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

@@ -3,6 +3,9 @@ name = "dusk"
 version = "0.1.0"
 edition = "2024"
 
+[workspace]
+members = [".", "userspace/*"]
+
 [dependencies]
 limine = "0.6.5"
 

Makefile

@@ -6,17 +6,17 @@ 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
@@ -27,6 +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)
@@ -50,7 +51,7 @@ endif
 
 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 -Zjson-target-spec
@@ -63,13 +64,20 @@ prepare-bin-files:
 		# Make bin/ and bin/iso_root
 		mkdir -p ${ARTIFACTS_PATH}
 		mkdir -p ${ISO_PATH}
-		# mkdir -p ${INITRAMFS_PATH}
+		mkdir -p ${INITRAMFS_PATH}
-run-scripts:
-		# Place the build ID into the binary so it can be read at runtime
-		@HASH=$$(md5sum ${KERNEL_FILE} | cut -c1-12) && \
-		sed -i "s/__BUILD_ID__/$${HASH}/" ${KERNEL_FILE}
 
-		#python scripts/initramfs-test.py 100 ${INITRAMFS_PATH}/
+compile-user:
+		RUSTFLAGS="-C relocation-model=static" cargo build --package omega3 ${USERSPACE_CARGO_OPTS}
+		RUSTFLAGS="-C relocation-model=static" cargo build --package client ${USERSPACE_CARGO_OPTS}
+		RUSTFLAGS="-C relocation-model=static" cargo build --package echo ${USERSPACE_CARGO_OPTS}
+
+copy-initramfs-files: compile-user
+		cp -v target/${ARCH}-unknown-none/${MODE}/omega3 ${INITRAMFS_PATH}/omega3.elf
+		cp -v target/${ARCH}-unknown-none/${MODE}/client ${INITRAMFS_PATH}/client.elf
+		cp -v target/${ARCH}-unknown-none/${MODE}/echo ${INITRAMFS_PATH}/echo.elf
+
+compile-initramfs: copy-initramfs-files
+		(cd ${INITRAMFS_PATH} && find . -mindepth 1 | cpio -o -H newc) > ${ARTIFACTS_PATH}/initramfs.img
 
 copy-iso-files:
 		# Limine files
@@ -81,7 +89,7 @@ copy-iso-files:
 
 		# 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
 
 build-esp: copy-iso-files
 		# Create and populate formatted FAT image for ESP partition (130048 1K-blocks = ~127MiB)
@@ -129,7 +137,7 @@ compile-binaries:
 
 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
 

limine.conf

@@ -4,4 +4,5 @@ timeout: 0
     protocol: limine
     
     path: boot():/boot/dusk.elf
+    module_path: boot():/boot/initramfs.img
 

src/arch/mod.rs

@@ -5,4 +5,7 @@ mod x86_64;
 pub use x86_64::*;
 
 #[cfg(target_arch = "x86_64")]
-pub(crate) use x86_64::{PageTableCreateError, PageTableMapError, PageTableUnmapError};
+pub(crate) use x86_64::{
+    PageTableCreateError, PageTableMapError, PageTableUnmapError, ThreadContext, set_kernel_stack,
+    switch_context,
+};

src/arch/x86_64/apic.rs

@@ -13,7 +13,6 @@ use crate::{
     memory::{
         AddressSpace, CachePolicy, FrameAllocator, PagePermissions, PhysicalAddr, VirtualAddr,
     },
-    println,
 };
 
 const APIC_ID: u32 = 0x20;
@@ -37,7 +36,6 @@ const APIC_TIMER_INITIAL_COUNT: u32 = 0x380;
 const APIC_TIMER_CURRENT_COUNT: u32 = 0x390;
 const APIC_TIMER_DIVIDE_CONFIG: u32 = 0x3E0;
 
-#[derive(Debug)]
 enum LocalApicAccess {
     X2Apic,
     XApic,
@@ -89,7 +87,6 @@ pub enum LocalApicError {
     NotBootSystemProcessor,
 }
 
-#[derive(Debug)]
 pub struct LocalApic {
     id: u32,
     access: LocalApicAccess,
@@ -184,28 +181,6 @@ impl LocalApic {
         self.access.write(APIC_TIMER_INITIAL_COUNT, 0);
     }
 
-    // pub fn delay_ticks(&self, count: u32) {
-    //     if count == 0 {
-    //         return;
-    //     }
-
-    //     APIC_TIMER_COUNT.store(0, Ordering::SeqCst);
-
-    //     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, count as u64);
-    //     self.access.write(APIC_LVT_TIMER, APIC_TIMER_VECTOR as u64);
-
-    //     while APIC_TIMER_COUNT.load(Ordering::SeqCst) == 0 {
-    //         unsafe {
-    //             core::arch::asm!("sti", "hlt", "cli", options(nomem, nostack));
-    //         }
-    //     }
-
-    //     self.stop_timer();
-    // }
-
     pub fn id(&self) -> u32 {
         self.id
     }

src/arch/x86_64/cpu.rs

@@ -1,13 +1,109 @@
-use core::arch::asm;
+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, Debug)]
+#[derive(Clone, Copy)]
 pub(crate) struct CpuFeatures {
     pub nx_supported: bool,
     pub nx_enabled: bool,
@@ -40,6 +136,18 @@ pub fn detect_features_and_enable() -> Result<CpuFeatures, CpuFeaturesError> {
 
