Fix register typo: rpb -> rbp

Fixes #746

Cargo.toml

@@ -33,3 +33,6 @@ test-args = [
     "-display", "none"
 ]
 test-success-exit-code = 33         # (0x10 << 1) | 1
+
+[profile.release]
+lto = true

src/allocator/bump.rs

@@ -0,0 +1,58 @@
+use super::{align_up, Locked};
+use alloc::alloc::{GlobalAlloc, Layout};
+use core::ptr;
+
+pub struct BumpAllocator {
+    heap_start: usize,
+    heap_end: usize,
+    next: usize,
+    allocations: usize,
+}
+
+impl BumpAllocator {
+    /// Creates a new empty bump allocator.
+    pub const fn new() -> Self {
+        BumpAllocator {
+            heap_start: 0,
+            heap_end: 0,
+            next: 0,
+            allocations: 0,
+        }
+    }
+
+    /// Initializes the bump allocator with the given heap bounds.
+    ///
+    /// This method is unsafe because the caller must ensure that the given
+    /// memory range is unused. Also, this method must be called only once.
+    pub unsafe fn init(&mut self, heap_start: usize, heap_size: usize) {
+        self.heap_start = heap_start;
+        self.heap_end = heap_start + heap_size;
+        self.next = heap_start;
+    }
+}
+
+unsafe impl GlobalAlloc for Locked<BumpAllocator> {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        let mut bump = self.lock(); // get a mutable reference
+
+        let alloc_start = align_up(bump.next, layout.align());
+        let alloc_end = alloc_start + layout.size();
+
+        if alloc_end > bump.heap_end {
+            ptr::null_mut() // out of memory
+        } else {
+            bump.next = alloc_end;
+            bump.allocations += 1;
+            alloc_start as *mut u8
+        }
+    }
+
+    unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {
+        let mut bump = self.lock(); // get a mutable reference
+
+        bump.allocations -= 1;
+        if bump.allocations == 0 {
+            bump.next = bump.heap_start;
+        }
+    }
+}

