            .checked_add(region.length)
+            .ok_or(FrameAllocatorInitError::AddressOverflow)?;
+
+        let start = align_up_to_frame(region.start.as_usize())
+            .ok_or(FrameAllocatorInitError::AddressOverflow)?;
+        let end = align_down_to_frame(region_end);
+
+        Ok((start / FRAME_SIZE)..(end / FRAME_SIZE))
+    }
+}
+
+#[repr(transparent)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct FrameAddr(PhysicalAddr);
+
+impl FrameAddr {
+    pub fn from_start_address(address: PhysicalAddr) -> Option<Self> {
+        if address.as_usize() % FRAME_SIZE != 0 {
+            return None;
+        }
+
+        Some(Self(address))
+    }
+
+    pub fn start_address(&self) -> PhysicalAddr {
+        self.0
+    }
+
+    fn from_index(index: usize) -> Self {
+        Self(PhysicalAddr::new(index * FRAME_SIZE))
+    }
+
+    fn index(&self) -> usize {
+        self.0.as_usize() / FRAME_SIZE
+    }
+}
+
+// specifically not Clone or Copy
+pub struct OwnedFrame {
+    frame: FrameAddr,
+}
+
+impl OwnedFrame {
+    fn new(frame: FrameAddr) -> Self {
+        Self { frame }
+    }
+
+    pub fn into_raw(self) -> FrameAddr {
+        self.frame
+    }
+
+    pub unsafe fn from_raw(frame: FrameAddr) -> Self {
+        Self { frame }
+    }
+
+    pub fn frame_address(&self) -> FrameAddr {
+        self.frame
+    }
+
+    pub fn index(&self) -> usize {
+        self.frame.index()
+    }
+}

src/memory/mod.rs

@@ -1,36 +1,128 @@
+mod address_space;
+mod frame;
+mod stack;
+mod user;
+
+use core::ops::Add;
+
+#[allow(unused)]
+pub use address_space::{
+    AddressSpace, AddressSpaceCreateError, AddressSpaceId, MapError, PageTableMapping, UnmapError,
+    init_kernel_address_space, insert_address_space, remove_address_space, with_address_space,
+    with_address_space_mut, with_kernel_address_space,
+};
+pub use frame::{
+    FRAME_SIZE, FrameAddr, FrameAllocator, OwnedFrame, alloc_frame, dealloc_frame,
+    init_global as init_frame_allocator, with_allocator,
+};
+#[allow(unused)]
+pub use stack::{KernelStack, KernelStackPool, StackCreateError, UserStack};
+#[allow(unused)]
+pub use user::*;
+
+pub struct BootString<const N: usize> {
+    bytes: [u8; N],
+    len: usize,
+}
+
+impl<const N: usize> BootString<N> {
+    pub fn from_bytes(bytes: &[u8]) -> Self {
+        let len = bytes.len();
+        let mut boot_string = Self { bytes: [0; N], len };
+        boot_string.bytes[..len].copy_from_slice(bytes);
+        boot_string
+    }
+
+    pub fn as_str(&self) -> &str {
+        core::str::from_utf8(&self.bytes[..self.len]).unwrap()
+    }
+}
+
+pub struct InitramfsImage {
+    pub start: VirtualAddr,
+    pub length: usize,
+}
+
+impl InitramfsImage {
+    pub fn data(&self) -> &[u8] {
+        unsafe { core::slice::from_raw_parts(self.start.as_ptr(), self.length) }
+    }
+}
+
+pub struct KernelSegment {
+    pub physical_base: PhysicalAddr,
+    pub virtual_base: VirtualAddr,
+    pub length: usize,
+    pub permissions: PagePermissions,
+}
+
+pub struct KernelMemoryLayout {
+    pub segments: [KernelSegment; 3],
+}
+
+#[derive(Clone, Copy, PartialEq, Eq)]
+pub struct PagePermissions {
+    pub writable: bool,
+    pub executable: bool,
+    pub user_accessible: bool,
+}
+
+impl PagePermissions {
+    pub const fn new(writable: bool, executable: bool, user_accessible: bool) -> Self {
+        Self {
+            writable,
+            executable,
+            user_accessible,
+        }
+    }
+}
+
 #[repr(transparent)]
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
-pub struct PhysicalAddr(u64);
+pub struct PhysicalAddr(usize);
 
 impl PhysicalAddr {
-    pub fn new(addr: u64) -> Self {
+    pub const fn new(addr: usize) -> Self {
         Self(addr)
     }
 
-    pub const fn as_u64(self) -> u64 {
+    pub const fn as_usize(self) -> usize {
         self.0
     }
 }
 
 #[repr(transparent)]
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
-pub struct VirtualAddr(u64);
+pub struct VirtualAddr(usize);
 
 impl VirtualAddr {
-    pub fn new(addr: u64) -> Self {
+    pub const fn new(addr: usize) -> Self {
         Self(addr)
     }
 
-    pub const fn as_u64(self) -> u64 {
+    pub const fn as_usize(self) -> usize {
-        self.0
+        self.0 as usize
     }
 
-    pub unsafe fn as_mut_ptr<T>(self) -> *mut T {
+    pub const unsafe fn as_mut_ptr<T>(self) -> *mut T {
-        self.as_u64() as *mut T
+        self.as_usize() as *mut T
+    }
+
+    pub const unsafe fn as_ptr<T>(self) -> *const T {
+        self.as_usize() as *const T
     }
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+impl Add<usize> for VirtualAddr {
+    type Output = Self;
+
+    fn add(self, rhs: usize) -> Self::Output {
+        Self(self.0 + rhs)
+    }
+}
+
+#[repr(u8)]
+#[derive(Clone, Copy, PartialEq, Eq)]
 pub enum MemoryRegionKind {
     Usable,
     Reserved,
@@ -43,9 +135,73 @@ pub enum MemoryRegionKind {
     MappedReserved,
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[derive(Clone, Copy)]
+pub enum CachePolicy {
+    Uncacheable,
+    WriteBack,
+}
+
+#[derive(Clone, Copy, PartialEq, Eq)]
 pub struct MemoryRegion {
     pub start: PhysicalAddr,
-    pub length: u64,
+    pub length: usize,
     pub kind: MemoryRegionKind,
 }
+
+#[derive(Clone, Copy)]
+pub struct DirectMap {
+    offset: usize,
+}
+
+impl DirectMap {
+    pub fn new(offset: usize) -> Self {
+        Self { offset }
+    }
+
+    pub fn translate(self, addr: PhysicalAddr) -> Option<VirtualAddr> {
+        addr.as_usize()
+            .checked_add(self.offset)
+            .map(VirtualAddr::new)
+    }
+}
+
+const EMPTY_MEMORY_REGION: MemoryRegion = MemoryRegion {
+    start: PhysicalAddr::new(0),
+    length: 0,
+    kind: MemoryRegionKind::Reserved,
+};
+const MAX_MEMORY_REGIONS: usize = 128;
+
+pub enum MemoryMapError {
+    TooManyRegions,
+}
+
+#[derive(Clone, Copy)]
+pub struct MemoryMap {
+    pub entries: [MemoryRegion; MAX_MEMORY_REGIONS],
+    pub len: usize,
+}
+
+impl MemoryMap {
+    pub const fn new() -> Self {
+        Self {
+            entries: [EMPTY_MEMORY_REGION; MAX_MEMORY_REGIONS],
+            len: 0,
+        }
+    }
+
+    pub fn push(&mut self, entry: MemoryRegion) -> Result<(), MemoryMapError> {
+        if self.len >= MAX_MEMORY_REGIONS {
+            return Err(MemoryMapError::TooManyRegions);
+        }
+
+        self.entries[self.len] = entry;
+        self.len += 1;
+
+        Ok(())
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item = MemoryRegion> + Clone + '_ {
+        self.entries[..self.len].iter().copied()
+    }
+}

src/memory/stack.rs

@@ -0,0 +1,249 @@
+use crate::memory::{
+    AddressSpace, CachePolicy, FRAME_SIZE, FrameAllocator, MapError, OwnedFrame, PagePermissions,
+    VirtualAddr,
+};
+
+const GUARD_PAGES: usize = 1; // 4KiB
+const KERNEL_STACK_PAGES: usize = 8; // 32KiB
+const USER_STACK_PAGES: usize = 16; // 64KiB
+const KERNEL_STACK_SIZE: usize = KERNEL_STACK_PAGES * FRAME_SIZE;
+const USER_STACK_SIZE: usize = USER_STACK_PAGES * FRAME_SIZE;
+
+const KERNEL_SLOT_SIZE: usize = (KERNEL_STACK_PAGES + GUARD_PAGES) * FRAME_SIZE;
+const MAX_KERNEL_STACKS: usize = 64;
+const KERNEL_STACK_BASE: usize = 0xFFFF_FFFE_0000_0000;
+
+const USER_STACK_TOP: VirtualAddr = VirtualAddr::new(0x0000_7FFF_FFFF_F000);
+
+#[derive(Debug)]
+pub enum StackCreateError {
+    AddressOverflow,
+    UnalignedStackTop,
+    OutOfFrames,
+    OutOfStacks,
+    GuardPageMapped,
+    Map(MapError),
+}
+
+#[derive(Debug)]
+struct StackMapping {
+    guard_page: VirtualAddr,
+    mapped_start: VirtualAddr,
+    stack_size: usize,
+    top: VirtualAddr,
+}
+
+impl StackMapping {
+    fn allocate(
+        address_space: &mut AddressSpace,
+        allocator: &mut FrameAllocator,
+        stack_size: usize,
+        top: VirtualAddr,
+        permissions: PagePermissions,
+    ) -> Result<Self, StackCreateError> {
+        if top.as_usize() % FRAME_SIZE != 0 {
+            return Err(StackCreateError::UnalignedStackTop);
+        }
+
+        let mapped_start = VirtualAddr::new(
+            top.as_usize()
+                .checked_sub(stack_size)
+                .ok_or(StackCreateError::AddressOverflow)?,
+        );
+        let guard_page = VirtualAddr::new(
+            mapped_start
+                .as_usize()
+                .checked_sub(FRAME_SIZE)
+                .ok_or(StackCreateError::AddressOverflow)?,
+        );
+
+        if address_space.to_physical(guard_page).is_some() {
+            return Err(StackCreateError::GuardPageMapped);
+        }
+
+        let mut mapped_pages = 0;
+
+        while mapped_pages < stack_size / FRAME_SIZE {
+            let virtual_address = VirtualAddr::new(
+                mapped_start
+                    .as_usize()
+                    .checked_add(mapped_pages * FRAME_SIZE)
+                    .ok_or(StackCreateError::AddressOverflow)?,
+            );
+            let frame = match allocator.alloc() {
+                Some(frame) => frame,
+                None => {
+                    Self::rollback(address_space, allocator, mapped_start, mapped_pages);
+                    return Err(StackCreateError::OutOfFrames);
+                }
+            };
+
+            if let Err(error) = address_space.map(
+                frame.frame_address().start_address(),
+                virtual_address,
+                permissions,
+                allocator,
+                CachePolicy::WriteBack,
+            ) {
+                unsafe { allocator.dealloc(frame) };
+                Self::rollback(address_space, allocator, mapped_start, mapped_pages);
+                return Err(StackCreateError::Map(error));
+            }
+
+            let _ = frame.into_raw();
+            mapped_pages += 1;
+        }
+
+        debug_assert!(address_space.to_physical(guard_page).is_none());
+
+        Ok(Self {
+            guard_page,
+            mapped_start,
+            stack_size,
+            top,
+        })
+    }
+
+    fn rollback(
+        address_space: &mut AddressSpace,
+        allocator: &mut FrameAllocator,
+        mapped_start: VirtualAddr,
+        mapped_pages: usize,
+    ) {
+        for page in (0..mapped_pages).rev() {
+            let virtual_address = VirtualAddr::new(mapped_start.as_usize() + page * FRAME_SIZE);
+            let frame = unsafe {
+                address_space
+                    .unmap(virtual_address, allocator)
+                    .expect("failed to roll back stack mapping")
+            };
+            unsafe { allocator.dealloc(OwnedFrame::from_raw(frame)) };
+        }
+    }
+
+    /// # Safety
+    ///
+    /// The caller must ensure this stack is not active on any CPU and cannot be accessed by any
+    /// kernel operation while it is being destroyed.
+    unsafe fn destroy(self, address_space: &mut AddressSpace, allocator: &mut FrameAllocator) {
+        for page in (0..self.stack_size / FRAME_SIZE).rev() {
+            let virtual_address =
+                VirtualAddr::new(self.mapped_start.as_usize() + page * FRAME_SIZE);
+            let frame = unsafe {
+                address_space
+                    .unmap(virtual_address, allocator)
+                    .expect("stack mapping was unexpectedly missing")
+            };
+            unsafe { allocator.dealloc(OwnedFrame::from_raw(frame)) };
+        }
+
+        debug_assert!(address_space.to_physical(self.guard_page).is_none());
+    }
+}
+
+#[derive(Debug)]
+pub struct KernelStack {
+    mapping: StackMapping,
+}
+
+impl KernelStack {
+    pub fn allocate(
+        address_space: &mut AddressSpace,
+        top: usize,
+        allocator: &mut FrameAllocator,
+    ) -> Result<Self, StackCreateError> {
+        let mapping = StackMapping::allocate(
+            address_space,
+            allocator,
+            KERNEL_STACK_SIZE,
+            VirtualAddr::new(top),
+            PagePermissions::new(true, false, false),
+        )?;
+
+        Ok(Self { mapping })
+    }
+
+    pub const fn top(&self) -> VirtualAddr {
+        self.mapping.top
+    }
+
+    /// # Safety
+    ///
+    /// The caller must ensure this stack is not active on any CPU and cannot be accessed by any
+    /// kernel operation while it is being destroyed.
+    pub unsafe fn destroy(self, address_space: &mut AddressSpace, allocator: &mut FrameAllocator) {
+        unsafe { self.mapping.destroy(address_space, allocator) };
+    }
+}
+
+pub struct UserStack {
+    mapping: StackMapping,
+}
+
+impl UserStack {
+    pub fn allocate(
+        address_space: &mut AddressSpace,
+        allocator: &mut FrameAllocator,
+    ) -> Result<Self, StackCreateError> {
+        let top = VirtualAddr::new(USER_STACK_TOP.as_usize());
+        let mapping = StackMapping::allocate(
+            address_space,
+            allocator,
+            USER_STACK_SIZE,
+            top,
+            PagePermissions::new(true, false, true),
+        )?;
+
+        Ok(Self { mapping })
+    }
+
+    pub const fn top(&self) -> VirtualAddr {
+        self.mapping.top
+    }
+
+    /// # Safety
+    ///
+    /// The caller must ensure this stack is not active in any thread and cannot be accessed while
+    /// it is being destroyed.
+    pub unsafe fn destroy(self, address_space: &mut AddressSpace, allocator: &mut FrameAllocator) {
+        unsafe { self.mapping.destroy(address_space, allocator) };
+    }
+}
+
+pub struct KernelStackPool {
+    free_slots: u64, // bitmap
+}
+
+impl KernelStackPool {
+    pub const fn new() -> Self {
+        Self { free_slots: 0 }
+    }
+
+    pub fn allocate(
+        &mut self,
+        address_space: &mut AddressSpace,
+        allocator: &mut FrameAllocator,
+    ) -> Result<KernelStack, StackCreateError> {
+        let mut slot = 0;
+        while slot < MAX_KERNEL_STACKS {
+            if self.free_slots & (1 << slot) == 0 {
+                self.free_slots |= 1 << slot;
+                let top = KERNEL_STACK_BASE + ((slot + 1) * KERNEL_SLOT_SIZE);
+                return KernelStack::allocate(address_space, top, allocator);
+            }
+            slot += 1;
+        }
+
+        Err(StackCreateError::OutOfStacks)
+    }
+
+    pub fn free(&mut self, stack: KernelStack) {
+        let slot = (stack.top().as_usize() - KERNEL_STACK_BASE) / KERNEL_SLOT_SIZE - 1;
+        crate::memory::with_kernel_address_space(|kernel_as| {
+            crate::memory::with_allocator(|allocator| unsafe {
+                stack.destroy(kernel_as, allocator)
+            });
+        });
+        self.free_slots &= !(1 << slot);
+    }
+}