     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;

src/arch/x86_64/gdt.rs

@@ -3,8 +3,8 @@ use core::arch::asm;
 use crate::memory::VirtualAddr;
 
 #[repr(C, align(8))]
-struct Gdt {
+pub(super) struct Gdt {
-    entries: [u64; 7],
+    pub entries: [u64; 7],
 }
 
 impl Gdt {
@@ -49,12 +49,12 @@ 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 USER_CODE_SELECTOR: u16 = (3 * 8) | 3;
+pub(super) const USER_DATA_SELECTOR: u16 = (3 * 8) | 3;
-pub(super) const USER_DATA_SELECTOR: u16 = (4 * 8) | 3;
+pub(super) const USER_CODE_SELECTOR: u16 = (4 * 8) | 3;
 pub(super) const TSS_SELECTOR: u16 = 5 * 8;
 
 #[repr(align(16))]
-#[allow(dead_code)] // field 0 is read, rust just cant tell
+#[allow(unused)] // field 0 is read, rust just cant tell
 struct ExceptionStack([u8; DOUBLE_FAULT_STACK_SIZE]);
 
 static mut GDT: Gdt = Gdt::new();
@@ -78,8 +78,10 @@ pub fn init() {
                 0,
                 kernel_code_descriptor(),
                 kernel_data_descriptor(),
-                user_code_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,
             ],

src/arch/x86_64/interrupts/apic_vectors.rs

@@ -1,35 +1,40 @@
-use crate::arch::{
+use super::idt::{self, InterruptFrame, stub_no_err};
-    apic, timer,
+use crate::arch::{apic, timer};
-    x86_64::interrupts::idt::{self, InterruptStackFrame},
-};
 
 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;
 
-extern "x86-interrupt" fn error_handler(_frame: InterruptStackFrame) {
+stub_no_err!(stub_pit_calibration, 0xF1);
-    apic::record_error();
+stub_no_err!(stub_apic_timer, 0xFD);
-    apic::end_of_interrupt();
+stub_no_err!(stub_apic_error, 0xFE);
-}
+stub_no_err!(stub_apic_spurious, 0xFF);
 
-extern "x86-interrupt" fn timer_handler(_frame: InterruptStackFrame) {
+pub(super) fn handle(frame: &mut InterruptFrame) {
-    apic::record_timer();
+    match frame.vector as u8 {
-    apic::end_of_interrupt();
+        PIT_CALIBRATION_VECTOR => {
-}
+            timer::record_pit_calibration();
-
+            apic::end_of_interrupt();
-extern "x86-interrupt" fn pit_calibration_handler(_frame: InterruptStackFrame) {
+        }
-    timer::record_pit_calibration();
+        APIC_TIMER_VECTOR => {
-    apic::end_of_interrupt();
+            apic::record_timer();
-}
+            apic::end_of_interrupt();
-
+        }
-extern "x86-interrupt" fn spurious_handler(_frame: InterruptStackFrame) {
+        APIC_ERROR_VECTOR => {
-    // No EOI
+            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, pit_calibration_handler, 0);
+    idt.set_handler(PIT_CALIBRATION_VECTOR, stub_pit_calibration, 0);
-    idt.set_handler(APIC_ERROR_VECTOR, error_handler, 0);
+    idt.set_handler(APIC_ERROR_VECTOR, stub_apic_error, 0);
-    idt.set_handler(APIC_TIMER_VECTOR, timer_handler, 0);
+    idt.set_handler(APIC_TIMER_VECTOR, stub_apic_timer, 0);
-    idt.set_handler(APIC_SPURIOUS_VECTOR, spurious_handler, 0);
+    idt.set_handler(APIC_SPURIOUS_VECTOR, stub_apic_spurious, 0);
 }

src/arch/x86_64/interrupts/exceptions.rs

@@ -1,79 +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);
+
+const EXCEPTION_NAMES: [&str; 32] = [
+    "DIVIDE ERROR",
+    "DEBUG",
+    "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",
+];
+
+pub(super) fn handle(frame: &mut InterruptFrame) {
+    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");
+
+    if !is_user {
+        if vector == 14 {
+            report_exception(name, &frame.stack_frame, Some(frame.error_code));
+            println!("Faulting address: {:#X}", read_cr2());
+            print_page_fault_error(frame.error_code);
+            hcf();
         }
-    };
-}
 