src/allocator/fixed_size_block.rs

@@ -0,0 +1,102 @@
+use super::Locked;
+use alloc::alloc::{GlobalAlloc, Layout};
+use core::{
+    mem,
+    ptr::{self, NonNull},
+};
+
+/// The block sizes to use.
+///
+/// The sizes must each be power of 2 because they are also used as
+/// the block alignment (alignments must be always powers of 2).
+const BLOCK_SIZES: &[usize] = &[8, 16, 32, 64, 128, 256, 512, 1024, 2048];
+
+/// Choose an appropriate block size for the given layout.
+///
+/// Returns an index into the `BLOCK_SIZES` array.
+fn list_index(layout: &Layout) -> Option<usize> {
+    let required_block_size = layout.size().max(layout.align());
+    BLOCK_SIZES.iter().position(|&s| s >= required_block_size)
+}
+
+struct ListNode {
+    next: Option<&'static mut ListNode>,
+}
+
+pub struct FixedSizeBlockAllocator {
+    list_heads: [Option<&'static mut ListNode>; BLOCK_SIZES.len()],
+    fallback_allocator: linked_list_allocator::Heap,
+}
+
+impl FixedSizeBlockAllocator {
+    /// Creates an empty FixedSizeBlockAllocator.
+    pub const fn new() -> Self {
+        FixedSizeBlockAllocator {
+            list_heads: [None; BLOCK_SIZES.len()],
+            fallback_allocator: linked_list_allocator::Heap::empty(),
+        }
+    }
+
+    /// Initialize the allocator with the given heap bounds.
+    ///
+    /// This function is unsafe because the caller must guarantee that the given
+    /// heap bounds are valid and that the heap is unused. This method must be
+    /// called only once.
+    pub unsafe fn init(&mut self, heap_start: usize, heap_size: usize) {
+        self.fallback_allocator.init(heap_start, heap_size);
+    }
+
+    /// Allocates using the fallback allocator.
+    fn fallback_alloc(&mut self, layout: Layout) -> *mut u8 {
+        match self.fallback_allocator.allocate_first_fit(layout) {
+            Ok(ptr) => ptr.as_ptr(),
+            Err(_) => ptr::null_mut(),
+        }
+    }
+}
+
+unsafe impl GlobalAlloc for Locked<FixedSizeBlockAllocator> {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        let mut allocator = self.lock();
+        match list_index(&layout) {
+            Some(index) => {
+                match allocator.list_heads[index].take() {
+                    Some(node) => {
+                        allocator.list_heads[index] = node.next.take();
+                        node as *mut ListNode as *mut u8
+                    }
+                    None => {
+                        // no block exists in list => allocate new block
+                        let block_size = BLOCK_SIZES[index];
+                        // only works if all block sizes are a power of 2
+                        let block_align = block_size;
+                        let layout = Layout::from_size_align(block_size, block_align).unwrap();
+                        allocator.fallback_alloc(layout)
+                    }
+                }
+            }
+            None => allocator.fallback_alloc(layout),
+        }
+    }
+
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+        let mut allocator = self.lock();
+        match list_index(&layout) {
+            Some(index) => {
+                let new_node = ListNode {
+                    next: allocator.list_heads[index].take(),
+                };
+                // verify that block has size and alignment required for storing node
+                assert!(mem::size_of::<ListNode>() <= BLOCK_SIZES[index]);
+                assert!(mem::align_of::<ListNode>() <= BLOCK_SIZES[index]);
+                let new_node_ptr = ptr as *mut ListNode;
+                new_node_ptr.write(new_node);
+                allocator.list_heads[index] = Some(&mut *new_node_ptr);
+            }
+            None => {
+                let ptr = NonNull::new(ptr).unwrap();
+                allocator.fallback_allocator.deallocate(ptr, layout);
+            }
+        }
+    }
+}