src/memory/user.rs

@@ -0,0 +1,123 @@
+use crate::{
+    memory::{FRAME_SIZE, VirtualAddr, address_space::AddressSpaceId, with_address_space},
+    syscall::Status,
+};
+
+pub const USER_SPACE_END: VirtualAddr = VirtualAddr::new(0x0000_8000_0000_0000);
+
+pub fn validate_user_range(
+    as_id: AddressSpaceId,
+    start: VirtualAddr,
+    len: usize,
+    writable: bool,
+) -> Result<(), Status> {
+    let start_addr = start.as_usize();
+    let end_addr = start_addr.checked_add(len).ok_or(Status::BadAddress)?;
+
+    if start_addr >= USER_SPACE_END.as_usize() || end_addr > USER_SPACE_END.as_usize() {
+        return Err(Status::BadAddress);
+    }
+
+    if len == 0 {
+        return Ok(());
+    }
+
+    let page_start = start_addr & !(FRAME_SIZE - 1);
+    for page in (page_start..end_addr).step_by(FRAME_SIZE) {
+        let is_valid = with_address_space(as_id, |address_space| {
+            address_space
+                .mapping(VirtualAddr::new(page))
+                .is_some_and(|mapping| {
+                    mapping.permissions.user_accessible
+                        && (!writable || mapping.permissions.writable)
+                })
+        })
+        .ok_or(Status::BadAddress)?;
+
+        if !is_valid {
+            return Err(Status::BadAddress);
+        }
+    }
+
+    Ok(())
+}
+
+/// # Safety
+///
+/// The caller must ensure that the user address range is valid, mapped, and user-accessible (e.g. via [`validate_user_range`]).
+pub unsafe fn copy_from_user(src: VirtualAddr, dst: &mut [u8]) -> Result<(), Status> {
+    let end = src
+        .as_usize()
+        .checked_add(dst.len())
+        .ok_or(Status::BadAddress)?;
+    if end > USER_SPACE_END.as_usize() {
+        return Err(Status::BadAddress);
+    }
+
+    unsafe {
+        core::ptr::copy_nonoverlapping(src.as_ptr(), dst.as_mut_ptr(), dst.len());
+    }
+
+    Ok(())
+}
+
+/// # Safety
+///
+/// The caller must ensure that the user address range is valid, mapped, user-accessible, and writable (e.g. via [`validate_user_range`]).
+pub unsafe fn copy_to_user(dst: VirtualAddr, src: &[u8]) -> Result<(), Status> {
+    let end = dst
+        .as_usize()
+        .checked_add(src.len())
+        .ok_or(Status::BadAddress)?;
+    if end > USER_SPACE_END.as_usize() {
+        return Err(Status::BadAddress);
+    }
+
+    unsafe {
+        core::ptr::copy_nonoverlapping(src.as_ptr(), dst.as_mut_ptr::<u8>(), src.len());
+    }
+
+    Ok(())
+}
+
+/// # Safety
+///
+/// The caller must ensure that the user address is valid, mapped, user-accessible, and writable (e.g. via [`validate_user_range`]).
+pub unsafe fn copy_val_to_user<T: Copy>(dst: VirtualAddr, val: &T) -> Result<(), Status> {
+    if dst.as_usize() % core::mem::align_of::<T>() != 0 {
+        return Err(Status::InvalidArgument);
+    }
+    let end = dst
+        .as_usize()
+        .checked_add(core::mem::size_of::<T>())
+        .ok_or(Status::BadAddress)?;
+    if end > USER_SPACE_END.as_usize() {
+        return Err(Status::BadAddress);
+    }
+
+    unsafe {
+        (dst.as_mut_ptr::<T>()).write(*val);
+    }
+
+    Ok(())
+}
+
+/// # Safety
+///
+/// The caller must ensure that the user address is valid, mapped, and user-accessible (e.g. via [`validate_user_range`]).
+#[allow(unused)]
+pub unsafe fn copy_val_from_user<T: Copy>(src: VirtualAddr) -> Result<T, Status> {
+    if src.as_usize() % core::mem::align_of::<T>() != 0 {
+        return Err(Status::InvalidArgument);
+    }
+    let end = src
+        .as_usize()
+        .checked_add(core::mem::size_of::<T>())
+        .ok_or(Status::BadAddress)?;
+    if end > USER_SPACE_END.as_usize() {
+        return Err(Status::BadAddress);
+    }
+
+    let val = unsafe { src.as_ptr::<T>().read() };
+    Ok(val)
+}