-macro_rules! fatal_with_error_code {
+        fatal_exception(name, &frame.stack_frame, Some(frame.error_code));
-    ($handler:ident, $name:literal) => {
-        extern "x86-interrupt" fn $handler(frame: InterruptStackFrame, error_code: u64) {
-            fatal_exception($name, &frame, Some(error_code));
-        }
-    };
-}
-
-extern "x86-interrupt" fn breakpoint_handler(frame: InterruptStackFrame) {
-    report_exception("BREAKPOINT", &frame, None);
-}
-
-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);
-    hcf();
-}
-
-fatal_without_error_code!(divide_error_handler, "DIVIDE ERROR");
-fatal_without_error_code!(debug_handler, "DEBUG EXCEPTION");
-fatal_without_error_code!(non_maskable_interrupt_handler, "NON-MASKABLE INTERRUPT");
-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!(double_fault_handler, "DOUBLE FAULT");
-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");
-
-extern "x86-interrupt" fn user_test_exit_handler(frame: InterruptStackFrame) {
-    if frame.code_segment & 0b11 != 3 {
-        panic!("user_test_exit_handler called from kernel");
     }
 
-    println!("User test exit");
+    let fault = match vector {
-    hcf();
+        // 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));
 }
 
 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_user_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, user_test_exit_handler, 0);
+    idt.set_user_handler(0x80, stub_user_test_exit, 0);
 }
 
 fn read_cr2() -> u64 {

src/arch/x86_64/interrupts/idt.rs

@@ -18,7 +18,7 @@ struct IdtEntry {
 
 impl IdtEntry {
     const fn missing() -> Self {
-        return Self {
+        Self {
             offset_low: 0,
             code_selector: 0,
             ist: 0,
@@ -26,7 +26,7 @@ impl IdtEntry {
             offset_middle: 0,
             offset_high: 0,
             reserved: 0,
-        };
+        }
     }
 }
 
@@ -38,7 +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,
@@ -46,14 +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],
@@ -66,20 +88,11 @@ 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, 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,
-        vector: u8,
-        handler: ErrorCodeHandler,
-        ist: u8,
-    ) {
-        self.set_handler_address(vector, handler as usize, ist, KERNEL_INTERRUPT_GATE);
-    }
-
-    pub(super) fn set_user_handler(&mut self, vector: u8, handler: Handler, ist: u8) {
         self.set_handler_address(vector, handler as usize, ist, USER_INTERRUPT_GATE);
     }
 
@@ -101,6 +114,110 @@ static mut IDT: Idt = Idt::new();
 const _: () = assert!(core::mem::size_of::<IdtEntry>() == 16);
 const _: () = assert!(core::mem::size_of::<IdtPointer>() == 10);
 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();

src/arch/x86_64/interrupts/mod.rs

@@ -6,6 +6,33 @@ mod idt;
 
 pub use idt::idt_init as init;
 
+#[inline(always)]
+pub fn disable_interrupts_and_save() -> u64 {
+    let flags: u64;
+
+    unsafe {
+        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 {

src/arch/x86_64/io_apic.rs

@@ -3,7 +3,6 @@ use crate::{
         AddressSpace, CachePolicy, FrameAllocator, PagePermissions, PhysicalAddr, VirtualAddr,
     },
     platform::acpi::{InterruptPolarity, TriggerMode},
-    println,
 };
 
 const IOWIN: usize = 0x10;
@@ -32,7 +31,6 @@ pub struct RedirectionConfig {
     pub trigger: TriggerMode,
 }
 
-#[derive(Debug)]
 pub struct IoApic {
     base: VirtualAddr,
     global_interrupt_base: u32,

src/arch/x86_64/mod.rs

@@ -1,16 +1,18 @@
-pub mod apic;
+pub(super) mod apic;
 mod cpu;
 mod gdt;
 mod interrupts;
-pub mod io_apic;
+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,
 };
@@ -22,11 +24,7 @@ pub struct ArchState {
 
 use crate::{
     KernelHandoff,
-    arch::{
+    arch::x86_64::cpu::BOOT_CPU,
-        apic::LocalApic,
-        io_apic::{IOAPIC_VIRTUAL_ADDRESS, IoApic},
-        x86_64::interrupts::apic_vectors::PIT_CALIBRATION_VECTOR,
-    },
     memory::{AddressSpace, FrameAllocator, VirtualAddr},
     platform::acpi::Madt,
     println,
@@ -45,6 +43,14 @@ pub fn init() -> ArchState {
     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 {
@@ -58,10 +64,9 @@ pub enum InterruptInitError {
     PitNotHandled,
 }
 
-#[derive(Debug)]
 pub struct InterruptController {
-    local_apic: LocalApic,
+    local_apic: apic::LocalApic,
-    io_apic: IoApic,
+    io_apic: io_apic::IoApic,
     local_timer_frequency: u64,
 }
 
@@ -89,7 +94,7 @@ pub fn init_interrupt_controller(
         io_apic_info.id,
         io_apic_info.apic_address,
         io_apic_info.global_system_interrupt_base,
-        IOAPIC_VIRTUAL_ADDRESS,
+        io_apic::IOAPIC_VIRTUAL_ADDRESS,
         allocator,
         address_space,
     )
@@ -107,7 +112,7 @@ pub fn init_interrupt_controller(
         .configure_masked(
             pit_route.gsi,
             io_apic::RedirectionConfig {
-                vector: PIT_CALIBRATION_VECTOR,
+                vector: interrupts::apic_vectors::PIT_CALIBRATION_VECTOR,
                 destination,
                 polarity: pit_route.polarity,
                 trigger: pit_route.trigger,
@@ -126,37 +131,6 @@ pub fn init_interrupt_controller(
     })
 }
 