src/allocator/linked_list.rs

@@ -0,0 +1,145 @@
+use super::{align_up, Locked};
+use alloc::alloc::{GlobalAlloc, Layout};
+use core::{mem, ptr};
+
+struct ListNode {
+    size: usize,
+    next: Option<&'static mut ListNode>,
+}
+
+impl ListNode {
+    const fn new(size: usize) -> Self {
+        ListNode { size, next: None }
+    }
+
+    fn start_addr(&self) -> usize {
+        self as *const Self as usize
+    }
+
+    fn end_addr(&self) -> usize {
+        self.start_addr() + self.size
+    }
+}
+
+pub struct LinkedListAllocator {
+    head: ListNode,
+}
+
+impl LinkedListAllocator {
+    /// Creates an empty LinkedListAllocator.
+    pub const fn new() -> Self {
+        Self {
+            head: ListNode::new(0),
+        }
+    }
+
+    /// Initialize the allocator with the given heap bounds.
+    ///
+    /// This function is unsafe because the caller must guarantee that the given
+    /// heap bounds are valid and that the heap is unused. This method must be
+    /// called only once.
+    pub unsafe fn init(&mut self, heap_start: usize, heap_size: usize) {
+        self.add_free_region(heap_start, heap_size);
+    }
+
+    /// Adds the given memory region to the front of the list.
+    unsafe fn add_free_region(&mut self, addr: usize, size: usize) {
+        // ensure that the freed region is capable of holding ListNode
+        assert!(align_up(addr, mem::align_of::<ListNode>()) == addr);
+        assert!(size >= mem::size_of::<ListNode>());
+
+        // create a new list node and append it at the start of the list
+        let mut node = ListNode::new(size);
+        node.next = self.head.next.take();
+        let node_ptr = addr as *mut ListNode;
+        node_ptr.write(node);
+        self.head.next = Some(&mut *node_ptr)
+    }
+
+    /// Looks for a free region with the given size and alignment and removes
+    /// it from the list.
+    ///
+    /// Returns a tuple of the list node and the start address of the allocation.
+    fn find_region(&mut self, size: usize, align: usize) -> Option<(&'static mut ListNode, usize)> {
+        // reference to current list node, updated for each iteration
+        let mut current = &mut self.head;
+        // look for a large enough memory region in linked list
+        while let Some(ref mut region) = current.next {
+            if let Ok(alloc_start) = Self::alloc_from_region(&region, size, align) {
+                // region suitable for allocation -> remove node from list
+                let next = region.next.take();
+                let ret = Some((current.next.take().unwrap(), alloc_start));
+                current.next = next;
+                return ret;
+            } else {
+                // region not suitable -> continue with next region
+                current = current.next.as_mut().unwrap();
+            }
+        }
+
+        // no suitable region found
+        None
+    }
+
+    /// Try to use the given region for an allocation with given size and alignment.
+    ///
+    /// Returns the allocation start address on success.
+    fn alloc_from_region(region: &ListNode, size: usize, align: usize) -> Result<usize, ()> {
+        let alloc_start = align_up(region.start_addr(), align);
+        let alloc_end = alloc_start + size;
+
+        if alloc_end > region.end_addr() {
+            // region too small
+            return Err(());
+        }
+
+        let excess_size = region.end_addr() - alloc_end;
+        if excess_size > 0 && excess_size < mem::size_of::<ListNode>() {
+            // rest of region too small to hold a ListNode (required because the
+            // allocation splits the region in a used and a free part)
+            return Err(());
+        }
+
+        // region suitable for allocation
+        Ok(alloc_start)
+    }
+
+    /// Adjust the given layout so that the resulting allocated memory
+    /// region is also capable of storing a `ListNode`.
+    ///
+    /// Returns the adjusted size and alignment as a (size, align) tuple.
+    fn size_align(layout: Layout) -> (usize, usize) {
+        let layout = layout
+            .align_to(mem::align_of::<ListNode>())
+            .expect("adjusting alignment failed")
+            .pad_to_align();
+        let size = layout.size().max(mem::size_of::<ListNode>());
+        (size, layout.align())
+    }
+}
+
+unsafe impl GlobalAlloc for Locked<LinkedListAllocator> {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        // perform layout adjustments
+        let (size, align) = LinkedListAllocator::size_align(layout);
+        let mut allocator = self.inner.lock();
+
+        if let Some((region, alloc_start)) = allocator.find_region(size, align) {
+            let alloc_end = alloc_start + size;
+            let excess_size = region.end_addr() - alloc_end;
+            if excess_size > 0 {
+                allocator.add_free_region(alloc_end, excess_size);
+            }
+            alloc_start as *mut u8
+        } else {
+            ptr::null_mut()
+        }
+    }
+
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+        // perform layout adjustments
+        let (size, _) = LinkedListAllocator::size_align(layout);
+
+        self.inner.lock().add_free_region(ptr as usize, size)
+    }
+}

src/allocator/mod.rs

@@ -1,6 +1,6 @@
 use alloc::alloc::{GlobalAlloc, Layout};
 use core::ptr::null_mut;
-use linked_list_allocator::LockedHeap;
+use fixed_size_block::FixedSizeBlockAllocator;
 use x86_64::{
     structures::paging::{
         mapper::MapToError, FrameAllocator, Mapper, Page, PageTableFlags, Size4KiB,
@@ -8,11 +8,15 @@ use x86_64::{
     VirtAddr,
 };
 
+pub mod bump;
+pub mod fixed_size_block;
+pub mod linked_list;
+
 pub const HEAP_START: usize = 0x_4444_4444_0000;
 pub const HEAP_SIZE: usize = 100 * 1024; // 100 KiB
 