src/platform/acpi.rs

@@ -0,0 +1,652 @@
+use crate::memory::{DirectMap, PhysicalAddr, VirtualAddr};
+
+#[derive(Debug)]
+pub enum AcpiError {
+    InvalidInput,
+    AddressOverflow,
+    InvalidSdtLength,
+    InvalidRootTableLength,
+    MalformedAcpiTable,
+    MalformedMadt,
+    MissingIoApic,
+    MultipleIoApicsUnsupported,
+}
+
+pub struct AcpiTables {
+    direct_map: DirectMap,
+    root: RootTable,
+}
+
+impl AcpiTables {
+    pub fn madt(&self) -> Result<Option<Madt<'_>>, AcpiError> {
+        if let Some(table) = self.find_table(*b"APIC")? {
+            return Ok(Some(Madt::parse(self, table)?));
+        };
+
+        return Ok(None);
+    }
+
+    fn read<T: Copy>(&self, physical_addr: PhysicalAddr) -> Result<T, AcpiError> {
+        let virtual_addr = self
+            .direct_map
+            .translate(physical_addr)
+            .ok_or(AcpiError::AddressOverflow)?;
+
+        Ok(unsafe { core::ptr::read_unaligned(virtual_addr.as_ptr::<T>()) })
+    }
+
+    fn checksum_valid(
+        &self,
+        physical_addr: PhysicalAddr,
+        length: usize,
+    ) -> Result<bool, AcpiError> {
+        let virtual_addr = self
+            .direct_map
+            .translate(physical_addr)
+            .ok_or(AcpiError::AddressOverflow)?;
+
+        let mut sum: u8 = 0;
+        for i in 0..length {
+            let byte_addr = virtual_addr
+                .as_usize()
+                .checked_add(i)
+                .ok_or(AcpiError::AddressOverflow)?;
+
+            let byte = unsafe { core::ptr::read_unaligned(byte_addr as *const u8) };
+            sum = sum.wrapping_add(byte);
+        }
+
+        Ok(sum == 0)
+    }
+
+    pub unsafe fn from_rsdp(
+        rsdp_ptr: VirtualAddr,
+        direct_map: DirectMap,
+    ) -> Result<Self, AcpiError> {
+        let rsdp = unsafe { core::ptr::read_unaligned(rsdp_ptr.as_ptr::<Rsdp>()) };
+
+        let mut sum: u8 = 0;
+        for i in 0..core::mem::size_of::<Rsdp>() as usize {
+            sum = sum.wrapping_add(unsafe {
+                core::ptr::read_unaligned::<u8>(rsdp_ptr.as_ptr::<u8>().add(i))
+            });
+        }
+
+        if sum != 0 {
+            return Err(AcpiError::MalformedAcpiTable);
+        }
+
+        if rsdp.signature != *b"RSD PTR " {
+            return Err(AcpiError::MalformedAcpiTable);
+        }
+
+        let root_table = if rsdp.revision >= 2 {
+            let xsdp = unsafe { core::ptr::read_unaligned(rsdp_ptr.as_ptr::<Xsdp>()) };
+
+            if xsdp.length < core::mem::size_of::<Xsdp>() as u32 {
+                return Err(AcpiError::MalformedAcpiTable);
+            }
+
+            sum = 0;
+            for i in 0..xsdp.length as usize {
+                sum = sum.wrapping_add(unsafe {
+                    core::ptr::read_unaligned::<u8>(rsdp_ptr.as_ptr::<u8>().add(i))
+                });
+            }
+
+            if sum != 0 {
+                return Err(AcpiError::MalformedAcpiTable);
+            }
+
+            let xsdt_address = PhysicalAddr::new(xsdp.xsdt_address as usize);
+            let xsdt_virtual_addr = unsafe {
+                direct_map
+                    .translate(xsdt_address)
+                    .ok_or(AcpiError::AddressOverflow)?
+                    .as_ptr::<SDTHeader>()
+            };
+            let xsdt = unsafe { core::ptr::read_unaligned(xsdt_virtual_addr) };
+
+            sum = 0;
+            for i in 0..xsdt.length as usize {
+                sum = sum.wrapping_add(unsafe {
+                    core::ptr::read_unaligned::<u8>(xsdt_virtual_addr.cast::<u8>().add(i))
+                });
+            }
+
+            if sum != 0 {
+                return Err(AcpiError::MalformedAcpiTable);
+            }
+
+            if &xsdt.signature != b"XSDT" {
+                return Err(AcpiError::MalformedAcpiTable);
+            }
+
+            RootTable::Xsdt(Sdt {
+                physical_addr: xsdt_address,
+                length: xsdt.length as usize,
+                signature: xsdt.signature,
+            })
+        } else {
+            let rsdt_address = PhysicalAddr::new(rsdp.rsdt_address as usize);
+            let rsdt_virtual_addr = unsafe {
+                direct_map
+                    .translate(rsdt_address)
+                    .ok_or(AcpiError::AddressOverflow)?
+                    .as_ptr::<SDTHeader>()
+            };
+
+            let rsdt = unsafe { core::ptr::read_unaligned(rsdt_virtual_addr) };
+
+            sum = 0;
+            for i in 0..rsdt.length as usize {
+                sum = sum.wrapping_add(unsafe {
+                    core::ptr::read_unaligned::<u8>(rsdt_virtual_addr.cast::<u8>().add(i))
+                });
+            }
+
+            if sum != 0 {
+                return Err(AcpiError::MalformedAcpiTable);
+            }
+
+            if &rsdt.signature != b"RSDT" {
+                return Err(AcpiError::MalformedAcpiTable);
+            }
+
+            RootTable::Rsdt(Sdt {
+                physical_addr: rsdt_address,
+                length: rsdt.length as usize,
+                signature: rsdt.signature,
+            })
+        };
+
+        Ok(Self {
+            direct_map: direct_map,
+            root: root_table,
+        })
+    }
+
+    pub fn find_table(&self, signature: [u8; 4]) -> Result<Option<Sdt>, AcpiError> {
+        let root = self.root.table();
+        let entry_width = self.root.entry_width();
+        let entries_start = root
+            .physical_addr
+            .as_usize()
+            .checked_add(size_of::<SDTHeader>())
+            .ok_or(AcpiError::AddressOverflow)?;
+
+        for i in 0..self.root.entry_count()? {
+            let entry_addr = entries_start
+                .checked_add(
+                    i.checked_mul(entry_width)
+                        .ok_or(AcpiError::AddressOverflow)?,
+                )
+                .ok_or(AcpiError::AddressOverflow)?;
+
+            let table_addr = match self.root {
+                RootTable::Rsdt(_) => self.read::<u32>(PhysicalAddr::new(entry_addr))? as usize,
+                RootTable::Xsdt(_) => self.read::<u64>(PhysicalAddr::new(entry_addr))? as usize,
+            };
+
+            let physical_addr = PhysicalAddr::new(table_addr);
+            let header = self.read::<SDTHeader>(physical_addr)?;
+
+            if header.length < size_of::<SDTHeader>() as u32 {
+                return Err(AcpiError::InvalidSdtLength);
+            }
+
+            if header.signature != signature {
+                continue;
+            }
+
+            if !self.checksum_valid(physical_addr, header.length as usize)? {
+                return Err(AcpiError::MalformedAcpiTable);
+            }
+
+            return Ok(Some(Sdt {
+                physical_addr,
+                length: header.length as usize,
+                signature: header.signature,
+            }));
+        }
+
+        Ok(None)
+    }
+}
+
+enum RootTable {
+    Rsdt(Sdt),
+    Xsdt(Sdt),
+}
+
+impl RootTable {
+    fn table(&self) -> &Sdt {
+        match self {
+            RootTable::Rsdt(sdt) | RootTable::Xsdt(sdt) => sdt,
+        }
+    }
+
+    const fn entry_width(&self) -> usize {
+        match self {
+            RootTable::Rsdt(_) => core::mem::size_of::<u32>(),
+            RootTable::Xsdt(_) => core::mem::size_of::<u64>(),
+        }
+    }
+
+    fn entry_count(&self) -> Result<usize, AcpiError> {
+        let payload_length = self
+            .table()
+            .length
+            .checked_sub(size_of::<SDTHeader>())
+            .ok_or(AcpiError::InvalidSdtLength)?;
+
+        if payload_length % self.entry_width() != 0 {
+            return Err(AcpiError::InvalidRootTableLength);
+        }
+
+        Ok(payload_length / self.entry_width())
+    }
+}
+
+#[repr(C, packed)]
+#[derive(Clone, Copy)]
+struct Rsdp {
+    signature: [u8; 8],
+    checksum: u8,
+    oem_id: [u8; 6],
+    revision: u8,
+    rsdt_address: u32,
+}
+
+#[repr(C, packed)]
+#[derive(Clone, Copy)]
+struct Xsdp {
+    rsdp: Rsdp,
+    length: u32,
+    xsdt_address: u64,
+    extended_checksum: u8,
+    reserved: [u8; 3],
+}
+
+#[repr(C, packed)]
+#[derive(Clone, Copy)]
+struct SDTHeader {
+    signature: [u8; 4],
+    length: u32,
+    revision: u8,
+    checksum: u8,
+    oem_id: [u8; 6],
+    oem_table_id: [u8; 8],
+    oem_revision: u32,
+    creator_id: u32,
+    creator_revision: u32,
+}
+
+pub struct Sdt {
+    physical_addr: PhysicalAddr,
+    length: usize,
+    signature: [u8; 4],
+}
+
+#[allow(unused)]
+pub struct Madt<'a> {
+    acpi: &'a AcpiTables,
+    table: Sdt,
+    pub local_apic_address: PhysicalAddr,
+    flags: u32,
+}
+
+#[repr(C, packed)]
+#[derive(Clone, Copy)]
+struct MadtBody {
+    local_apic_address: u32,
+    flags: u32,
+}
+
+const MADT_ENTRIES_OFFSET: usize = size_of::<SDTHeader>() + size_of::<MadtBody>();
+
+impl<'a> Madt<'a> {
+    pub fn entries(
+        &self,
+    ) -> Result<impl Iterator<Item = Result<MadtEntry, AcpiError>> + '_, AcpiError> {
+        Ok(MadtEntries {
+            acpi: self.acpi,
+            current: PhysicalAddr::new(
+                self.table
+                    .physical_addr
+                    .as_usize()
+                    .checked_add(MADT_ENTRIES_OFFSET)
+                    .ok_or(AcpiError::AddressOverflow)?,
+            ),
+            end: PhysicalAddr::new(
+                self.table
+                    .physical_addr
+                    .as_usize()
+                    .checked_add(self.table.length)
+                    .ok_or(AcpiError::AddressOverflow)?,
+            ),
+        })
+    }
+
+    pub fn parse(acpi: &'a AcpiTables, table: Sdt) -> Result<Self, AcpiError> {
+        if table.signature != *b"APIC" {
+            return Err(AcpiError::InvalidInput);
+        }
+
+        if table.length < size_of::<SDTHeader>() + size_of::<MadtBody>() {
+            return Err(AcpiError::MalformedAcpiTable);
+        }
+
+        let body_addr = table
+            .physical_addr
+            .as_usize()
+            .checked_add(size_of::<SDTHeader>())
+            .ok_or(AcpiError::AddressOverflow)?;
+        let body = acpi.read::<MadtBody>(PhysicalAddr::new(body_addr))?;
+
+        Ok(Madt {
+            acpi,
+            table,
+            local_apic_address: PhysicalAddr::new(body.local_apic_address as usize),
+            flags: body.flags,
+        })
+    }
+
+    pub fn effective_local_apic_address(&self) -> Result<PhysicalAddr, AcpiError> {
+        let mut address = self.local_apic_address;
+
+        for result in self.entries()? {
+            let entry = result?;
+
+            match entry {
+                MadtEntry::LocalApicAddressOverride(local_apic_address_override) => {
+                    address =
+                        PhysicalAddr::new(local_apic_address_override.local_apic_address as usize);
+                }
+                _ => {}
+            }
+        }
+
+        Ok(address)
+    }
+
+    pub fn io_apics(
+        &self,
+    ) -> Result<impl Iterator<Item = Result<IoApicInfo, AcpiError>> + '_, AcpiError> {
+        Ok(self.entries()?.filter_map(|result| match result {
+            Ok(MadtEntry::IoApic(io_apic)) => Some(Ok(IoApicInfo {
+                id: io_apic.id,
+                apic_address: PhysicalAddr::new(io_apic.apic_address as usize),
+                global_system_interrupt_base: io_apic.global_system_interrupt_base,
+            })),
+            Ok(_) => None,
+            Err(error) => Some(Err(error)),
+        }))
+    }
+
+    pub fn sole_io_apic(&self) -> Result<IoApicInfo, AcpiError> {
+        let mut entries = self.io_apics()?;
+
+        let first = entries
+            .next()
+            .transpose()?
+            .ok_or(AcpiError::MissingIoApic)?;
+
+        if entries.next().transpose()?.is_some() {
+            return Err(AcpiError::MultipleIoApicsUnsupported);
+        }
+
+        Ok(first)
+    }
+
+    pub fn isa_irq_route(&self, irq: u8) -> Result<IsaIrqRoute, AcpiError> {
+        for entry in self.entries()? {
+            match entry? {
+                MadtEntry::InterruptSourceOverride(interrupt_source_override) => {
+                    if interrupt_source_override.bus != 0 {
+                        continue;
+                    }
+
+                    if interrupt_source_override.source != irq {
+                        continue;
+                    }
+
+                    return Ok(IsaIrqRoute {
+                        gsi: interrupt_source_override.global_interrupt,
+                        polarity: match interrupt_source_override.flags & 0b11 {
+                            0 => InterruptPolarity::ActiveHigh,
+                            1 => InterruptPolarity::ActiveHigh,
+                            3 => InterruptPolarity::ActiveLow,
+                            _ => Err(AcpiError::MalformedMadt)?,
+                        },
+                        trigger: match interrupt_source_override.flags >> 2 & 0b11 {
+                            0 => TriggerMode::Edge,
+                            1 => TriggerMode::Edge,
+                            3 => TriggerMode::Level,
+                            _ => Err(AcpiError::MalformedMadt)?,
+                        },
+                    });
+                }
+                _ => {}
+            }
+        }
+
+        Ok(IsaIrqRoute {
+            gsi: irq as u32,
+            polarity: InterruptPolarity::ActiveHigh,
+            trigger: TriggerMode::Edge,
+        })
+    }
+}
+
+pub struct MadtEntries<'a> {
+    acpi: &'a AcpiTables,
+    current: PhysicalAddr,
+    end: PhysicalAddr,
+}
+
+impl MadtEntries<'_> {
+    fn read_body<T: Copy>(&self, header: &MadtEntryHeader) -> Result<T, AcpiError> {
+        let required_length = size_of::<MadtEntryHeader>() + size_of::<T>();
+
+        if (header.length as usize) < required_length {
+            return Err(AcpiError::MalformedAcpiTable);
+        }
+
+        let body_addr = self
+            .current
+            .as_usize()
+            .checked_add(size_of::<MadtEntryHeader>())
+            .ok_or(AcpiError::AddressOverflow)?;
+
+        self.acpi.read::<T>(PhysicalAddr::new(body_addr))
+    }
+
+    fn read_next(&mut self) -> Result<MadtEntry, AcpiError> {
+        let header = self.acpi.read::<MadtEntryHeader>(self.current)?;
+
+        let entry = match header.kind {
+            0 => MadtEntry::LocalApic(self.read_body(&header)?),
+            1 => MadtEntry::IoApic(self.read_body(&header)?),
+            2 => MadtEntry::InterruptSourceOverride(self.read_body(&header)?),
+            3 => MadtEntry::IoApicNmi(self.read_body(&header)?),
+            4 => MadtEntry::LocalApicNmi(self.read_body(&header)?),
+            5 => MadtEntry::LocalApicAddressOverride(self.read_body(&header)?),
+            9 => MadtEntry::LocalX2Apic(self.read_body(&header)?),
+            _ => MadtEntry::Unknown {
+                kind: header.kind,
+                length: header.length,
+            },
+        };
+
+        self.current = PhysicalAddr::new(
+            self.current
+                .as_usize()
+                .checked_add(header.length as usize)
+                .ok_or(AcpiError::AddressOverflow)?,
+        );
+
+        Ok(entry)
+    }
+}
+
+impl<'a> Iterator for MadtEntries<'a> {
+    type Item = Result<MadtEntry, AcpiError>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let current = self.current.as_usize();
+        let end = self.end.as_usize();
+
+        if current == end {
+            return None;
+        }
+
+        if current > end {
+            self.current = self.end;
+            return Some(Err(AcpiError::MalformedAcpiTable));
+        }
+
+        let remaining = end - current;
+
+        if remaining < size_of::<MadtEntryHeader>() {
+            self.current = self.end;
+            return Some(Err(AcpiError::MalformedAcpiTable));
+        }
+
+        let header = match self.acpi.read::<MadtEntryHeader>(self.current) {
+            Ok(header) => header,
+            Err(error) => {
+                self.current = self.end;
+                return Some(Err(error));
+            }
+        };
+
+        let length = header.length as usize;
+
+        if length < size_of::<MadtEntryHeader>() || length > remaining {
+            self.current = self.end;
+            return Some(Err(AcpiError::MalformedAcpiTable));
+        }
+
+        let res = self.read_next();
+
+        if res.is_err() {
+            self.current = self.end;
+        }
+
+        Some(res)
+    }
+}
+
+#[repr(C, packed)]
+#[derive(Clone, Copy)]
+pub struct MadtEntryHeader {
+    kind: u8,
+    length: u8,
+}
+
+#[repr(C, packed)]
+#[derive(Clone, Copy)]
+pub struct LocalApicEntry {
+    processor_id: u8,
+    id: u8,
+    flags: u32,
+}
+
+pub struct IoApicInfo {
+    pub id: u8,
+    pub apic_address: PhysicalAddr,
+    pub global_system_interrupt_base: u32,
+}
+
+#[repr(C, packed)]
+#[derive(Clone, Copy)]
+pub struct IoApicEntry {
+    id: u8,
+    reserved: u8,
+    apic_address: u32,
+    global_system_interrupt_base: u32,
+}
+
+#[repr(C, packed)]
+#[derive(Clone, Copy)]
+pub struct InterruptSourceOverride {
+    bus: u8,
+    source: u8,
+    global_interrupt: u32,
+    flags: u16,
+}
+
+#[derive(Clone, Copy)]
+pub enum InterruptPolarity {
+    ActiveHigh,
+    ActiveLow,
+}
+
+#[derive(Clone, Copy)]
+pub enum TriggerMode {
+    Edge,
+    Level,
+}
+
+#[derive(Clone, Copy)]
+pub struct IsaIrqRoute {
+    pub gsi: u32,
+    pub polarity: InterruptPolarity,
+    pub trigger: TriggerMode,
+}
+
+#[repr(C, packed)]
+#[derive(Clone, Copy)]
+pub struct IoApicNmiEntry {
+    nmi_source: u8,
+    reserved: u8,
+    flags: u16,
+    global_system_interrupt: u32,
+}
+
+#[repr(C, packed)]
+#[derive(Clone, Copy)]
+pub struct LocalApicNmiEntry {
+    processor_id: u8,
+    flags: u16,
+    lint_num: u8,
+}
+
+#[repr(C, packed)]
+#[derive(Clone, Copy)]
+pub struct LocalApicAddressOverride {
+    reserved: u16,
+    local_apic_address: u64,
+}
+
+#[repr(C, packed)]
+#[derive(Clone, Copy)]
+pub struct LocalX2ApicEntry {
+    reserved: u16,
+    local_x2apic_id: u32,
+    flags: u32,
+    acpi_processor_uid: u32,
+}
+
+#[derive(Clone, Copy)]
+#[allow(unused)]
+pub enum MadtEntry {
+    LocalApic(LocalApicEntry),
+    IoApic(IoApicEntry),
+    InterruptSourceOverride(InterruptSourceOverride),
+    IoApicNmi(IoApicNmiEntry),
+    LocalApicNmi(LocalApicNmiEntry),
+    LocalApicAddressOverride(LocalApicAddressOverride),
+    LocalX2Apic(LocalX2ApicEntry),
+    Unknown { kind: u8, length: u8 },
+}
+
+pub fn init(
+    boot_info: &crate::boot::BootInfo,
+    direct_map: DirectMap,
+) -> Result<AcpiTables, AcpiError> {
+    let acpi_table = unsafe { AcpiTables::from_rsdp(boot_info.rsdp, direct_map)? };
+
+    Ok(acpi_table)
+}

src/platform/mod.rs

@@ -0,0 +1 @@
+pub mod acpi;

src/syscall/mod.rs

@@ -0,0 +1,92 @@
+mod table;
+
+use table::*;
+
+use crate::task::tcb::{ExitReason, Fault};
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[repr(u64)]
+pub enum Status {
+    // Status::Success = 0
+    InvalidArgument = 1,   // EINVAL
+    BadAddress = 2,        // EFAULT
+    BadFileDescriptor = 3, // EBADF
+    NoSuchTask = 4,        // ESRCH
+    OutOfMemory = 5,       // ENOMEM
+    BadHandle = 6,         // EBADH
+}
+
+#[derive(Clone, Copy, PartialEq, Eq)]
+#[repr(u64)]
+pub enum SyscallNumber {
+    Yield = 1,
+    Exit = 2,
+    Write = 3,
+    Send = 4,
+    Recv = 5,
+    FrameAlloc = 6,
+    FrameDealloc = 7,
+    AsCreate = 8,
+    Map = 9,
+    Unmap = 10,
+    TaskCreate = 11,
+}
+
+impl TryFrom<u64> for SyscallNumber {
+    type Error = ();
+    fn try_from(val: u64) -> Result<Self, Self::Error> {
+        match val {
+            1 => Ok(Self::Yield),
+            2 => Ok(Self::Exit),
+            3 => Ok(Self::Write),
+            4 => Ok(Self::Send),
+            5 => Ok(Self::Recv),
+            6 => Ok(Self::FrameAlloc),
+            7 => Ok(Self::FrameDealloc),
+            8 => Ok(Self::AsCreate),
+            9 => Ok(Self::Map),
+            10 => Ok(Self::Unmap),
+            11 => Ok(Self::TaskCreate),
+            _ => Err(()),
+        }
+    }
+}
+
+pub fn handle(num: u64, arg0: u64, arg1: u64, arg2: u64, arg3: u64, arg4: u64, _arg5: u64) -> u64 {
+    let result = (|| -> Result<(), Status> {
+        let syscall = SyscallNumber::try_from(num).unwrap_or_else(|_| {
+            crate::task::scheduler::exit_current(ExitReason::Fault(Fault::BadSystemCall))
+        });
+
+        match syscall {
+            SyscallNumber::Yield => sys_yield(),
+            SyscallNumber::Exit => sys_exit(arg0 as usize),
+            SyscallNumber::Write => {
+                sys_write(arg0 as usize, arg1 as usize, arg2 as usize, arg3 as usize)
+            }
+            SyscallNumber::Send => sys_send(arg0 as usize, arg1 as usize, arg2 as usize),
+            SyscallNumber::Recv => {
+                sys_recv(arg0 as usize, arg1 as usize, arg2 as usize, arg3 as usize)
+            }
+            SyscallNumber::FrameAlloc => sys_frame_alloc(arg0 as usize),
+            SyscallNumber::FrameDealloc => sys_frame_dealloc(arg0 as usize),
+            SyscallNumber::AsCreate => sys_as_create(arg0 as usize),
+            SyscallNumber::Map => sys_map(
+                arg0 as usize,
+                arg1 as usize,
+                arg2 as usize,
+                arg3 as usize,
+                arg4 as usize,
+            ),
+            SyscallNumber::Unmap => sys_unmap(arg0 as usize),
+            SyscallNumber::TaskCreate => {
+                sys_task_create(arg0 as usize, arg1 as usize, arg2 as usize, arg3 as usize)
+            }
+        }
+    })();
+
+    match result {
+        Ok(()) => 0,
+        Err(err) => err as u64,
+    }
+}