-// #[derive(Debug)]
-// pub enum TimerError {
-//     DurationOverflow,
-// }
-
-// impl InterruptController {
-//     pub fn delay(&self, duration: core::time::Duration) -> Result<(), TimerError> {
-//         let nanoseconds = duration.as_nanos();
-
-//         let ticks = (nanoseconds
-//             .checked_mul(self.local_timer_frequency as u128)
-//             .ok_or(TimerError::DurationOverflow)?
-//             + 999_999_999)
-//             / 1_000_000_000;
-
-//         if ticks == 0 {
-//             return Ok(());
-//         }
-
-//         let mut remaining = ticks;
-
-//         while remaining > 0 {
-//             let chunk = remaining.min(u32::MAX as u128) as u32;
-//             self.local_apic.delay_ticks(chunk);
-//             remaining -= chunk as u128;
-//         }
-
-//         Ok(())
-//     }
-// }
-
 /// # Safety
 ///
 /// The caller must ensure:
@@ -165,6 +139,9 @@ 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",
@@ -186,20 +163,39 @@ pub unsafe fn enter_user(
     user_instruction_pointer: VirtualAddr,
     user_stack_pointer: VirtualAddr,
 ) -> ! {
-    println!("Entering user mode");
-
     unsafe {
         asm!(
             "mov ds, {user_data_selector:x}",
             "mov es, {user_data_selector:x}",
             "mov fs, {user_data_selector:x}",
-            "mov gs, {user_data_selector:x}", // ss is handled by iretq
 
             "push {user_data_selector}",
             "push {user_stack_pointer}",
-            "pushfq",
+            "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,

src/arch/x86_64/paging.rs

@@ -4,14 +4,14 @@ use crate::{
     arch::x86_64::cpu::CpuFeatures,
     memory::{
         CachePolicy, DirectMap, FrameAddr, FrameAllocator, OwnedFrame, PagePermissions,
-        PhysicalAddr, VirtualAddr,
+        PageTableMapping, PhysicalAddr, VirtualAddr,
     },
 };
 
 pub const PAGE_SIZE: usize = 4096;
 pub const PAGE_TABLE_ENTRIES: usize = 512;
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub struct PagingConfig {
     physical_address_bits: u8,
     global_pages: bool,
@@ -42,7 +42,7 @@ impl PagingConfig {
     }
 }
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 enum PagingMode {
     FourLevel,
     FiveLevel,
@@ -77,7 +77,7 @@ impl PagingMode {
     }
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[derive(Clone, Copy, PartialEq, Eq)]
 enum PageTableLevel {
     Pml5,
     Pml4,
@@ -106,14 +106,13 @@ impl PageTableLevel {
     }
 }
 
-#[derive(Debug)]
 enum PageTableEntryError {
     PhysicalAddressTooLarge,
     NoExecuteUnsupported,
 }
 
 #[repr(transparent)]
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[derive(Clone, Copy, PartialEq, Eq)]
 struct PageTableEntry(u64);
 
 impl PageTableEntry {
@@ -200,6 +199,14 @@ impl PageTableEntry {
         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
     }
@@ -227,9 +234,16 @@ impl PageTableEntry {
 
         FrameAddr::from_start_address(self.physical_address(config))
     }
+
+    fn permissions(&self) -> PagePermissions {
+        PagePermissions::new(
+            self.writable(),
+            self.executable(),
+            self.is_user_accessible(),
+        )
+    }
 }
 
-#[derive(Debug)]
 pub(crate) enum MapError {
     InvalidVirtualAddress,
     VirtualAddressUnaligned,
@@ -270,6 +284,14 @@ pub struct PageTable {
     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,
@@ -357,6 +379,52 @@ impl PageTable {
         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,
@@ -381,16 +449,6 @@ impl PageTable {
             .ok_or(UnmapError::PageNotMapped)
     }
 
-    fn discard_private_tables(
-        frames: &mut [Option<OwnedFrame>; MAX_INTERMEDIATE_LEVELS],
-        count: usize,
-        allocator: &mut FrameAllocator,
-    ) {
-        for frame in frames[..count].iter_mut().rev().filter_map(Option::take) {
-            unsafe { allocator.dealloc(frame) };
-        }
-    }
-
     fn apply_user_upgrades(
         &mut self,
         upgrades: &[Option<EntryLocation>; MAX_INTERMEDIATE_LEVELS],
@@ -500,34 +558,18 @@ impl PageTable {
         let private_table_count = levels.len() - missing_depth;
         let mut private_tables: [Option<OwnedFrame>; MAX_INTERMEDIATE_LEVELS] =
             core::array::from_fn(|_| None);
-        let mut allocated_count = 0;
-
-        while allocated_count < private_table_count {
-            let private_frame = match allocator.alloc() {
-                Some(frame) => frame,
-                None => {
-                    Self::discard_private_tables(&mut private_tables, allocated_count, allocator);
-                    return Err(MapError::OutOfFrames);
-                }
-            };
-
-            if PageTableEntry::new_table(
-                private_frame.frame_address(),
-                permissions.user_accessible,
-                self.config,
-            )
-            .is_err()
-            {
-                unsafe { allocator.dealloc(private_frame) };
-                Self::discard_private_tables(&mut private_tables, allocated_count, allocator);
-                return Err(MapError::PhysicalAddressTooLarge);
-            }
-
-            private_tables[allocated_count] = Some(private_frame);
-            allocated_count += 1;
-        }
 