 #[global_allocator]
-static ALLOCATOR: LockedHeap = LockedHeap::empty();
+static ALLOCATOR: Locked<FixedSizeBlockAllocator> = Locked::new(FixedSizeBlockAllocator::new());
 
 pub fn init_heap(
     mapper: &mut impl Mapper<Size4KiB>,
@@ -52,3 +56,29 @@ unsafe impl GlobalAlloc for Dummy {
         panic!("dealloc should be never called")
     }
 }
+
+/// A wrapper around spin::Mutex to permit trait implementations.
+pub struct Locked<A> {
+    inner: spin::Mutex<A>,
+}
+
+impl<A> Locked<A> {
+    pub const fn new(inner: A) -> Self {
+        Locked {
+            inner: spin::Mutex::new(inner),
+        }
+    }
+
+    pub fn lock(&self) -> spin::MutexGuard<A> {
+        self.inner.lock()
+    }
+}
+
+fn align_up(addr: usize, align: usize) -> usize {
+    let remainder = addr % align;
+    if remainder == 0 {
+        addr // addr already aligned
+    } else {
+        addr - remainder + align
+    }
+}

src/interrupts.rs

@@ -77,6 +77,7 @@ extern "x86-interrupt" fn timer_interrupt_handler(_stack_frame: &mut InterruptSt
         PICS.lock()
             .notify_end_of_interrupt(InterruptIndex::Timer.as_u8());
     }
+    crate::multitasking::invoke_scheduler();
 }
 
 extern "x86-interrupt" fn keyboard_interrupt_handler(_stack_frame: &mut InterruptStackFrame) {

src/lib.rs

@@ -3,6 +3,15 @@
 #![feature(custom_test_frameworks)]
 #![feature(abi_x86_interrupt)]
 #![feature(alloc_error_handler)]
+#![feature(const_fn)]
+#![feature(alloc_layout_extra)]
+#![feature(const_in_array_repeat_expressions)]
+#![feature(global_asm)]
+#![feature(asm)]
+#![feature(raw)]
+#![feature(never_type)]
+#![feature(naked_functions)]
+#![feature(option_expect_none)]
 #![test_runner(crate::test_runner)]
 #![reexport_test_harness_main = "test_main"]
 
@@ -14,6 +23,7 @@ pub mod allocator;
 pub mod gdt;
 pub mod interrupts;
 pub mod memory;
+pub mod multitasking;
 pub mod serial;
 pub mod vga_buffer;
 

src/main.rs

@@ -7,7 +7,8 @@
 extern crate alloc;
 
 use alloc::{boxed::Box, rc::Rc, vec, vec::Vec};
-use blog_os::println;
+use blog_os::multitasking::{self, thread::Thread, with_scheduler};
+use blog_os::{print, println};
 use bootloader::{entry_point, BootInfo};
 use core::panic::PanicInfo;
 
@@ -54,14 +55,45 @@ fn kernel_main(boot_info: &'static BootInfo) -> ! {
     #[cfg(test)]
     test_main();
 
+    let idle_thread = Thread::create(idle_thread, 2, &mut mapper, &mut frame_allocator).unwrap();
+    with_scheduler(|s| s.set_idle_thread(idle_thread));
+
+    for _ in 0..10 {
+        let thread = Thread::create(thread_entry, 2, &mut mapper, &mut frame_allocator).unwrap();
+        with_scheduler(|s| s.add_new_thread(thread));
+    }
+    let thread =
+        Thread::create_from_closure(|| thread_entry(), 2, &mut mapper, &mut frame_allocator)
+            .unwrap();
+    with_scheduler(|s| s.add_new_thread(thread));
+
     println!("It did not crash!");
-    blog_os::hlt_loop();
+    thread_entry();
+}
+
+fn idle_thread() -> ! {
+    loop {
+        x86_64::instructions::hlt();
+        multitasking::yield_now();
+    }
+}
+
+fn thread_entry() -> ! {
+    let thread_id = with_scheduler(|s| s.current_thread_id()).as_u64();
+    for _ in 0..=thread_id {
+        print!("{}", thread_id);
+        x86_64::instructions::hlt();
+    }
+    multitasking::exit_thread();
 }
 