src/syscall/table.rs

@@ -0,0 +1,647 @@
+use crate::{
+    memory::{
+        FRAME_SIZE, MapError, PagePermissions, USER_SPACE_END, VirtualAddr, copy_from_user,
+        copy_to_user, copy_val_to_user, validate_user_range,
+    },
+    println,
+    task::{
+        scheduler::TaskId,
+        tcb::{BlockReason, ExitReason, Handle, KernelObject, MAX_MSG_SIZE, Message, Rights},
+    },
+};
+
+use super::Status;
+
+pub fn sys_yield() -> Result<(), Status> {
+    crate::task::scheduler::yield_current();
+    Ok(())
+}
+
+pub fn sys_exit(exit_code: usize) -> ! {
+    crate::task::scheduler::exit_current(ExitReason::Exited(exit_code));
+}
+
+pub fn sys_write(fd: usize, buf_ptr: usize, len: usize, out_ptr: usize) -> Result<(), Status> {
+    if fd != 1 && fd != 2 {
+        return Err(Status::BadFileDescriptor);
+    }
+
+    crate::task::scheduler::with_task(crate::task::scheduler::current(), |current_task| {
+        validate_user_range(current_task.as_id, VirtualAddr::new(buf_ptr), len, false)?;
+
+        if out_ptr != 0 {
+            if out_ptr % core::mem::align_of::<usize>() != 0 {
+                return Err(Status::InvalidArgument);
+            }
+            validate_user_range(
+                current_task.as_id,
+                VirtualAddr::new(out_ptr),
+                core::mem::size_of::<usize>(),
+                true,
+            )?;
+        }
+
+        Ok(())
+    })
+    .expect("failed to resolve self task")?;
+
+    let mut chunk = [0u8; 128];
+    let mut written = 0;
+    while written < len {
+        let n = (len - written).min(chunk.len());
+        unsafe {
+            copy_from_user(VirtualAddr::new(buf_ptr + written), &mut chunk[..n])?;
+        }
+        crate::debug::serial::write_bytes(&chunk[..n]);
+        written += n;
+    }
+
+    if out_ptr != 0 {
+        unsafe {
+            copy_val_to_user(VirtualAddr::new(out_ptr), &written)?;
+        }
+    }
+
+    Ok(())
+}
+
+pub fn sys_send(dest_task_id: usize, msg_ptr: usize, len: usize) -> Result<(), Status> {
+    let dest_task_id = TaskId::new(dest_task_id);
+
+    if len > MAX_MSG_SIZE {
+        return Err(Status::InvalidArgument);
+    }
+
+    let sender = crate::task::scheduler::current();
+    crate::task::scheduler::with_task(sender, |current_task| {
+        validate_user_range(current_task.as_id, VirtualAddr::new(msg_ptr), len, false)
+    })
+    .expect("failed to resolve self task")?;
+
+    let mut msg_buf = [0u8; MAX_MSG_SIZE];
+    unsafe { copy_from_user(VirtualAddr::new(msg_ptr), &mut msg_buf[..len])? };
+
+    let msg = Message {
+        sender,
+        length: len,
+        data: msg_buf,
+    };
+
+    let should_unblock = crate::task::scheduler::with_task_mut(dest_task_id, |dest_task| {
+        if !dest_task.mailbox.push(msg) {
+            return Err(Status::OutOfMemory);
+        }
+        Ok(matches!(
+            dest_task.state,
+            crate::task::tcb::ThreadState::Blocked(BlockReason::Recv)
+        ))
+    })
+    .ok_or(Status::NoSuchTask)??;
+
+    if should_unblock {
+        crate::task::scheduler::unblock(dest_task_id);
+    }
+
+    Ok(())
+}
+
+pub fn sys_recv(
+    out_ptr: usize,
+    max_len: usize,
+    out_actual_len: usize,
+    out_sender: usize,
+) -> Result<(), Status> {
+    if out_ptr == 0 {
+        return Err(Status::InvalidArgument);
+    }
+
+    crate::task::scheduler::with_task(crate::task::scheduler::current(), |current_task| {
+        if current_task.mailbox.len == 0 {
+            crate::task::scheduler::block_current(BlockReason::Recv);
+        }
+
+        validate_user_range(current_task.as_id, VirtualAddr::new(out_ptr), max_len, true)?;
+
+        if out_actual_len != 0 {
+            if out_actual_len % core::mem::align_of::<usize>() != 0 {
+                return Err(Status::InvalidArgument);
+            }
+            validate_user_range(
+                current_task.as_id,
+                VirtualAddr::new(out_actual_len),
+                core::mem::size_of::<usize>(),
+                true,
+            )?;
+        }
+
+        if out_sender != 0 {
+            if out_sender % core::mem::align_of::<usize>() != 0 {
+                return Err(Status::InvalidArgument);
+            }
+            validate_user_range(
+                current_task.as_id,
+                VirtualAddr::new(out_sender),
+                core::mem::size_of::<usize>(),
+                true,
+            )?;
+        }
+
+        Ok(())
+    })
+    .expect("failed to resolve self task")?;
+
+    let msg =
+        crate::task::scheduler::with_task_mut(crate::task::scheduler::current(), |current_task| {
+            current_task.mailbox.pop().ok_or(Status::NoSuchTask)
+        })
+        .expect("failed to resolve self task")?;
+
+    unsafe {
+        copy_to_user(
+            VirtualAddr::new(out_ptr),
+            &msg.data[..msg.length.min(max_len)],
+        )?;
+        if out_actual_len != 0 {
+            copy_val_to_user(VirtualAddr::new(out_actual_len), &msg.length)?;
+        }
+        if out_sender != 0 {
+            copy_val_to_user(VirtualAddr::new(out_sender), &msg.sender)?;
+        }
+    }
+
+    Ok(())
+}
+
+pub fn sys_frame_alloc(out_handle: usize) -> Result<(), Status> {
+    if out_handle == 0 {
+        return Err(Status::InvalidArgument);
+    }
+
+    if out_handle % core::mem::align_of::<usize>() != 0 {
+        return Err(Status::InvalidArgument);
+    }
+
+    crate::task::scheduler::with_task(crate::task::scheduler::current(), |current_task| {
+        validate_user_range(
+            current_task.as_id,
+            VirtualAddr::new(out_handle),
+            core::mem::size_of::<usize>(),
+            true,
+        )
+    })
+    .expect("failed to resolve self task")?;
+
+    let frame = crate::memory::alloc_frame().ok_or(Status::OutOfMemory)?;
+
+    let handle = Handle {
+        object: KernelObject::Frame(frame),
+        rights: Rights::READ | Rights::WRITE | Rights::EXECUTE | Rights::MAP,
+    };
+
+    crate::task::scheduler::with_task_mut(crate::task::scheduler::current(), |current_task| {
+        let handle_id = match current_task.handles.push(handle) {
+            Ok(id) => id,
+            Err(handle) => {
+                let frame = match handle.object {
+                    KernelObject::Frame(frame) => frame,
+                    _ => unreachable!("pushed handle was not a frame"),
+                };
+                unsafe { crate::memory::dealloc_frame(frame) };
+                return Err(Status::OutOfMemory);
+            }
+        };
+
+        unsafe { copy_val_to_user(VirtualAddr::new(out_handle), &handle_id) }
+    })
+    .expect("failed to resolve self task")
+}
+
+pub fn sys_frame_dealloc(frame_handle_id: usize) -> Result<(), Status> {
+    crate::task::scheduler::with_task_mut(crate::task::scheduler::current(), |task| {
+        let frame_handle = task
+            .handles
+            .take(frame_handle_id)
+            .ok_or(Status::BadHandle)?;
+        match frame_handle.object {
+            KernelObject::Frame(frame_addr) => {
+                unsafe { crate::memory::dealloc_frame(frame_addr) };
+
+                Ok(())
+            }
+            _ => {
+                // Wrong-type operations must not consume the handle
+                match task.handles.put(frame_handle_id, frame_handle) {
+                    Ok(_) => {}
+                    Err(_) => panic!("taken handle was unexpectedly occupied"),
+                }
+
+                return Err(Status::InvalidArgument);
+            }
+        }
+    })
+    .expect("failed to resolve self task")
+}
+
+pub fn sys_as_create(out_handle: usize) -> Result<(), Status> {
+    if out_handle == 0 {
+        return Err(Status::InvalidArgument);
+    }
+
+    if out_handle % core::mem::align_of::<usize>() != 0 {
+        return Err(Status::InvalidArgument);
+    }
+
+    let new_as =
+        crate::task::scheduler::with_task(crate::task::scheduler::current(), |current_task| {
+            validate_user_range(
+                current_task.as_id,
+                VirtualAddr::new(out_handle),
+                core::mem::size_of::<usize>(),
+                true,
+            )?;
+
+            let new_as = match crate::memory::with_address_space(current_task.as_id, |caller_as| {
+                crate::memory::with_allocator(|allocator| caller_as.new_user(allocator))
+            }) {
+                Some(Ok(as_space)) => as_space,
+                _ => return Err(Status::OutOfMemory),
+            };
+
+            Ok(new_as)
+        })
+        .expect("failed to resolve self task")?;
+
+    let as_id = match crate::memory::insert_address_space(new_as) {
+        Ok(id) => id,
+        Err(addr_space) => {
+            crate::memory::with_allocator(|allocator| unsafe { addr_space.destroy(allocator) });
+            return Err(Status::OutOfMemory);
+        }
+    };
+
+    let handle = Handle {
+        object: KernelObject::AddressSpace(as_id),
+        rights: Rights::READ | Rights::WRITE | Rights::EXECUTE,
+    };
+
+    crate::task::scheduler::with_task_mut(crate::task::scheduler::current(), |current_task| {
+        let handle_id = match current_task.handles.push(handle) {
+            Ok(id) => id,
+            Err(handle) => {
+                let address_space = match handle.object {
+                    KernelObject::AddressSpace(as_id) => crate::memory::remove_address_space(as_id)
+                        .expect("address space was just inserted"),
+                    _ => unreachable!("pushed handle was not an address space"),
+                };
+                crate::memory::with_allocator(|allocator| unsafe {
+                    address_space.destroy(allocator)
+                });
+                return Err(Status::OutOfMemory);
+            }
+        };
+
+        unsafe { copy_val_to_user(VirtualAddr::new(out_handle), &handle_id) }
+    })
+    .expect("failed to resolve self task")
+}
+
+pub fn sys_map(
+    as_handle: usize,
+    frame_handle: usize,
+    virtual_addr: usize,
+    permissions: usize,
+    out_handle: usize,
+) -> Result<(), Status> {
+    if out_handle == 0 || out_handle % core::mem::align_of::<usize>() != 0 {
+        return Err(Status::InvalidArgument);
+    }
+
+    if virtual_addr % FRAME_SIZE != 0 || permissions & !0b11 != 0 {
+        return Err(Status::InvalidArgument);
+    }
+
+    let writable = permissions & (1 << 0) != 0;
+    let executable = permissions & (1 << 1) != 0;
+
+    let end = virtual_addr
+        .checked_add(FRAME_SIZE)
+        .ok_or(Status::InvalidArgument)?;
+    if end > USER_SPACE_END.as_usize() {
+        return Err(Status::InvalidArgument);
+    }
+
+    let current_task = crate::task::scheduler::current();
+    let as_id = crate::task::scheduler::with_task(current_task, |task| {
+        validate_user_range(
+            task.as_id,
+            VirtualAddr::new(out_handle),
+            core::mem::size_of::<usize>(),
+            true,
+        )?;
+
+        let as_handle = task.handles.get(as_handle).ok_or(Status::BadHandle)?;
+        let as_id = match as_handle.object {
+            KernelObject::AddressSpace(as_id) => as_id,
+            _ => return Err(Status::InvalidArgument),
+        };
+        if as_handle.rights.0 & Rights::WRITE.0 == 0 {
+            return Err(Status::InvalidArgument);
+        }
+
+        let frame_handle = task.handles.get(frame_handle).ok_or(Status::BadHandle)?;
+        if !matches!(frame_handle.object, KernelObject::Frame(_)) {
+            return Err(Status::InvalidArgument);
+        }
+
+        let mut required_rights = Rights::READ | Rights::MAP;
+        if writable {
+            required_rights = required_rights | Rights::WRITE;
+        }
+        if executable {
+            required_rights = required_rights | Rights::EXECUTE;
+        }
+        if frame_handle.rights.0 & required_rights.0 != required_rights.0 {
+            return Err(Status::InvalidArgument);
+        }
+
+        Ok(as_id)
+    })
+    .expect("failed to resolve self task")?;
+
+    let handle = crate::task::scheduler::with_task_mut(current_task, |task| {
+        task.handles.take(frame_handle).ok_or(Status::BadHandle)
+    })
+    .expect("failed to resolve self task")?;
+
+    let Handle { object, rights } = handle;
+    let KernelObject::Frame(frame) = object else {
+        panic!("validated frame handle changed before it was taken");
+    };
+
+    let permissions = PagePermissions::new(writable, executable, true);
+    let virtual_addr = VirtualAddr::new(virtual_addr);
+
+    let map_result = crate::memory::with_address_space_mut(as_id, |target_as| {
+        crate::memory::with_allocator(|allocator| {
+            target_as.map(
+                frame.frame_address().start_address(),
+                virtual_addr,
+                permissions,
+                allocator,
+                crate::memory::CachePolicy::WriteBack,
+            )
+        })
+    })
+    .expect("failed to resolve self address space");
+
+    match map_result {
+        Ok(_) => {
+            crate::task::scheduler::with_task_mut(crate::task::scheduler::current(), |task| {
+                match task.handles.put(
+                    frame_handle,
+                    Handle {
+                        object: KernelObject::Mapping {
+                            frame,
+                            address_space: as_id,
+                            virtual_addr,
+                        },
+                        rights,
+                    },
+                ) {
+                    Ok(_) => {}
+                    Err(_) => panic!("taken handle was unexpectedly occupied"),
+                }
+            });
+
+            unsafe {
+                copy_val_to_user(VirtualAddr::new(out_handle), &frame_handle)
+                    .expect("out_handle has already been checked")
+            }
+
+            Ok(())
+        }
+        Err(err) => {
+            crate::task::scheduler::with_task_mut(current_task, |task| {
+                match task.handles.put(
+                    frame_handle,
+                    Handle {
+                        object: KernelObject::Frame(frame),
+                        rights,
+                    },
+                ) {
+                    Ok(_) => {}
+                    Err(_) => panic!("taken handle was unexpectedly occupied"),
+                }
+            })
+            .expect("failed to resolve self task");
+
+            match err {
+                MapError::AlreadyMapped | MapError::UnsupportedPermissions => {
+                    Err(Status::InvalidArgument)
+                }
+                MapError::OutOfMemory => Err(Status::OutOfMemory),
+                MapError::InvalidVirtualAddress
+                | MapError::VirtualAddressUnaligned
+                | MapError::PhysicalAddressTooLarge
+                | MapError::PhysicalAddressUnaligned
+                | MapError::RangeLengthUnaligned
+                | MapError::AddressOverflow
+                | MapError::MappingConflict
+                | MapError::PageTableUnavailable
+                | MapError::CorruptedPageTable
+                | MapError::InvalidUserAddress
+                | MapError::InvalidUserMap => {
+                    panic!("validated user mapping failed with an impossible error: {err:?}")
+                }
+            }
+        }
+    }
+}
+
+pub fn sys_unmap(mapping_handle: usize) -> Result<(), Status> {
+    let handle = crate::task::scheduler::with_task_mut(crate::task::scheduler::current(), |task| {
+        task.handles.take(mapping_handle).ok_or(Status::BadHandle)
+    })
+    .expect("failed to resolve self task")?;
+
+    let Handle { object, rights } = handle;
+
+    let KernelObject::Mapping {
+        frame,
+        address_space,
+        virtual_addr,
+    } = object
+    else {
+        // Wrong-type operations must not consume the handle
+        crate::task::scheduler::with_task_mut(crate::task::scheduler::current(), |task| match task
+            .handles
+            .put(mapping_handle, Handle { object, rights })
+        {
+            Ok(_) => {}
+            Err(_) => panic!("taken handle was unexpectedly occupied"),
+        })
+        .expect("failed to resolve self task");
+
+        return Err(Status::InvalidArgument);
+    };
+
+    let unmap_result = crate::memory::with_address_space_mut(address_space, |target_as| {
+        crate::memory::with_allocator(|allocator| unsafe {
+            target_as.unmap(virtual_addr, allocator)
+        })
+    });
+
+    match unmap_result {
+        Some(Ok(unmapped_frame)) => {
+            if unmapped_frame == frame.frame_address() {
+                crate::task::scheduler::with_task_mut(crate::task::scheduler::current(), |task| {
+                    match task.handles.put(
+                        mapping_handle,
+                        Handle {
+                            object: KernelObject::Frame(frame),
+                            rights,
+                        },
+                    ) {
+                        Ok(_) => {}
+                        Err(_) => panic!("taken handle was unexpectedly occupied"),
+                    }
+                });
+
+                Ok(())
+            } else {
+                panic!("unmap resulted in a frame that was not the one we expected")
+            }
+        }
+        Some(Err(err)) => {
+            // every unmapping error should be impossible to occur
+            panic!("failed to unmap: {err:?}");
+        }
+        None => {
+            crate::task::scheduler::with_task_mut(crate::task::scheduler::current(), |task| {
+                match task.handles.put(
+                    mapping_handle,
+                    Handle {
+                        object: KernelObject::Mapping {
+                            frame: frame,
+                            address_space,
+                            virtual_addr,
+                        },
+                        rights,
+                    },
+                ) {
+                    Ok(_) => {}
+                    Err(_) => panic!("taken handle was unexpectedly occupied"),
+                }
+            });
+
+            Err(Status::BadHandle)
+        }
+    }
+}
+
+pub fn sys_task_create(
+    as_handle: usize,
+    entry: usize,
+    user_stack: usize,
+    out_task_handle: usize,
+) -> Result<(), Status> {
+    if entry == 0 || user_stack == 0 || out_task_handle == 0 {
+        return Err(Status::InvalidArgument);
+    }
+
+    if entry >= USER_SPACE_END.as_usize() || user_stack > USER_SPACE_END.as_usize() {
+        return Err(Status::BadAddress);
+    }
+
+    if out_task_handle % core::mem::align_of::<usize>() != 0 {
+        return Err(Status::InvalidArgument);
+    }
+
+    let as_id =
+        crate::task::scheduler::with_task(crate::task::scheduler::current(), |current_task| {
+            validate_user_range(
+                current_task.as_id,
+                VirtualAddr::new(out_task_handle),
+                core::mem::size_of::<usize>(),
+                true,
+            )?;
+
+            let as_handle = current_task
+                .handles
+                .get(as_handle)
+                .ok_or(Status::BadHandle)?;
+            let as_id = match as_handle.object {
+                KernelObject::AddressSpace(as_id) => as_id,
+                _ => return Err(Status::InvalidArgument),
+            };
+
+            if as_handle.rights.0 & Rights::EXECUTE.0 == 0 {
+                return Err(Status::InvalidArgument);
+            }
+
+            Ok(as_id)
+        })
+        .expect("failed to resolve self task")?;
+
+    if crate::memory::with_address_space(as_id, |_| ()).is_none() {
+        return Err(Status::BadHandle);
+    }
+
+    let stack_probe = user_stack.checked_sub(1).ok_or(Status::BadAddress)?;
+    validate_user_range(as_id, VirtualAddr::new(stack_probe), 1, true)?;
+
+    let entry_is_valid = crate::memory::with_address_space(as_id, |address_space| {
+        address_space
+            .mapping(VirtualAddr::new(entry))
+            .is_some_and(|mapping| {
+                mapping.permissions.user_accessible && mapping.permissions.executable
+            })
+    })
+    .ok_or(Status::BadHandle)?;
+
+    if !entry_is_valid {
+        return Err(Status::BadAddress);
+    }
+
+    let kernel_stack = match crate::memory::with_kernel_address_space(|kernel_as| {
+        crate::memory::with_allocator(|allocator| {
+            crate::task::scheduler::allocate_kernel_stack(kernel_as, allocator)
+        })
+    }) {
+        Ok(stack) => stack,
+        Err(err) => {
+            println!("Failed to allocate kernel stack: {:?}", err);
+            return Err(Status::OutOfMemory);
+        }
+    };
+
+    let new_tcb = crate::task::tcb::Tcb::new_user(
+        as_id,
+        kernel_stack,
+        VirtualAddr::new(entry),
+        VirtualAddr::new(user_stack),
+    );
+
+    let new_task_id = match crate::task::scheduler::add_task(new_tcb) {
+        Ok(id) => id,
+        Err(_) => return Err(Status::OutOfMemory),
+    };
+
+    let handle = Handle {
+        object: KernelObject::Thread(new_task_id),
+        rights: Rights::READ | Rights::WRITE | Rights::EXECUTE,
+    };
+
+    crate::task::scheduler::with_task_mut(crate::task::scheduler::current(), |current_task| {
+        let handle_id = match current_task.handles.push(handle) {
+            Ok(id) => id,
+            Err(_) => {
+                crate::task::scheduler::remove_task(new_task_id);
+                return Err(Status::OutOfMemory);
+            }
+        };
+
+        unsafe { copy_val_to_user(VirtualAddr::new(out_task_handle), &handle_id) }
+    })
+    .expect("failed to resolve self task")
+}