         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()
@@ -563,7 +605,9 @@ impl PageTable {
         let publication_entry = match prepare_result {
             Ok(entry) => entry,
             Err(error) => {
-                Self::discard_private_tables(&mut private_tables, allocated_count, allocator);
+                for frame in private_tables.into_iter().rev().flatten() {
+                    unsafe { allocator.dealloc(frame) };
+                }
                 return Err(error);
             }
         };
@@ -574,10 +618,7 @@ impl PageTable {
             [publication_location.index] = publication_entry;
 
         // The published page table now owns these frames.
-        for frame in private_tables[..allocated_count]
+        for frame in private_tables.into_iter().flatten() {
-            .iter_mut()
-            .flat_map(Option::take)
-        {
             let _ = frame.into_raw();
         }
 

src/arch/x86_64/pit.rs

@@ -13,15 +13,11 @@ const BINARY: u8 = 0;
 pub const PIT_FREQUENCY: u64 = 1_193_182;
 pub const PIT_CALIBRATION_COUNT: u16 = u16::MAX;
 
-pub struct Pit;
+pub fn start_pit_one_shot(count: u16) {
+    unsafe {
+        write_u8(PIT_COMMAND, CHANNEL_0 | LOW_HIGH | MODE_0 | BINARY);
 
-impl Pit {
+        write_u8(PIT_CHANNEL_0, count as u8);
-    pub fn start_one_shot(count: u16) {
+        write_u8(PIT_CHANNEL_0, (count >> 8) as u8);
-        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/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

@@ -7,7 +7,7 @@ use crate::{
         io_apic::IoApic,
         x86_64::{
             interrupts::enable_interrupts,
-            pit::{PIT_CALIBRATION_COUNT, PIT_FREQUENCY, Pit},
+            pit::{PIT_CALIBRATION_COUNT, PIT_FREQUENCY, start_pit_one_shot},
         },
     },
     platform::acpi::IsaIrqRoute,
@@ -37,7 +37,7 @@ pub fn calibrate_local_apic(
         .map_err(|_| TimerCalibrationError::IoApicNotHandled)?;
     local_apic.start_calibration_counter();
 
-    Pit::start_one_shot(PIT_CALIBRATION_COUNT);
+    start_pit_one_shot(PIT_CALIBRATION_COUNT);
 
     enable_interrupts();
 

src/boot/limine.rs

@@ -1,13 +1,13 @@
 use ::limine as limine_api;
 use limine::paging::PagingMode;
-use limine::request::{PagingModeRequest, RsdpRequest};
+use limine::request::{ExecutableCmdlineRequest, ModulesRequest, PagingModeRequest, RsdpRequest};
 
 use limine_api::request::{ExecutableAddressRequest, HhdmRequest, MemmapRequest};
 use limine_api::{BaseRevision, RequestsEndMarker, RequestsStartMarker};
 
 use crate::memory::{
-    KernelMemoryLayout, KernelSegment, MemoryMap, MemoryRegion, MemoryRegionKind, PagePermissions,
+    BootString, InitramfsImage, KernelMemoryLayout, KernelSegment, MemoryMap, MemoryRegion,
-    PhysicalAddr, VirtualAddr,
+    MemoryRegionKind, PagePermissions, PhysicalAddr, VirtualAddr,
 };
 
 /// Sets the base revision to the latest revision supported by the crate.
@@ -22,6 +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();
@@ -58,8 +66,12 @@ unsafe extern "C" {
     static __data_end: u64;
 }
 
+const MAX_COMMAND_LINE_LENGTH: usize = 512;
+
 pub struct BootInfo {
     pub kernel_layout: KernelMemoryLayout,
+    pub command_line: BootString<MAX_COMMAND_LINE_LENGTH>,
+    pub initramfs: InitramfsImage,
     pub hhdm_offset: usize,
     pub memory_map: MemoryMap,
     pub rsdp: VirtualAddr,
@@ -75,6 +87,9 @@ impl BootInfo {
 pub enum BootError {
     UnsupportedBaseRevision,
     FailedToGetKernelAddress,
+    FailedToGetKernelCmdline,
+    FailedToGetModules,
+    FailedToGetInitramfs,
     FailedToGetHHDMAddress,
     FailedToGetMemmap,
     TooManyMemoryRegions,
@@ -90,10 +105,28 @@ pub fn load_boot_info() -> Result<BootInfo, BootError> {
     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;
+        .offset as usize;
 
     let rsdp = RSDP_REQUEST.response().ok_or(BootError::FailedToGetRsdp)?;
 