 /// This function is called on panic.
 #[cfg(not(test))]
 #[panic_handler]
 fn panic(info: &PanicInfo) -> ! {
+    unsafe {
+        blog_os::vga_buffer::WRITER.force_unlock();
+    }
     println!("{}", info);
     blog_os::hlt_loop();
 }

src/memory.rs

@@ -1,7 +1,7 @@
 use bootloader::bootinfo::{MemoryMap, MemoryRegionType};
 use x86_64::{
     structures::paging::{
-        FrameAllocator, Mapper, OffsetPageTable, Page, PageTable, PhysFrame, Size4KiB,
+        mapper, FrameAllocator, Mapper, OffsetPageTable, Page, PageTable, PhysFrame, Size4KiB,
         UnusedPhysFrame,
     },
     PhysAddr, VirtAddr,
@@ -36,6 +36,54 @@ unsafe fn active_level_4_table(physical_memory_offset: VirtAddr) -> &'static mut
     &mut *page_table_ptr // unsafe
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct StackBounds {
+    start: VirtAddr,
+    end: VirtAddr,
+}
+
+impl StackBounds {
+    pub fn start(&self) -> VirtAddr {
+        self.start
+    }
+
+    pub fn end(&self) -> VirtAddr {
+        self.end
+    }
+}
+
+pub fn alloc_stack(
+    size_in_pages: u64,
+    mapper: &mut impl Mapper<Size4KiB>,
+    frame_allocator: &mut impl FrameAllocator<Size4KiB>,
+) -> Result<StackBounds, mapper::MapToError> {
+    use core::sync::atomic::{AtomicU64, Ordering};
+    use x86_64::structures::paging::PageTableFlags as Flags;
+
+    static STACK_ALLOC_NEXT: AtomicU64 = AtomicU64::new(0x_5555_5555_0000);
+
+    let guard_page_start = STACK_ALLOC_NEXT.fetch_add(
+        (size_in_pages + 1) * Page::<Size4KiB>::SIZE,
+        Ordering::SeqCst,
+    );
+    let guard_page = Page::from_start_address(VirtAddr::new(guard_page_start))
+        .expect("`STACK_ALLOC_NEXT` not page aligned");
+
+    let stack_start = guard_page + 1;
+    let stack_end = stack_start + size_in_pages;
+    let flags = Flags::PRESENT | Flags::WRITABLE;
+    for page in Page::range(stack_start, stack_end) {
+        let frame = frame_allocator
+            .allocate_frame()
+            .ok_or(mapper::MapToError::FrameAllocationFailed)?;
+        mapper.map_to(page, frame, flags, frame_allocator)?.flush();
+    }
+    Ok(StackBounds {
+        start: stack_start.start_address(),
+        end: stack_end.start_address(),
+    })
+}
+
 /// Creates an example mapping for the given page to frame `0xb8000`.
 pub fn create_example_mapping(
     page: Page,