src/task/bootstrap.rs

@@ -0,0 +1,140 @@
+use crate::{
+    format,
+    memory::{
+        self, AddressSpace, DirectMap, FRAME_SIZE, FrameAllocator, InitramfsImage, PagePermissions,
+        UserStack, VirtualAddr,
+    },
+    task::{scheduler::TaskId, tcb::Tcb},
+};
+
+pub fn spawn(
+    name: &str,
+    initramfs: &InitramfsImage,
+    kernel_as: &mut AddressSpace,
+    allocator: &mut FrameAllocator,
+    direct_map: DirectMap,
+) -> TaskId {
+    let bytes = format::cpio::find_file(initramfs.data(), name)
+        .unwrap_or_else(|| panic!("{name} missing from initramfs"));
+    let kernel_stack = crate::task::scheduler::allocate_kernel_stack(kernel_as, allocator)
+        .expect("kernel stack allocation failed");
+
+    let initramfs_physical_addr = kernel_as
+        .to_physical(initramfs.start)
+        .expect("failed to translate initramfs start address");
+
+    let mut address_space = kernel_as
+        .new_user(allocator)
+        .expect("address space allocation failed");
+    address_space
+        .map_range(
+            initramfs_physical_addr,
+            VirtualAddr::new(0x4000_0000),
+            ((initramfs.length) + 0xFFF) & !0xFFF,
+            PagePermissions::new(true, false, true),
+            allocator,
+            memory::CachePolicy::WriteBack,
+        )
+        .expect("failed to map initramfs");
+    let user_stack =
+        UserStack::allocate(&mut address_space, allocator).expect("user stack allocation failed");
+
+    let entry = load_elf(bytes, &mut address_space, allocator, direct_map).expect("invalid ELF");
+
+    let as_id = match crate::memory::insert_address_space(address_space) {
+        Ok(id) => id,
+        Err(_) => {
+            panic!("address space table is full");
+        }
+    };
+
+    let task = Tcb::new_user(as_id, kernel_stack, entry, user_stack.top());
+    crate::task::scheduler::add_task(task).expect("scheduler is full")
+}
+
+#[derive(Debug)]
+enum ElfLoadError {
+    AddressTranslationFailed,
+    FailedToMapSegment,
+    OutOfMemory,
+    InvalidElf,
+}
+
+fn load_elf(
+    bytes: &[u8],
+    user_address_space: &mut AddressSpace,
+    allocator: &mut FrameAllocator,
+    direct_map: DirectMap,
+) -> Result<VirtualAddr, ElfLoadError> {
+    let program = format::elf::Elf::parse(bytes).map_err(|_| ElfLoadError::InvalidElf)?;
+    let mut executable_entry = false;
+
+    for segment in program.segments() {
+        let segment = segment.map_err(|_| ElfLoadError::InvalidElf)?;
+        let end = segment
+            .address
+            .checked_add(segment.memory_size)
+            .filter(|&end| end <= memory::USER_SPACE_END.as_usize())
+            .ok_or(ElfLoadError::InvalidElf)?;
+        if segment.memory_size == 0 {
+            continue;
+        }
+        executable_entry |= segment.executable && (segment.address..end).contains(&program.entry);
+
+        let page_start = segment.address & !(FRAME_SIZE - 1);
+        let file_end = segment.address + segment.data.len();
+        let permissions = PagePermissions::new(segment.writable, segment.executable, true);
+
+        // Overlapping segment pages are rejected by map(), including stack/archive collisions.
+        for page in (page_start..end).step_by(FRAME_SIZE) {
+            let frame = allocator.alloc_nozero().ok_or(ElfLoadError::OutOfMemory)?;
+            let physical = frame.frame_address().start_address();
+            let Some(destination) = direct_map.translate(physical) else {
+                unsafe { allocator.dealloc(frame) };
+                return Err(ElfLoadError::AddressTranslationFailed);
+            };
+
+            let copy_start = page.max(segment.address).min(page + FRAME_SIZE);
+            let copy_end = (page + FRAME_SIZE).min(file_end).max(copy_start);
+            let prefix = copy_start - page;
+            let copied = copy_end - copy_start;
+            unsafe {
+                let destination = destination.as_mut_ptr::<u8>();
+                // Initialize padding and BSS, but don't zero bytes we're about to overwrite.
+                core::ptr::write_bytes(destination, 0, prefix);
+                if copied != 0 {
+                    core::ptr::copy_nonoverlapping(
+                        segment.data.as_ptr().add(copy_start - segment.address),
+                        destination.add(prefix),
+                        copied,
+                    );
+                }
+                core::ptr::write_bytes(
+                    destination.add(prefix + copied),
+                    0,
+                    FRAME_SIZE - prefix - copied,
+                );
+            }
+
+            if user_address_space
+                .map(
+                    physical,
+                    VirtualAddr::new(page),
+                    permissions,
+                    allocator,
+                    memory::CachePolicy::WriteBack,
+                )
+                .is_err()
+            {
+                unsafe { allocator.dealloc(frame) };
+                return Err(ElfLoadError::FailedToMapSegment);
+            }
+            let _ = frame.into_raw();
+        }
+    }
+
+    if !executable_entry {
+        return Err(ElfLoadError::InvalidElf);
+    }
+    Ok(VirtualAddr::new(program.entry))
+}

src/task/mod.rs

@@ -0,0 +1,3 @@
+pub mod bootstrap;
+pub mod scheduler;
+pub mod tcb;