@@ -204,7 +237,9 @@ pub fn load_boot_info() -> Result<BootInfo, BootError> {
                 kernel_data_segment,
             ],
         },
-        hhdm_offset: hhdm_offset as usize,
+        command_line,
+        initramfs,
+        hhdm_offset,
         memory_map,
         rsdp: VirtualAddr::new(rsdp.address as usize),
     })

src/debug/serial.rs

@@ -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]
@@ -6,14 +5,15 @@
 mod arch;
 mod boot;
 mod debug;
+mod format;
 mod memory;
 mod platform;
+mod syscall;
+mod task;
 
 use crate::{
     debug::serial,
-    memory::{
+    memory::{AddressSpace, MemoryRegionKind, init_frame_allocator, init_kernel_address_space},
-        AddressSpace, KernelStack, MemoryRegionKind, PagePermissions, UserStack, VirtualAddr,
-    },
 };
 
 pub struct KernelHandoff {
@@ -49,8 +49,9 @@ pub extern "C" fn _start() -> ! {
 
     println!("Entering kernel main...");
 
-    let bootstrap_stack = KernelStack::allocate(&mut address_space, &mut allocator)
+    let kernel_stack =
-        .expect("failed to allocate bootstrap stack");
+        crate::task::scheduler::allocate_kernel_stack(&mut address_space, &mut allocator)
+            .expect("failed to allocate bootstrap stack");
 
     let handoff_frame = allocator
         .alloc()
@@ -59,7 +60,7 @@ pub extern "C" fn _start() -> ! {
         .to_virtual(handoff_frame.frame_address().start_address())
         .expect("failed to map kernel handoff");
 
-    let bootstrap_stack_top = bootstrap_stack.top();
+    let bootstrap_stack_top = kernel_stack.top();
     let handoff = KernelHandoff {
         allocator,
         address_space,
@@ -106,56 +107,31 @@ pub unsafe extern "C" fn kernel_main(handoff: *mut KernelHandoff) -> ! {
 
     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");
 
-    let interrupt_controller =
+    println!("Initializing interrupt controller...");
+
+    let _interrupt_controller =
         arch::init_interrupt_controller(&madt, &mut allocator, &mut address_space)
             .expect("failed to initialize interrupt controller");
 
-    let mut user_addr_space = address_space
+    task::bootstrap::spawn(
-        .new_user(&mut allocator)
+        "omega3.elf",
-        .expect("failed to create user address space");
+        &boot_info.initramfs,
+        &mut address_space,
+        &mut allocator,
+        direct_map,
+    );
 
-    let user_stack = UserStack::allocate(&mut user_addr_space, &mut allocator)
+    init_frame_allocator(allocator);
-        .expect("failed to allocate user stack");
+    init_kernel_address_space(address_space);
 
-    let user_instruction_pointer = VirtualAddr::new(0x8000);
+    task::scheduler::start();
-    let user_code_page = allocator
-        .alloc()
-        .expect("failed to allocate user code page")
-        .frame_address();
-    user_addr_space
-        .map(
-            user_code_page.start_address(),
-            user_instruction_pointer,
-            PagePermissions::new(true, true, true),
-            &mut allocator,
-            memory::CachePolicy::WriteBack,
-        )
-        .expect("failed to map user code page");
-
-    unsafe {
-        user_addr_space.activate();
-
-        let user_code: [u8; 5] = [
-            0xCC, // INT3
-            0xCD, 0x80, // INT 0x80
-            0xEB, 0xFE, // JMP -2 (loop forever if exit returns)
-        ];
-
-        core::ptr::copy_nonoverlapping(
-            user_code.as_ptr(),
-            user_instruction_pointer.as_mut_ptr::<u8>(),
-            user_code.len(),
-        );
-
-        arch::enter_user(user_instruction_pointer, user_stack.top());
-    }
-
-    hcf();
 }
 
 #[panic_handler]

src/memory/address_space.rs

@@ -1,11 +1,118 @@
+use core::cell::UnsafeCell;
+
 use crate::{
     arch::{PageTable, PageTableCreateError, PageTableMapError, PageTableUnmapError, PagingConfig},
     memory::{
         CachePolicy, DirectMap, FRAME_SIZE, FrameAddr, FrameAllocator, KernelMemoryLayout,
-        MemoryRegion, MemoryRegionKind, PagePermissions, PhysicalAddr, VirtualAddr,
+        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,
@@ -82,12 +189,17 @@ impl From<PageTableCreateError> for AddressSpaceCreateError {
     }
 }
 
-#[derive(Debug, PartialEq, Eq)]
+pub struct PageTableMapping {
+    pub permissions: PagePermissions,
+}
+
+#[derive(PartialEq, Eq)]
 enum AddressSpaceKind {
     Kernel,
     User,
 }
 