src/multitasking/context_switch.rs

@@ -0,0 +1,105 @@
+use super::{with_scheduler, SwitchReason};
+use crate::multitasking::thread::ThreadId;
+use alloc::boxed::Box;
+use core::mem;
+use core::raw::TraitObject;
+use x86_64::VirtAddr;
+
+pub struct Stack {
+    pointer: VirtAddr,
+}
+
+impl Stack {
+    pub unsafe fn new(stack_pointer: VirtAddr) -> Self {
+        Stack {
+            pointer: stack_pointer,
+        }
+    }
+
+    pub fn get_stack_pointer(self) -> VirtAddr {
+        self.pointer
+    }
+
+    pub fn set_up_for_closure(&mut self, closure: Box<dyn FnOnce() -> !>) {
+        let trait_object: TraitObject = unsafe { mem::transmute(closure) };
+        unsafe { self.push(trait_object.data) };
+        unsafe { self.push(trait_object.vtable) };
+
+        self.set_up_for_entry_point(call_closure_entry);
+    }
+
+    pub fn set_up_for_entry_point(&mut self, entry_point: fn() -> !) {
+        unsafe { self.push(entry_point) };
+        let rflags: u64 = 0x200;
+        unsafe { self.push(rflags) };
+    }
+
+    unsafe fn push<T>(&mut self, value: T) {
+        self.pointer -= core::mem::size_of::<T>();
+        let ptr: *mut T = self.pointer.as_mut_ptr();
+        ptr.write(value);
+    }
+}
+
+pub unsafe fn context_switch_to(
+    new_stack_pointer: VirtAddr,
+    prev_thread_id: ThreadId,
+    switch_reason: SwitchReason,
+) {
+    asm!(
+        "call asm_context_switch"
+        :
+        : "{rdi}"(new_stack_pointer), "{rsi}"(prev_thread_id), "{rdx}"(switch_reason as u64)
+        : "rax", "rbx", "rcx", "rdx", "rsi", "rdi", "rbp", "r8", "r9", "r10",
+        "r11", "r12", "r13", "r14", "r15", "rflags", "memory"
+        : "intel", "volatile"
+    );
+}
+
+global_asm!(
+    "
+    .intel_syntax noprefix
+
+    // asm_context_switch(stack_pointer: u64, thread_id: u64)
+    asm_context_switch:
+        pushfq
+
+        mov rax, rsp
+        mov rsp, rdi
+
+        mov rdi, rax
+        call add_paused_thread
+
+        popfq
+        ret
+"
+);
+
+#[no_mangle]
+pub extern "C" fn add_paused_thread(
+    paused_stack_pointer: VirtAddr,
+    paused_thread_id: ThreadId,
+    switch_reason: SwitchReason,
+) {
+    with_scheduler(|s| s.add_paused_thread(paused_stack_pointer, paused_thread_id, switch_reason));
+}
+
+#[naked]
+fn call_closure_entry() -> ! {
+    unsafe {
+        asm!("
+        pop rsi
+        pop rdi
+        call call_closure
+    " ::: "mem" : "intel", "volatile")
+    };
+    unreachable!();
+}
+
+// no_mangle required because of https://github.com/rust-lang/rust/issues/68136
+#[no_mangle]
+extern "C" fn call_closure(data: *mut (), vtable: *mut ()) -> ! {
+    let trait_object = TraitObject { data, vtable };
+    let f: Box<dyn FnOnce() -> !> = unsafe { mem::transmute(trait_object) };
+    f()
+}

src/multitasking/mod.rs

@@ -0,0 +1,57 @@
+use scheduler::Scheduler;
+
+pub mod context_switch;
+pub mod scheduler;
+pub mod thread;
+
+static SCHEDULER: spin::Mutex<Option<Scheduler>> = spin::Mutex::new(None);
+
+#[repr(u64)]
+pub enum SwitchReason {
+    Paused,
+    Yield,
+    Blocked,
+    Exit,
+}
+
+pub fn invoke_scheduler() {
+    let next = SCHEDULER
+        .try_lock()
+        .and_then(|mut scheduler| scheduler.as_mut().and_then(|s| s.schedule()));
+    if let Some((next_stack_pointer, prev_thread_id)) = next {
+        unsafe {
+            context_switch::context_switch_to(
+                next_stack_pointer,
+                prev_thread_id,
+                SwitchReason::Paused,
+            )
+        };
+    }
+}
+
+pub fn exit_thread() -> ! {
+    synchronous_context_switch(SwitchReason::Exit).expect("can't exit last thread");
+    unreachable!("finished thread continued");
+}
+
+pub fn yield_now() {
+    let _ = synchronous_context_switch(SwitchReason::Yield);
+}
+
+fn synchronous_context_switch(reason: SwitchReason) -> Result<(), ()> {
+    let next = with_scheduler(|s| s.schedule());
+    match next {
+        Some((next_stack_pointer, prev_thread_id)) => unsafe {
+            context_switch::context_switch_to(next_stack_pointer, prev_thread_id, reason);
+            Ok(())
+        },
+        None => Err(()),
+    }
+}
+
+pub fn with_scheduler<F, T>(f: F) -> T
+where
+    F: FnOnce(&mut Scheduler) -> T,
+{
+    f(SCHEDULER.lock().get_or_insert_with(Scheduler::new))
+}