src/task/scheduler.rs

@@ -0,0 +1,372 @@
+use core::cell::UnsafeCell;
+
+use crate::{
+    arch::ThreadContext,
+    memory::{
+        AddressSpace, AddressSpaceId, FrameAllocator, KernelStack, KernelStackPool,
+        StackCreateError, VirtualAddr,
+    },
+    println,
+    task::tcb::{BlockReason, ExitReason, Handle, KernelObject, Rights, Tcb, ThreadState},
+};
+
+const MAX_TASKS: usize = 32;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[repr(transparent)]
+pub struct TaskId(usize);
+
+impl TaskId {
+    pub const fn new(id: usize) -> Self {
+        Self(id)
+    }
+}
+
+struct Scheduler {
+    current: Option<TaskId>,
+    tasks: [Option<Tcb>; MAX_TASKS],
+    ready: ReadyQueue,
+    stacks: KernelStackPool,
+}
+
+impl Scheduler {
+    const fn new() -> Self {
+        Self {
+            current: None,
+            tasks: [const { None }; MAX_TASKS],
+            ready: ReadyQueue::new(),
+            stacks: KernelStackPool::new(),
+        }
+    }
+
+    fn make_switch(&mut self, current_id: TaskId, next_id: TaskId) -> Switch {
+        assert_ne!(current_id, next_id);
+
+        let current = self.tasks[current_id.0].as_mut().unwrap();
+        let prev_ctx = &mut current.context as *mut ThreadContext;
+        let prev_as_id = current.as_id;
+
+        let next = self.tasks[next_id.0].as_ref().unwrap();
+        let next_ctx = &next.context as *const ThreadContext;
+        let next_as_id = next.as_id;
+        let next_kernel_stack = next.kernel_stack.top();
+
+        Switch {
+            previous_context: prev_ctx,
+            next_context: next_ctx,
+            next_as_id,
+            next_kernel_stack,
+            activate_address_space: next_as_id != prev_as_id,
+        }
+    }
+}
+
+struct Switch {
+    previous_context: *mut ThreadContext,
+    next_context: *const ThreadContext,
+    next_as_id: AddressSpaceId,
+    next_kernel_stack: VirtualAddr,
+    activate_address_space: bool,
+}
+
+impl Switch {
+    unsafe fn perform(self) {
+        if self.activate_address_space {
+            crate::memory::with_address_space(self.next_as_id, |as_ref| unsafe {
+                as_ref.activate();
+            });
+        }
+
+        crate::arch::set_kernel_stack(self.next_kernel_stack);
+
+        unsafe {
+            crate::arch::switch_context(self.previous_context, self.next_context);
+        }
+    }
+}
+
+struct ReadyQueue {
+    entries: [TaskId; MAX_TASKS],
+    head: usize,
+    len: usize,
+}
+
+impl ReadyQueue {
+    pub const fn new() -> Self {
+        Self {
+            entries: [TaskId(0); MAX_TASKS],
+            head: 0,
+            len: 0,
+        }
+    }
+
+    pub fn push_back(&mut self, task: TaskId) -> bool {
+        if self.len == MAX_TASKS {
+            return false;
+        }
+
+        let tail = (self.head + self.len) % MAX_TASKS;
+        self.entries[tail] = task;
+        self.len += 1;
+
+        true
+    }
+
+    pub fn pop_front(&mut self) -> Option<TaskId> {
+        if self.len == 0 {
+            return None;
+        }
+
+        let task = self.entries[self.head];
+        self.head = (self.head + 1) % MAX_TASKS;
+        self.len -= 1;
+
+        Some(task)
+    }
+
+    pub fn remove(&mut self, task: TaskId) -> bool {
+        for i in 0..self.len {
+            let idx = (self.head + i) % MAX_TASKS;
+            if self.entries[idx] == task {
+                for j in i..(self.len - 1) {
+                    let from = (self.head + j + 1) % MAX_TASKS;
+                    let to = (self.head + j) % MAX_TASKS;
+                    self.entries[to] = self.entries[from];
+                }
+                self.len -= 1;
+                return true;
+            }
+        }
+
+        false
+    }
+}
+
+struct GlobalScheduler(UnsafeCell<Scheduler>);
+
+unsafe impl Sync for GlobalScheduler {}
+
+static SCHEDULER: GlobalScheduler = GlobalScheduler(UnsafeCell::new(Scheduler::new()));
+
+pub fn add_task(mut task: Tcb) -> Result<TaskId, Tcb> {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+
+    let result = {
+        let scheduler = unsafe { &mut *SCHEDULER.0.get() };
+
+        match scheduler.tasks.iter().position(Option::is_none) {
+            Some(id) => {
+                let id = TaskId(id);
+
+                task.id = id;
+
+                match task.handles.push(Handle {
+                    object: KernelObject::Thread(id),
+                    rights: Rights::READ | Rights::WRITE | Rights::EXECUTE,
+                }) {
+                    Ok(_) => {}
+                    Err(_) => unreachable!("Cant push root thread handle"),
+                }
+
+                task.state = ThreadState::Ready;
+                scheduler.tasks[id.0] = Some(task);
+                assert!(scheduler.ready.push_back(id));
+                Ok(id)
+            }
+            None => Err(task),
+        }
+    };
+
+    crate::arch::restore_interrupts(interrupt_state);
+    result
+}
+
+pub fn start() -> ! {
+    crate::arch::disable_interrupts();
+
+    let mut bootstrap_context = ThreadContext::empty();
+
+    let switch = {
+        let scheduler = unsafe { &mut *SCHEDULER.0.get() };
+        let next_id = scheduler.ready.pop_front().expect("no tasks to run");
+
+        let next = scheduler.tasks[next_id.0]
+            .as_mut()
+            .expect("ready task is missing");
+
+        next.state = ThreadState::Running;
+        scheduler.current = Some(next_id);
+
+        Switch {
+            previous_context: &mut bootstrap_context,
+            next_context: &next.context,
+            next_as_id: next.as_id,
+            next_kernel_stack: next.kernel_stack.top(),
+            activate_address_space: true,
+        }
+    };
+
+    unsafe {
+        switch.perform();
+    }
+
+    panic!("scheduler returned to bootstrap context");
+}
+
+pub fn allocate_kernel_stack(
+    address_space: &mut AddressSpace,
+    allocator: &mut FrameAllocator,
+) -> Result<KernelStack, StackCreateError> {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+    let scheduler = unsafe { &mut *SCHEDULER.0.get() };
+    let result = scheduler.stacks.allocate(address_space, allocator);
+    crate::arch::restore_interrupts(interrupt_state);
+    result
+}
+
+pub fn remove_task(id: TaskId) -> bool {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+
+    let result = {
+        let scheduler = unsafe { &mut *SCHEDULER.0.get() };
+
+        if scheduler.current == Some(id) {
+            false
+        } else if let Some(task) = scheduler.tasks.get_mut(id.0).and_then(Option::take) {
+            scheduler.ready.remove(id);
+            scheduler.stacks.free(task.kernel_stack);
+            true
+        } else {
+            false
+        }
+    };
+
+    crate::arch::restore_interrupts(interrupt_state);
+    result
+}
+
+pub fn with_task<R>(id: TaskId, f: impl FnOnce(&Tcb) -> R) -> Option<R> {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+    let scheduler = unsafe { &*SCHEDULER.0.get() };
+    let res = scheduler.tasks.get(id.0).and_then(Option::as_ref).map(f);
+    crate::arch::restore_interrupts(interrupt_state);
+    res
+}
+
+pub fn with_task_mut<R>(id: TaskId, f: impl FnOnce(&mut Tcb) -> R) -> Option<R> {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+    let scheduler = unsafe { &mut *SCHEDULER.0.get() };
+    let res = scheduler
+        .tasks
+        .get_mut(id.0)
+        .and_then(Option::as_mut)
+        .map(f);
+    crate::arch::restore_interrupts(interrupt_state);
+    res
+}
+
+pub fn current() -> TaskId {
+    let scheduler = unsafe { &mut *SCHEDULER.0.get() };
+    scheduler.current.expect("no current task")
+}
+
+pub fn block_current(reason: BlockReason) {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+
+    let switch = {
+        let scheduler = unsafe { &mut *SCHEDULER.0.get() };
+
+        let Some(next_id) = scheduler.ready.pop_front() else {
+            println!("Deadlock: all tasks blocked");
+            crate::hcf();
+        };
+
+        let current_id = scheduler.current.expect("no current task");
+
+        scheduler.tasks[current_id.0].as_mut().unwrap().state = ThreadState::Blocked(reason);
+        // explicitly do NOT push back the current task, because it is not ready
+
+        scheduler.tasks[next_id.0].as_mut().unwrap().state = ThreadState::Running;
+        scheduler.current = Some(next_id);
+
+        scheduler.make_switch(current_id, next_id)
+    };
+
+    unsafe {
+        switch.perform();
+    }
+
+    // this runs when this task is selected to run again
+    crate::arch::restore_interrupts(interrupt_state);
+}
+
+pub fn unblock(id: TaskId) {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+
+    let scheduler = unsafe { &mut *SCHEDULER.0.get() };
+    if let Some(task) = scheduler.tasks[id.0].as_mut() {
+        if matches!(task.state, ThreadState::Blocked(_)) {
+            task.state = ThreadState::Ready;
+            assert!(scheduler.ready.push_back(id));
+        }
+    }
+
+    crate::arch::restore_interrupts(interrupt_state);
+}
+
+pub fn yield_current() {
+    let interrupt_state = crate::arch::disable_interrupts_and_save();
+
+    let switch = {
+        let scheduler = unsafe { &mut *SCHEDULER.0.get() };
+
+        let Some(next_id) = scheduler.ready.pop_front() else {
+            crate::arch::restore_interrupts(interrupt_state);
+            return;
+        };
+
+        let current_id = scheduler.current.expect("no current task");
+
+        scheduler.tasks[current_id.0].as_mut().unwrap().state = ThreadState::Ready;
+        assert!(scheduler.ready.push_back(current_id));
+
+        scheduler.tasks[next_id.0].as_mut().unwrap().state = ThreadState::Running;
+        scheduler.current = Some(next_id);
+
+        scheduler.make_switch(current_id, next_id)
+    };
+
+    unsafe {
+        switch.perform();
+    }
+
+    // this runs when this task is selected to run again
+    crate::arch::restore_interrupts(interrupt_state);
+}
+
+pub fn exit_current(reason: ExitReason) -> ! {
+    crate::arch::disable_interrupts();
+
+    let switch = {
+        let scheduler = unsafe { &mut *SCHEDULER.0.get() };
+        let current_id = scheduler.current.expect("no current task");
+        let Some(next_id) = scheduler.ready.pop_front() else {
+            println!("All tasks exited");
+            crate::hcf();
+        };
+
+        let current = scheduler.tasks[current_id.0].as_mut().unwrap();
+        current.state = ThreadState::Dead(reason);
+
+        scheduler.tasks[next_id.0].as_mut().unwrap().state = ThreadState::Running;
+        scheduler.current = Some(next_id);
+
+        scheduler.make_switch(current_id, next_id)
+    };
+
+    unsafe {
+        switch.perform();
+    }
+
+    panic!("dead task was scheduled again");
+}

src/task/tcb.rs

@@ -0,0 +1,222 @@
+use core::ops::BitOr;
+
+use crate::{
+    arch::ThreadContext,
+    memory::{AddressSpaceId, KernelStack, OwnedFrame, VirtualAddr},
+    task::scheduler::TaskId,
+};
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum Fault {
+    SegmentationFault,
+    IllegalInstruction,
+    Abort,
+    BadSystemCall,
+}
+
+// Thread Control Block
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum ExitReason {
+    Exited(usize),
+    Killed,
+    Fault(Fault),
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum BlockReason {
+    Recv,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum ThreadState {
+    Ready,
+    Running,
+    Blocked(BlockReason),
+    Dead(ExitReason),
+}
+
+pub const MAX_MSG_SIZE: usize = 128;
+pub const MAILBOX_CAPACITY: usize = 4;
+
+#[derive(Clone, Copy)]
+pub struct Message {
+    pub sender: TaskId,
+    pub length: usize,
+    pub data: [u8; MAX_MSG_SIZE],
+}
+
+pub struct Mailbox {
+    pub messages: [Option<Message>; MAILBOX_CAPACITY],
+    pub head: usize,
+    pub len: usize,
+}
+
+impl Mailbox {
+    const fn new() -> Self {
+        Self {
+            messages: [None; MAILBOX_CAPACITY],
+            head: 0,
+            len: 0,
+        }
+    }
+
+    pub fn pop(&mut self) -> Option<Message> {
+        if self.len == 0 {
+            return None;
+        }
+
+        let msg = self.messages[self.head];
+        self.head = (self.head + 1) % MAILBOX_CAPACITY;
+        self.len -= 1;
+        msg
+    }
+
+    pub fn push(&mut self, msg: Message) -> bool {
+        if self.len == MAILBOX_CAPACITY {
+            return false;
+        }
+
+        let tail = (self.head + self.len) % MAILBOX_CAPACITY;
+        self.messages[tail] = Some(msg);
+        self.len += 1;
+        true
+    }
+}
+
+const MAX_HANDLES: usize = 32;
+
+pub enum KernelObject {
+    AddressSpace(AddressSpaceId),
+    Frame(OwnedFrame),
+    Mapping {
+        frame: OwnedFrame,
+        address_space: AddressSpaceId,
+        virtual_addr: VirtualAddr,
+    },
+    Thread(TaskId),
+}
+
+pub struct Handle {
+    pub object: KernelObject,
+    pub rights: Rights,
+}
+
+#[derive(Clone, Copy)]
+pub struct Rights(pub u32);
+
+impl Rights {
+    pub const READ: Self = Self(1 << 0);
+    pub const WRITE: Self = Self(1 << 1);
+    pub const EXECUTE: Self = Self(1 << 2);
+    pub const MAP: Self = Self(1 << 3);
+}
+
+impl BitOr for Rights {
+    type Output = Self;
+
+    fn bitor(self, rhs: Self) -> Self::Output {
+        Self(self.0 | rhs.0)
+    }
+}
+
+pub struct HandleTable {
+    handles: [Option<Handle>; MAX_HANDLES],
+}
+
+impl HandleTable {
+    pub const fn new() -> Self {
+        Self {
+            handles: [const { None }; MAX_HANDLES],
+        }
+    }
+
+    pub fn push(&mut self, handle: Handle) -> Result<usize, Handle> {
+        for (i, slot) in self.handles.iter_mut().enumerate() {
+            if slot.is_none() {
+                *slot = Some(handle);
+                return Ok(i);
+            }
+        }
+
+        Err(handle)
+    }
+
+    pub fn remove(&mut self, id: usize) -> Option<Handle> {
+        self.handles.get_mut(id).and_then(Option::take)
+    }
+
+    pub fn take(&mut self, id: usize) -> Option<Handle> {
+        self.handles.get_mut(id)?.take()
+    }
+
+    pub fn put(&mut self, id: usize, handle: Handle) -> Result<(), Handle> {
+        let Some(slot) = self.handles.get_mut(id) else {
+            return Err(handle);
+        };
+
+        if slot.is_some() {
+            return Err(handle);
+        }
+
+        *slot = Some(handle);
+        Ok(())
+    }
+
+    pub fn get(&self, id: usize) -> Option<&Handle> {
+        self.handles.get(id).and_then(Option::as_ref)
+    }
+
+    pub fn get_mut(&mut self, id: usize) -> Option<&mut Handle> {
+        self.handles.get_mut(id).and_then(Option::as_mut)
+    }
+}
+
+pub struct Tcb {
+    pub id: TaskId,
+    pub as_id: AddressSpaceId,
+    pub state: ThreadState,
+    pub kernel_stack: KernelStack,
+    pub context: ThreadContext,
+    pub mailbox: Mailbox,
+    pub handles: HandleTable,
+}
+
+impl core::fmt::Debug for Tcb {
+    fn fmt(&self, formatter: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        formatter
+            .debug_struct("Tcb")
+            .field("id", &self.id)
+            .field("as_id", &self.as_id)
+            .field("state", &self.state)
+            .finish_non_exhaustive()
+    }
+}
+
+impl Tcb {
+    pub fn new_user(
+        as_id: AddressSpaceId,
+        kernel_stack: KernelStack,
+        entry: VirtualAddr,
+        user_stack: VirtualAddr,
+    ) -> Self {
+        let context = ThreadContext::new(entry, user_stack, kernel_stack.top());
+        let mut handles = HandleTable::new();
+        match handles.push(Handle {
+            object: KernelObject::AddressSpace(as_id),
+            rights: Rights::READ | Rights::WRITE | Rights::EXECUTE,
+        }) {
+            Ok(_) => {}
+            Err(_) => unreachable!("Cant push root address space handle"),
+        };
+
+        Self {
+            id: TaskId::new(0),
+            as_id,
+            state: ThreadState::Ready,
+            kernel_stack,
+            context,
+            mailbox: Mailbox::new(),
+            handles,
+        }
+    }
+}

userspace/client/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "client"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+dusk-sys = { path = "../dusk-sys" }
+
+[[bin]]
+name = "client"
+test = false
+bench = false