+#[derive(PartialEq, Eq)]
 pub struct AddressSpace {
     root: PageTable,
     kind: AddressSpaceKind,
@@ -112,13 +224,18 @@ impl AddressSpace {
             // undermind the permissions of the explicitly mapped kernel image
             if matches!(
                 region.kind,
-                MemoryRegionKind::Reserved
+                MemoryRegionKind::Reserved | MemoryRegionKind::BadMemory
-                    | MemoryRegionKind::BadMemory
-                    | MemoryRegionKind::KernelAndModules
             ) {
                 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
@@ -194,7 +311,7 @@ impl AddressSpace {
         let global = self.kind == AddressSpaceKind::Kernel;
 
         if self.kind == AddressSpaceKind::User {
-            if virtual_addr.as_usize() >= 0x0000_8000_0000_0000 {
+            if virtual_addr.as_usize() >= USER_SPACE_END.as_usize() {
                 return Err(MapError::InvalidUserAddress);
             }
 
@@ -320,6 +437,10 @@ impl AddressSpace {
         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() }
     }

src/memory/frame.rs

@@ -1,3 +1,5 @@
+use core::cell::UnsafeCell;
+
 use crate::memory::{DirectMap, MemoryRegion, MemoryRegionKind, PhysicalAddr, VirtualAddr};
 
 pub const FRAME_SIZE: usize = 4096;
@@ -12,15 +14,57 @@ pub fn align_down_to_frame(addr: usize) -> usize {
 }
 
 #[repr(u8)]
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[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
-#[derive(Debug)]
 struct Bitmap {
     start: VirtualAddr,
     frame_count: usize,
@@ -70,7 +114,6 @@ pub enum FrameAllocatorInitError {
 }
 
 // very very simple bitmap frame/page allocator
-#[derive(Debug)]
 pub struct FrameAllocator {
     bitmap: Bitmap,
     next_search: usize,
@@ -327,7 +370,6 @@ impl FrameAddr {
 }
 
 // specifically not Clone or Copy
-#[derive(Debug)]
 pub struct OwnedFrame {
     frame: FrameAddr,
 }

src/memory/mod.rs

@@ -1,11 +1,53 @@
 mod address_space;
 mod frame;
-pub mod stack;
+mod stack;
+mod user;
+
+use core::ops::Add;
 
 #[allow(unused)]
-pub use address_space::{AddressSpace, AddressSpaceCreateError, MapError, UnmapError};
+pub use address_space::{
-pub use frame::{FRAME_SIZE, FrameAddr, FrameAllocator, OwnedFrame};
+    AddressSpace, AddressSpaceCreateError, AddressSpaceId, MapError, PageTableMapping, UnmapError,
-pub use stack::{KernelStack, StackCreateError, UserStack};
+    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,
@@ -18,7 +60,7 @@ pub struct KernelMemoryLayout {
     pub segments: [KernelSegment; 3],
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[derive(Clone, Copy, PartialEq, Eq)]
 pub struct PagePermissions {
     pub writable: bool,
     pub executable: bool,
@@ -71,8 +113,16 @@ impl VirtualAddr {
     }
 }
 
+impl Add<usize> for VirtualAddr {
+    type Output = Self;
+
+    fn add(self, rhs: usize) -> Self::Output {
+        Self(self.0 + rhs)
+    }
+}
+
 #[repr(u8)]
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[derive(Clone, Copy, PartialEq, Eq)]
 pub enum MemoryRegionKind {
     Usable,
     Reserved,
@@ -85,20 +135,20 @@ pub enum MemoryRegionKind {
     MappedReserved,
 }
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub enum CachePolicy {
     Uncacheable,
     WriteBack,
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[derive(Clone, Copy, PartialEq, Eq)]
 pub struct MemoryRegion {
     pub start: PhysicalAddr,
     pub length: usize,
     pub kind: MemoryRegionKind,
 }
 
-#[derive(Debug, Clone, Copy)]
+#[derive(Clone, Copy)]
 pub struct DirectMap {
     offset: usize,
 }

src/memory/stack.rs

@@ -3,10 +3,16 @@ use crate::memory::{
     VirtualAddr,
 };
 
-const STACK_PAGES: usize = 16;
+const GUARD_PAGES: usize = 1; // 4KiB
-const STACK_SIZE: usize = STACK_PAGES * FRAME_SIZE;
+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 KERNEL_STACK_TOP: VirtualAddr = VirtualAddr::new(0xFFFF_FFFE_0000_0000);
 const USER_STACK_TOP: VirtualAddr = VirtualAddr::new(0x0000_7FFF_FFFF_F000);
 
 #[derive(Debug)]
@@ -14,13 +20,16 @@ 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,
 }
 
@@ -28,6 +37,7 @@ impl StackMapping {
     fn allocate(
         address_space: &mut AddressSpace,
         allocator: &mut FrameAllocator,
+        stack_size: usize,
         top: VirtualAddr,
         permissions: PagePermissions,
     ) -> Result<Self, StackCreateError> {
@@ -37,7 +47,7 @@ impl StackMapping {
 
         let mapped_start = VirtualAddr::new(
             top.as_usize()
-                .checked_sub(STACK_SIZE)
+                .checked_sub(stack_size)
                 .ok_or(StackCreateError::AddressOverflow)?,
         );
         let guard_page = VirtualAddr::new(
@@ -53,7 +63,7 @@ impl StackMapping {
 
         let mut mapped_pages = 0;
 