src/multitasking/scheduler.rs

@@ -0,0 +1,122 @@
+use super::SwitchReason;
+use crate::multitasking::thread::{Thread, ThreadId};
+use alloc::collections::{BTreeMap, BTreeSet, VecDeque};
+use core::mem;
+use x86_64::VirtAddr;
+
+pub struct Scheduler {
+    threads: BTreeMap<ThreadId, Thread>,
+    idle_thread_id: Option<ThreadId>,
+    current_thread_id: ThreadId,
+    paused_threads: VecDeque<ThreadId>,
+    blocked_threads: BTreeSet<ThreadId>,
+    wakeups: BTreeSet<ThreadId>,
+}
+
+impl Scheduler {
+    pub fn new() -> Self {
+        let root_thread = Thread::create_root_thread();
+        let root_id = root_thread.id();
+        let mut threads = BTreeMap::new();
+        threads
+            .insert(root_id, root_thread)
+            .expect_none("map is not empty after creation");
+        Scheduler {
+            threads,
+            current_thread_id: root_id,
+            paused_threads: VecDeque::new(),
+            blocked_threads: BTreeSet::new(),
+            wakeups: BTreeSet::new(),
+            idle_thread_id: None,
+        }
+    }
+
+    fn next_thread(&mut self) -> Option<ThreadId> {
+        self.paused_threads.pop_front()
+    }
+
+    pub fn schedule(&mut self) -> Option<(VirtAddr, ThreadId)> {
+        let mut next_thread_id = self.next_thread();
+        if next_thread_id.is_none() && Some(self.current_thread_id) != self.idle_thread_id {
+            next_thread_id = self.idle_thread_id
+        }
+        if let Some(next_id) = next_thread_id {
+            let next_thread = self
+                .threads
+                .get_mut(&next_id)
+                .expect("next thread does not exist");
+            let next_stack_pointer = next_thread
+                .stack_pointer()
+                .take()
+                .expect("paused thread has no stack pointer");
+            let prev_thread_id = mem::replace(&mut self.current_thread_id, next_thread.id());
+            Some((next_stack_pointer, prev_thread_id))
+        } else {
+            None
+        }
+    }
+
+    pub(super) fn add_paused_thread(
+        &mut self,
+        paused_stack_pointer: VirtAddr,
+        paused_thread_id: ThreadId,
+        switch_reason: SwitchReason,
+    ) {
+        let paused_thread = self
+            .threads
+            .get_mut(&paused_thread_id)
+            .expect("paused thread does not exist");
+        paused_thread
+            .stack_pointer()
+            .replace(paused_stack_pointer)
+            .expect_none("running thread should have stack pointer set to None");
+        if Some(paused_thread_id) == self.idle_thread_id {
+            return; // do nothing
+        }
+        match switch_reason {
+            SwitchReason::Paused | SwitchReason::Yield => {
+                self.paused_threads.push_back(paused_thread_id)
+            }
+            SwitchReason::Blocked => {
+                self.blocked_threads.insert(paused_thread_id);
+                self.check_for_wakeup(paused_thread_id);
+            }
+            SwitchReason::Exit => {
+                let thread = self
+                    .threads
+                    .remove(&paused_thread_id)
+                    .expect("thread not found");
+                // TODO: free stack memory again
+            }
+        }
+    }
+
+    pub fn add_new_thread(&mut self, thread: Thread) {
+        let thread_id = thread.id();
+        self.threads
+            .insert(thread_id, thread)
+            .expect_none("thread already exists");
+        self.paused_threads.push_back(thread_id);
+    }
+
+    pub fn set_idle_thread(&mut self, thread: Thread) {
+        let thread_id = thread.id();
+        self.threads
+            .insert(thread_id, thread)
+            .expect_none("thread already exists");
+        self.idle_thread_id
+            .replace(thread_id)
+            .expect_none("idle thread should be set only once");
+    }
+
+    pub fn current_thread_id(&self) -> ThreadId {
+        self.current_thread_id
+    }
+
+    fn check_for_wakeup(&mut self, thread_id: ThreadId) {
+        if self.wakeups.remove(&thread_id) {
+            assert!(self.blocked_threads.remove(&thread_id));
+            self.paused_threads.push_back(thread_id);
+        }
+    }
+}