userspace/client/src/main.rs

@@ -0,0 +1,27 @@
+#![no_std]
+#![no_main]
+
+use dusk_sys::{println, sys_exit, sys_recv, sys_send};
+
+#[unsafe(no_mangle)]
+pub extern "C" fn _start() -> ! {
+    let msg = "Hello from client!";
+    println!("[client] Sent: {}", msg);
+    // TODO: we assume the echo server is task 1 (spawned by omega3)
+    sys_send(1, msg.as_bytes()).unwrap();
+
+    let mut out = [0u8; 128];
+    let (actual_len, _) = sys_recv(&mut out).unwrap();
+    println!(
+        "[client] Received: {}",
+        core::str::from_utf8(&out[..actual_len]).unwrap()
+    );
+
+    sys_exit(0);
+}
+
+#[panic_handler]
+fn panic(info: &core::panic::PanicInfo) -> ! {
+    println!("{info}");
+    sys_exit(1);
+}

userspace/dusk-sys/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "dusk-sys"
+version = "0.1.0"
+edition = "2024"
+
+[lib]
+test = false
+bench = false

userspace/dusk-sys/src/lib.rs

@@ -0,0 +1,327 @@
+#![no_std]
+
+use core::arch::asm;
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum Status {
+    // Success = 0,
+    InvalidArgument = 1,
+    BadAddress = 2,
+    BadFileDescriptor = 3,
+    NoSuchTask = 4,
+    OutOfMemory = 5,
+    BadHandle = 6,
+}
+
+// Opaque handle type
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct AddressSpaceHandle(usize);
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct FrameHandle(usize);
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct MappingHandle(usize);
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct ThreadHandle(usize);
+
+// our own address space and thread handle are always given to us
+pub const SELF_AS: AddressSpaceHandle = AddressSpaceHandle(0);
+pub const SELF_THREAD: ThreadHandle = ThreadHandle(1);
+
+impl From<usize> for Status {
+    fn from(value: usize) -> Self {
+        match value {
+            1 => Self::InvalidArgument,
+            2 => Self::BadAddress,
+            3 => Self::BadFileDescriptor,
+            4 => Self::NoSuchTask,
+            5 => Self::OutOfMemory,
+            6 => Self::BadHandle,
+            _ => Self::InvalidArgument,
+        }
+    }
+}
+
+#[repr(u64)]
+pub enum SyscallNumber {
+    Yield = 1,
+    Exit = 2,
+    Write = 3,
+    Send = 4,
+    Recv = 5,
+    FrameAlloc = 6,
+    FrameDealloc = 7,
+    AsCreate = 8,
+    Map = 9,
+    Unmap = 10,
+    TaskCreate = 11,
+}
+
+pub fn sys_yield() {
+    unsafe {
+        asm!(
+            "syscall",
+            in("rax") 1usize,
+            lateout("rcx") _,
+            lateout("r11") _,
+        );
+    }
+}
+
+fn debug_write(buf: &str) -> Result<(), Status> {
+    unsafe {
+        let status: usize;
+
+        asm!(
+            "syscall",
+            in("rdi") 1,
+            in("rsi") buf.as_ptr(),
+            in("rdx") buf.len(),
+            in("r10") 0,
+            inlateout("rax") SyscallNumber::Write as usize => status,
+            lateout("rcx") _,
+            lateout("r11") _
+        );
+
+        if status != 0 {
+            Err(status.into())
+        } else {
+            Ok(())
+        }
+    }
+}
+
+struct DebugWriter;
+
+impl core::fmt::Write for DebugWriter {
+    fn write_str(&mut self, value: &str) -> core::fmt::Result {
+        debug_write(value).map_err(|_| core::fmt::Error)
+    }
+}
+
+#[doc(hidden)]
+pub fn __print(arguments: core::fmt::Arguments<'_>) {
+    use core::fmt::Write;
+
+    let _ = DebugWriter.write_fmt(arguments);
+}
+
+#[macro_export]
+macro_rules! print {
+    ($($arg:tt)*) => {{
+        $crate::__print(core::format_args!($($arg)*));
+    }};
+}
+
+#[macro_export]
+macro_rules! println {
+    () => {{
+        $crate::print!("\n");
+    }};
+    ($($arg:tt)*) => {{
+        $crate::print!("{}\n", core::format_args!($($arg)*));
+    }};
+}
+
+pub fn sys_exit(exit_code: usize) -> ! {
+    unsafe {
+        asm!(
+            "syscall",
+            in("rdi") exit_code,
+            in("rax") SyscallNumber::Exit as usize,
+            options(noreturn)
+        );
+    }
+}
+
+pub fn sys_send(dest_task_id: usize, msg: &[u8]) -> Result<(), Status> {
+    unsafe {
+        let status: usize;
+
+        asm!(
+            "syscall",
+            in("rdi") dest_task_id,
+            in("rsi") msg.as_ptr(),
+            in("rdx") msg.len(),
+            inlateout("rax") SyscallNumber::Send as usize => status,
+            lateout("rcx") _,
+            lateout("r11") _,
+        );
+
+        if status != 0 {
+            Err(status.into())
+        } else {
+            Ok(())
+        }
+    }
+}
+
+pub fn sys_recv(buf: &mut [u8]) -> Result<(usize, usize), Status> {
+    let mut actual_len: usize = 0;
+    let mut sender: usize = 0;
+
+    unsafe {
+        let status: usize;
+
+        asm!(
+            "syscall",
+            in("rdi") buf.as_mut_ptr(),
+            in("rsi") buf.len(),
+            in("rdx") &raw mut actual_len as usize,
+            in("r10") &raw mut sender as usize,
+            inlateout("rax") SyscallNumber::Recv as usize => status,
+            lateout("rcx") _,
+            lateout("r11") _,
+        );
+
+        if status != 0 {
+            Err(status.into())
+        } else {
+            Ok((actual_len, sender))
+        }
+    }
+}
+
+pub fn sys_frame_alloc() -> Result<FrameHandle, Status> {
+    let mut handle: usize = 0;
+    unsafe {
+        let status: usize;
+
+        asm!(
+            "syscall",
+            in("rdi") &raw mut handle as usize,
+            inlateout("rax") SyscallNumber::FrameAlloc as usize => status,
+            lateout("rcx") _,
+            lateout("r11") _,
+        );
+
+        if status != 0 {
+            Err(status.into())
+        } else {
+            Ok(FrameHandle(handle))
+        }
+    }
+}
+
+pub fn sys_frame_dealloc(frame_handle: FrameHandle) -> Result<(), Status> {
+    unsafe {
+        let status: usize;
+
+        asm!(
+            "syscall",
+            in("rdi") frame_handle.0,
+            inlateout("rax") SyscallNumber::FrameDealloc as usize => status,
+            lateout("rcx") _,
+            lateout("r11") _,
+        );
+
+        if status != 0 {
+            Err(status.into())
+        } else {
+            Ok(())
+        }
+    }
+}
+
+pub fn sys_as_create() -> Result<AddressSpaceHandle, Status> {
+    let mut handle: usize = 0;
+    unsafe {
+        let status: usize;
+
+        asm!(
+            "syscall",
+            in("rdi") &raw mut handle as usize,
+            inlateout("rax") SyscallNumber::AsCreate as usize => status,
+            lateout("rcx") _,
+            lateout("r11") _,
+        );
+
+        if status != 0 {
+            Err(status.into())
+        } else {
+            Ok(AddressSpaceHandle(handle))
+        }
+    }
+}
+
+pub fn sys_map(
+    as_handle: AddressSpaceHandle,
+    frame_handle: FrameHandle,
+    virtual_addr: usize,
+    permissions: usize,
+) -> Result<MappingHandle, Status> {
+    let mut handle: usize = 0;
+
+    unsafe {
+        let status: usize;
+
+        asm!(
+            "syscall",
+            in("rdi") as_handle.0,
+            in("rsi") frame_handle.0,
+            in("rdx") virtual_addr,
+            in("r10") permissions,
+            in("r8") &raw mut handle as usize,
+            inlateout("rax") SyscallNumber::Map as usize => status,
+            lateout("rcx") _,
+            lateout("r11") _,
+        );
+
+        if status != 0 {
+            Err(status.into())
+        } else {
+            Ok(MappingHandle(handle))
+        }
+    }
+}
+
+pub fn sys_unmap(mapping_handle: MappingHandle) -> Result<(), Status> {
+    unsafe {
+        let status: usize;
+
+        asm!(
+            "syscall",
+            in("rdi") mapping_handle.0,
+            inlateout("rax") SyscallNumber::Unmap as usize => status,
+            lateout("rcx") _,
+            lateout("r11") _,
+        );
+
+        if status != 0 {
+            Err(status.into())
+        } else {
+            Ok(())
+        }
+    }
+}
+
+pub fn sys_task_create(
+    as_handle: AddressSpaceHandle,
+    entry: usize,
+    user_stack: usize,
+) -> Result<ThreadHandle, Status> {
+    let mut handle: usize = 0;
+
+    unsafe {
+        let status: usize;
+
+        asm!(
+            "syscall",
+            in("rdi") as_handle.0,
+            in("rsi") entry,
+            in("rdx") user_stack,
+            in("r10") &raw mut handle as usize,
+            inlateout("rax") SyscallNumber::TaskCreate as usize => status,
+            lateout("rcx") _,
+            lateout("r11") _,
+        );
+
+        if status != 0 {
+            Err(status.into())
+        } else {
+            Ok(ThreadHandle(handle))
+        }
+    }
+}

userspace/echo/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "echo"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+dusk-sys = { path = "../dusk-sys" }
+
+[[bin]]
+name = "echo"
+test = false
+bench = false

userspace/echo/src/main.rs

@@ -0,0 +1,23 @@
+#![no_std]
+#![no_main]
+
+use dusk_sys::{println, sys_exit, sys_recv, sys_send};
+
+#[unsafe(no_mangle)]
+pub extern "C" fn _start() -> ! {
+    let mut out = [0u8; 128];
+    loop {
+        let (actual_len, sender) = sys_recv(&mut out).unwrap();
+        println!(
+            "[echo] Received: {}",
+            core::str::from_utf8(&out[..actual_len]).unwrap()
+        );
+        sys_send(sender, &out[..actual_len]).unwrap();
+    }
+}
+
+#[panic_handler]
+fn panic(info: &core::panic::PanicInfo) -> ! {
+    println!("{info}");
+    sys_exit(1);
+}

userspace/omega3/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "omega3"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+dusk-sys = { path = "../dusk-sys" }
+
+[[bin]]
+name = "omega3"
+test = false
+bench = false

userspace/omega3/src/cpio.rs

@@ -0,0 +1,73 @@
+// CPIO newc archive parser
+
+#[repr(C)]
+struct Header {
+    pub c_magic: [u8; 6],
+    pub c_ino: [u8; 8],
+    pub c_mode: [u8; 8],
+    pub c_uid: [u8; 8],
+    pub c_gid: [u8; 8],
+    pub c_nlink: [u8; 8],
+    pub c_mtime: [u8; 8],
+    pub c_filesize: [u8; 8],
+    pub c_devmajor: [u8; 8],
+    pub c_devminor: [u8; 8],
+    pub c_rdevmajor: [u8; 8],
+    pub c_rdevminor: [u8; 8],
+    pub c_namesize: [u8; 8],
+    pub c_check: [u8; 8],
+}
+
+impl Header {
+    pub fn from_bytes(bytes: &[u8]) -> Option<Self> {
+        if bytes.len() < core::mem::size_of::<Header>() {
+            return None;
+        }
+
+        let header: Header = unsafe { core::ptr::read(bytes.as_ptr() as *const Header) };
+
+        if header.c_magic != *b"070701" {
+            return None;
+        }
+
+        Some(header)
+    }
+}
+
+pub fn find_file<'a>(archive: *const u8, target: &str) -> Option<&'a [u8]> {
+    let mut offset = 0;
+
+    loop {
+        let header = Header::from_bytes(&unsafe {
+            core::slice::from_raw_parts(archive.add(offset), core::mem::size_of::<Header>())
+        })?;
+        let header_start = offset;
+        offset += core::mem::size_of::<Header>();
+
+        let file_len =
+            usize::from_str_radix(core::str::from_utf8(&header.c_filesize).ok()?, 16).ok()?;
+        let name_len =
+            usize::from_str_radix(core::str::from_utf8(&header.c_namesize).ok()?, 16).ok()?;
+
+        let name_bytes = &unsafe { core::slice::from_raw_parts(archive.add(offset), name_len) };
+        let name = core::str::from_utf8(name_bytes)
+            .ok()?
+            .trim_end_matches('\0');
+
+        if name == "TRAILER!!!" {
+            break;
+        }
+
+        let data_start = header_start + ((core::mem::size_of::<Header>() + name_len + 3) & !3);
+
+        if name == target {
+            return Some(&unsafe {
+                core::slice::from_raw_parts(archive.add(data_start), file_len)
+            });
+        }
+
+        offset = data_start + ((file_len + 3) & !3);
+    }
+
+    None
+}