-        while mapped_pages < STACK_PAGES {
+        while mapped_pages < stack_size / FRAME_SIZE {
             let virtual_address = VirtualAddr::new(
                 mapped_start
                     .as_usize()
@@ -89,6 +99,7 @@ impl StackMapping {
         Ok(Self {
             guard_page,
             mapped_start,
+            stack_size,
             top,
         })
     }
@@ -115,7 +126,7 @@ impl StackMapping {
     /// 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..STACK_PAGES).rev() {
+        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 {
@@ -130,6 +141,7 @@ impl StackMapping {
     }
 }
 
+#[derive(Debug)]
 pub struct KernelStack {
     mapping: StackMapping,
 }
@@ -137,12 +149,14 @@ pub struct KernelStack {
 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_TOP,
+            KERNEL_STACK_SIZE,
+            VirtualAddr::new(top),
             PagePermissions::new(true, false, false),
         )?;
 
@@ -175,6 +189,7 @@ impl UserStack {
         let mapping = StackMapping::allocate(
             address_space,
             allocator,
+            USER_STACK_SIZE,
             top,
             PagePermissions::new(true, false, true),
         )?;
@@ -194,3 +209,41 @@ impl UserStack {
         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

@@ -12,7 +12,6 @@ pub enum AcpiError {
     MultipleIoApicsUnsupported,
 }
 
-#[derive(Debug)]
 pub struct AcpiTables {
     direct_map: DirectMap,
     root: RootTable,
@@ -215,7 +214,6 @@ impl AcpiTables {
     }
 }
 
-#[derive(Debug)]
 enum RootTable {
     Rsdt(Sdt),
     Xsdt(Sdt),
@@ -251,7 +249,7 @@ impl RootTable {
 }
 
 #[repr(C, packed)]
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 struct Rsdp {
     signature: [u8; 8],
     checksum: u8,
@@ -261,7 +259,7 @@ struct Rsdp {
 }
 
 #[repr(C, packed)]
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 struct Xsdp {
     rsdp: Rsdp,
     length: u32,
@@ -271,7 +269,7 @@ struct Xsdp {
 }
 
 #[repr(C, packed)]
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 struct SDTHeader {
     signature: [u8; 4],
     length: u32,
@@ -284,14 +282,12 @@ struct SDTHeader {
     creator_revision: u32,
 }
 
-#[derive(Debug)]
 pub struct Sdt {
     physical_addr: PhysicalAddr,
     length: usize,
     signature: [u8; 4],
 }
 
-#[derive(Debug)]
 #[allow(unused)]
 pub struct Madt<'a> {
     acpi: &'a AcpiTables,
@@ -301,7 +297,7 @@ pub struct Madt<'a> {
 }
 
 #[repr(C, packed)]
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 struct MadtBody {
     local_apic_address: u32,
     flags: u32,
@@ -543,21 +539,20 @@ impl<'a> Iterator for MadtEntries<'a> {
 }
 
 #[repr(C, packed)]
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub struct MadtEntryHeader {
     kind: u8,
     length: u8,
 }
 
 #[repr(C, packed)]
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub struct LocalApicEntry {
     processor_id: u8,
     id: u8,
     flags: u32,
 }
 
-#[derive(Debug)]
 pub struct IoApicInfo {
     pub id: u8,
     pub apic_address: PhysicalAddr,
@@ -565,7 +560,7 @@ pub struct IoApicInfo {
 }
 
 #[repr(C, packed)]
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub struct IoApicEntry {
     id: u8,
     reserved: u8,
@@ -574,7 +569,7 @@ pub struct IoApicEntry {
 }
 
 #[repr(C, packed)]
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub struct InterruptSourceOverride {
     bus: u8,
     source: u8,
@@ -582,19 +577,19 @@ pub struct InterruptSourceOverride {
     flags: u16,
 }
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub enum InterruptPolarity {
     ActiveHigh,
     ActiveLow,
 }
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub enum TriggerMode {
     Edge,
     Level,
 }
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub struct IsaIrqRoute {
     pub gsi: u32,
     pub polarity: InterruptPolarity,
@@ -602,7 +597,7 @@ pub struct IsaIrqRoute {
 }
 
 #[repr(C, packed)]
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub struct IoApicNmiEntry {
     nmi_source: u8,
     reserved: u8,
@@ -611,7 +606,7 @@ pub struct IoApicNmiEntry {
 }
 
 #[repr(C, packed)]
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub struct LocalApicNmiEntry {
     processor_id: u8,
     flags: u16,
@@ -619,14 +614,14 @@ pub struct LocalApicNmiEntry {
 }
 
 #[repr(C, packed)]
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub struct LocalApicAddressOverride {
     reserved: u16,
     local_apic_address: u64,
 }
 
 #[repr(C, packed)]
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub struct LocalX2ApicEntry {
     reserved: u16,
     local_x2apic_id: u32,
@@ -634,7 +629,7 @@ pub struct LocalX2ApicEntry {
     acpi_processor_uid: u32,
 }
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 #[allow(unused)]
 pub enum MadtEntry {
     LocalApic(LocalApicEntry),

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);
+}