userspace/omega3/src/elf.rs

@@ -0,0 +1,425 @@
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum ElfIsa {
+    None,
+    Sparc,
+    X86,
+    Mips,
+    Ppc,
+    Arm,
+    SuperH,
+    Ia64,
+    Amd64,
+    AArch64,
+    Riscv,
+}
+
+impl ElfIsa {
+    fn from_u16(value: u16) -> Result<Self, ElfError> {
+        match value {
+            0x00 => Ok(Self::None),
+            0x02 => Ok(Self::Sparc),
+            0x03 => Ok(Self::X86),
+            0x08 => Ok(Self::Mips),
+            0x14 => Ok(Self::Ppc),
+            0x28 => Ok(Self::Arm),
+            0x2A => Ok(Self::SuperH),
+            0x32 => Ok(Self::Ia64),
+            0x3E => Ok(Self::Amd64),
+            0xB7 => Ok(Self::AArch64),
+            0xF3 => Ok(Self::Riscv),
+            _ => Err(ElfError::InvalidElf),
+        }
+    }
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum ElfClass {
+    Elf32,
+    Elf64,
+}
+
+impl ElfClass {
+    fn from_u8(value: u8) -> Result<Self, ElfError> {
+        match value {
+            1 => Ok(Self::Elf32),
+            2 => Ok(Self::Elf64),
+            _ => Err(ElfError::InvalidElf),
+        }
+    }
+
+    const fn header_size(self) -> u16 {
+        match self {
+            Self::Elf32 => 52,
+            Self::Elf64 => 64,
+        }
+    }
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum Endianness {
+    Little,
+    Big,
+}
+
+impl Endianness {
+    fn from_u8(value: u8) -> Result<Self, ElfError> {
+        match value {
+            1 => Ok(Self::Little),
+            2 => Ok(Self::Big),
+            _ => Err(ElfError::InvalidElf),
+        }
+    }
+}
+
+#[repr(u16)]
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum ElfType {
+    Relocatable = 1,
+    Executable = 2,
+    SharedObject = 3,
+    Core = 4,
+}
+
+impl ElfType {
+    fn from_u16(value: u16) -> Result<Self, ElfError> {
+        match value {
+            1 => Ok(Self::Relocatable),
+            2 => Ok(Self::Executable),
+            3 => Ok(Self::SharedObject),
+            4 => Ok(Self::Core),
+            _ => Err(ElfError::InvalidElf),
+        }
+    }
+}
+
+#[derive(Debug)]
+#[allow(unused)]
+pub struct ElfHeader {
+    magic: [u8; 4],
+    pub class: ElfClass,
+    endianness: Endianness,
+    version: u8,
+    os_abi: u8,
+    _reserved: [u8; 8],
+    pub object_type: ElfType,
+    pub machine: ElfIsa,
+    version_1: u32,
+    entry: u64,                     // 2115136
+    program_header_offset: u64,     // 64
+    section_header_offset: u64,     // 2759752
+    flags: u32,                     // 0
+    header_size: u16,               // 64
+    program_header_entry_size: u16, // 56
+    program_header_count: u16,      // 6
+    section_header_entry_size: u16, // 64
+    section_header_count: u16,      // 17
+    section_name_index: u16,        // 15
+}
+
+#[derive(Debug)]
+pub enum ElfError {
+    InvalidElf,
+}
+
+impl ElfHeader {
+    pub fn parse(bytes: &[u8]) -> Result<Self, ElfError> {
+        let mut reader = Reader::new(bytes);
+
+        let magic = reader.read_array()?;
+        if magic != *b"\x7fELF" {
+            return Err(ElfError::InvalidElf);
+        }
+
+        let class = ElfClass::from_u8(reader.read_u8()?)?;
+        let endianness = Endianness::from_u8(reader.read_u8()?)?;
+        reader.set_endianness(endianness);
+
+        let version = reader.read_u8()?;
+        let os_abi = reader.read_u8()?;
+        let reserved = reader.read_array()?;
+        let object_type = ElfType::from_u16(reader.read_u16()?)?;
+        let machine = ElfIsa::from_u16(reader.read_u16()?)?;
+        let version_1 = reader.read_u32()?;
+        let entry = reader.read_word(class)?;
+        let program_header_offset = reader.read_word(class)?;
+        let section_header_offset = reader.read_word(class)?;
+        let flags = reader.read_u32()?;
+        let header_size = reader.read_u16()?;
+        let program_header_entry_size = reader.read_u16()?;
+        let program_header_count = reader.read_u16()?;
+        let section_header_entry_size = reader.read_u16()?;
+        let section_header_count = reader.read_u16()?;
+        let section_name_index = reader.read_u16()?;
+
+        if header_size != class.header_size() {
+            return Err(ElfError::InvalidElf);
+        }
+
+        Ok(Self {
+            magic,
+            class,
+            endianness,
+            version,
+            os_abi,
+            _reserved: reserved,
+            object_type,
+            machine,
+            version_1,
+            entry,
+            program_header_offset,
+            section_header_offset,
+            flags,
+            header_size,
+            program_header_entry_size,
+            program_header_count,
+            section_header_entry_size,
+            section_header_count,
+            section_name_index,
+        })
+    }
+}
+
+#[repr(u32)]
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum ProgramHeaderType {
+    Null = 0,
+    Load = 1,
+    Dynamic = 2,
+    Interpreter = 3,
+    Note = 4,
+    Shlib = 5,
+    Phdr = 6,
+    GnuStack = 0x6474e551,
+    Relro = 0x6474e552,
+    Other(u32),
+}
+
+impl ProgramHeaderType {
+    fn from_u32(value: u32) -> Self {
+        match value {
+            0 => Self::Null,
+            1 => Self::Load,
+            2 => Self::Dynamic,
+            3 => Self::Interpreter,
+            4 => Self::Note,
+            5 => Self::Shlib,
+            6 => Self::Phdr,
+            0x6474e551 => Self::GnuStack,
+            0x6474e552 => Self::Relro,
+            _ => Self::Other(value),
+        }
+    }
+}
+
+#[derive(Debug)]
+#[allow(unused)]
+pub struct ProgramHeader {
+    pub segment_type: ProgramHeaderType,
+    pub flags: u32,
+    pub file_offset: u64,
+    pub virtual_address: u64,
+    _physical_address: u64,
+    pub file_size: u64,
+    pub memory_size: u64,
+    pub alignment: u64,
+}
+
+impl ProgramHeader {
+    pub fn parse(bytes: &[u8], class: ElfClass, endianness: Endianness) -> Result<Self, ElfError> {
+        let mut reader = Reader::new(bytes);
+        reader.set_endianness(endianness);
+
+        let segment_type = ProgramHeaderType::from_u32(reader.read_u32()?);
+
+        let flags = if class == ElfClass::Elf64 {
+            reader.read_u32()?
+        } else {
+            0
+        };
+
+        let file_offset = reader.read_word(class)?;
+        let virtual_address = reader.read_word(class)?;
+        let physical_address = reader.read_word(class)?;
+        let file_size = reader.read_word(class)?;
+        let memory_size = reader.read_word(class)?;
+
+        let flags = if class == ElfClass::Elf32 {
+            reader.read_u32()?
+        } else {
+            flags
+        };
+
+        let alignment = reader.read_word(class)?;
+
+        Ok(Self {
+            segment_type,
+            flags,
+            file_offset,
+            virtual_address,
+            _physical_address: physical_address,
+            file_size,
+            memory_size,
+            alignment,
+        })
+    }
+}
+
+pub struct Elf<'a> {
+    bytes: &'a [u8],
+    header: ElfHeader,
+}
+
+impl<'a> Elf<'a> {
+    pub fn parse(bytes: &'a [u8]) -> Result<Self, ElfError> {
+        let header = ElfHeader::parse(bytes)?;
+
+        Ok(Self { bytes, header })
+    }
+
+    pub fn program_headers(&self) -> Result<ProgramHeaders<'_>, ElfError> {
+        let offset =
+            usize::try_from(self.header.program_header_offset).map_err(|_| ElfError::InvalidElf)?;
+        let entry_size = usize::from(self.header.program_header_entry_size);
+        let count = usize::from(self.header.program_header_count);
+
+        let expected_entry_size = match self.header.class {
+            ElfClass::Elf32 => 32,
+            ElfClass::Elf64 => 56,
+        };
+
+        if entry_size != expected_entry_size {
+            return Err(ElfError::InvalidElf);
+        }
+
+        let table_size = entry_size.checked_mul(count).ok_or(ElfError::InvalidElf)?;
+        let table_end = offset.checked_add(table_size).ok_or(ElfError::InvalidElf)?;
+        let bytes = self
+            .bytes
+            .get(offset..table_end)
+            .ok_or(ElfError::InvalidElf)?;
+
+        Ok(ProgramHeaders {
+            bytes,
+            class: self.header.class,
+            endianness: self.header.endianness,
+            entry_size,
+            remaining: count,
+        })
+    }
+
+    pub fn bytes(&self) -> &[u8] {
+        self.bytes
+    }
+
+    #[allow(unused)]
+    pub fn machine(&self) -> ElfIsa {
+        self.header.machine
+    }
+
+    pub fn entry(&self) -> usize {
+        self.header.entry as usize
+    }
+}
+
+pub struct ProgramHeaders<'a> {
+    bytes: &'a [u8],
+    class: ElfClass,
+    endianness: Endianness,
+    entry_size: usize,
+    remaining: usize,
+}
+
+impl<'a> Iterator for ProgramHeaders<'a> {
+    type Item = Result<ProgramHeader, ElfError>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.remaining == 0 {
+            return None;
+        }
+
+        let entry = match self.bytes.get(..self.entry_size) {
+            Some(entry) => entry,
+            None => {
+                self.remaining = 0;
+                return None;
+            }
+        };
+
+        self.bytes = &self.bytes[self.entry_size..];
+        self.remaining -= 1;
+
+        Some(ProgramHeader::parse(entry, self.class, self.endianness))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.remaining, Some(self.remaining))
+    }
+}
+
+impl ExactSizeIterator for ProgramHeaders<'_> {}
+
+struct Reader<'a> {
+    bytes: &'a [u8],
+    offset: usize,
+    endianness: Endianness,
+}
+
+impl<'a> Reader<'a> {
+    const fn new(bytes: &'a [u8]) -> Self {
+        Self {
+            bytes,
+            offset: 0,
+            endianness: Endianness::Little,
+        }
+    }
+
+    fn set_endianness(&mut self, endianness: Endianness) {
+        self.endianness = endianness;
+    }
+
+    fn read_array<const N: usize>(&mut self) -> Result<[u8; N], ElfError> {
+        let end = self.offset.checked_add(N).ok_or(ElfError::InvalidElf)?;
+        let bytes = self
+            .bytes
+            .get(self.offset..end)
+            .ok_or(ElfError::InvalidElf)?;
+        self.offset = end;
+
+        bytes.try_into().map_err(|_| ElfError::InvalidElf)
+    }
+
+    fn read_u8(&mut self) -> Result<u8, ElfError> {
+        Ok(self.read_array::<1>()?[0])
+    }
+
+    fn read_u16(&mut self) -> Result<u16, ElfError> {
+        let bytes = self.read_array()?;
+        Ok(match self.endianness {
+            Endianness::Little => u16::from_le_bytes(bytes),
+            Endianness::Big => u16::from_be_bytes(bytes),
+        })
+    }
+
+    fn read_u32(&mut self) -> Result<u32, ElfError> {
+        let bytes = self.read_array()?;
+        Ok(match self.endianness {
+            Endianness::Little => u32::from_le_bytes(bytes),
+            Endianness::Big => u32::from_be_bytes(bytes),
+        })
+    }
+
+    fn read_u64(&mut self) -> Result<u64, ElfError> {
+        let bytes = self.read_array()?;
+        Ok(match self.endianness {
+            Endianness::Little => u64::from_le_bytes(bytes),
+            Endianness::Big => u64::from_be_bytes(bytes),
+        })
+    }
+
+    fn read_word(&mut self, class: ElfClass) -> Result<u64, ElfError> {
+        match class {
+            ElfClass::Elf32 => Ok(u64::from(self.read_u32()?)),
+            ElfClass::Elf64 => self.read_u64(),
+        }
+    }
+}

userspace/omega3/src/main.rs

@@ -0,0 +1,113 @@
+#![no_std]
+#![no_main]
+
+mod cpio;
+mod elf;
+
+use dusk_sys::{
+    AddressSpaceHandle, SELF_AS, println, sys_as_create, sys_exit, sys_frame_alloc, sys_map,
+    sys_task_create, sys_unmap, sys_yield,
+};
+
+// Mapped into the root task's address space by the kernel.
+static INITRAMFS_START: usize = 0x4000_0000;
+const SCRATCH_PAGE: usize = 0x8000_0000;
+const STACK_TOP: usize = 0x0000_7FFF_FFFF_F000;
+const STACK_PAGES: usize = 4;
+
+#[unsafe(no_mangle)]
+pub extern "C" fn _start() -> ! {
+    println!(r#"-----------------------------"#);
+    println!(r#"   .d88888888b.   .d88888b.  "#);
+    println!(r#"  d88P"    "Y88b 88P"  "Y88  "#);
+    println!(r#"  888        888     .od88P  "#);
+    println!(r#"  Y88b      d88P      "Y88b  "#);
+    println!(r#"   "88bo  od88"  88b    d88  "#);
+    println!(r#"  d88888  88888b  "Y88888P"  "#);
+    println!(r#"----- Omega3 Dusk Root Server"#);
+
+    let echo_bytes = cpio::find_file(INITRAMFS_START as *const u8, "echo.elf").unwrap();
+    let echo_elf = elf::Elf::parse(echo_bytes).unwrap();
+    let echo_as = sys_as_create().unwrap();
+    let echo_entry = load_elf(&echo_elf, echo_as);
+    map_stack(echo_as, STACK_TOP, STACK_PAGES);
+    let _ = sys_task_create(echo_as, echo_entry, STACK_TOP).unwrap();
+
+    let client_bytes = cpio::find_file(INITRAMFS_START as *const u8, "client.elf").unwrap();
+    let client_elf = elf::Elf::parse(client_bytes).unwrap();
+    let client_as = sys_as_create().unwrap();
+    let client_entry = load_elf(&client_elf, client_as);
+    map_stack(client_as, STACK_TOP, STACK_PAGES);
+    let _ = sys_task_create(client_as, client_entry, STACK_TOP).unwrap();
+
+    // call a bogus system call
+    unsafe {
+        core::arch::asm!("syscall", in("rax") 134);
+    }
+
+    sys_exit(0);
+}
+
+fn load_elf(elf: &elf::Elf, target_as: AddressSpaceHandle) -> usize {
+    for header in elf.program_headers().unwrap() {
+        let header = header.unwrap();
+        if header.segment_type != elf::ProgramHeaderType::Load || header.memory_size == 0 {
+            continue;
+        }
+
+        let mut perms = 0;
+        if header.flags & 2 != 0 {
+            perms |= 1 << 0;
+        }
+        if header.flags & 1 != 0 {
+            perms |= 1 << 1;
+        }
+
+        let segment_start = header.virtual_address as usize;
+        let segment_end = segment_start + header.memory_size as usize;
+        let file_end = segment_start + header.file_size as usize;
+        let page_start = segment_start & !0xFFF;
+
+        for page in (page_start..segment_end).step_by(0x1000) {
+            let frame = sys_frame_alloc().unwrap();
+
+            let scratch_handle = sys_map(SELF_AS, frame, SCRATCH_PAGE, 0b01).unwrap();
+            unsafe {
+                core::ptr::write_bytes(SCRATCH_PAGE as *mut u8, 0, 0x1000);
+
+                let copy_start = page.max(segment_start).min(page + 0x1000);
+                let copy_end = (page + 0x1000).min(file_end).max(copy_start);
+                if copy_end > copy_start {
+                    let page_offset = copy_start - page;
+                    let file_offset = header.file_offset as usize + (copy_start - segment_start);
+                    let len = copy_end - copy_start;
+
+                    core::ptr::copy_nonoverlapping(
+                        elf.bytes().as_ptr().add(file_offset),
+                        (SCRATCH_PAGE as *mut u8).add(page_offset),
+                        len,
+                    );
+                }
+            }
+            sys_unmap(scratch_handle).unwrap();
+
+            sys_map(target_as, frame, page, perms).unwrap();
+        }
+    }
+
+    elf.entry()
+}
+
+fn map_stack(target_as: AddressSpaceHandle, stack_top: usize, pages: usize) {
+    for i in 1..=pages {
+        let frame = sys_frame_alloc().unwrap();
+        let page_addr = stack_top - i * 0x1000;
+        sys_map(target_as, frame, page_addr, 0b01).unwrap();
+    }
+}
+
+#[panic_handler]
+fn panic(info: &core::panic::PanicInfo) -> ! {
+    println!("{info}");
+    sys_exit(1);
+}