src/multitasking/thread.rs

@@ -0,0 +1,82 @@
+use crate::memory::{alloc_stack, StackBounds};
+use crate::multitasking::context_switch::Stack;
+use alloc::boxed::Box;
+use x86_64::{
+    structures::paging::{mapper, FrameAllocator, Mapper, Size4KiB},
+    VirtAddr,
+};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct ThreadId(u64);
+
+impl ThreadId {
+    pub fn as_u64(&self) -> u64 {
+        self.0
+    }
+
+    fn new() -> Self {
+        use core::sync::atomic::{AtomicU64, Ordering};
+        static NEXT_THREAD_ID: AtomicU64 = AtomicU64::new(1);
+        ThreadId(NEXT_THREAD_ID.fetch_add(1, Ordering::SeqCst))
+    }
+}
+
+#[derive(Debug)]
+pub struct Thread {
+    id: ThreadId,
+    stack_pointer: Option<VirtAddr>,
+    stack_bounds: Option<StackBounds>,
+}
+
+impl Thread {
+    pub fn create(
+        entry_point: fn() -> !,
+        stack_size: u64,
+        mapper: &mut impl Mapper<Size4KiB>,
+        frame_allocator: &mut impl FrameAllocator<Size4KiB>,
+    ) -> Result<Self, mapper::MapToError> {
+        let stack_bounds = alloc_stack(stack_size, mapper, frame_allocator)?;
+        let mut stack = unsafe { Stack::new(stack_bounds.end()) };
+        stack.set_up_for_entry_point(entry_point);
+        Ok(Self::new(stack.get_stack_pointer(), stack_bounds))
+    }
+
+    pub fn create_from_closure<F>(
+        closure: F,
+        stack_size: u64,
+        mapper: &mut impl Mapper<Size4KiB>,
+        frame_allocator: &mut impl FrameAllocator<Size4KiB>,
+    ) -> Result<Self, mapper::MapToError>
+    where
+        F: FnOnce() -> ! + 'static + Send + Sync,
+    {
+        let stack_bounds = alloc_stack(stack_size, mapper, frame_allocator)?;
+        let mut stack = unsafe { Stack::new(stack_bounds.end()) };
+        stack.set_up_for_closure(Box::new(closure));
+        Ok(Self::new(stack.get_stack_pointer(), stack_bounds))
+    }
+
+    fn new(stack_pointer: VirtAddr, stack_bounds: StackBounds) -> Self {
+        Thread {
+            id: ThreadId::new(),
+            stack_pointer: Some(stack_pointer),
+            stack_bounds: Some(stack_bounds),
+        }
+    }
+
+    pub(super) fn create_root_thread() -> Self {
+        Thread {
+            id: ThreadId(0),
+            stack_pointer: None,
+            stack_bounds: None,
+        }
+    }
+
+    pub fn id(&self) -> ThreadId {
+        self.id
+    }
+
+    pub(super) fn stack_pointer(&mut self) -> &mut Option<VirtAddr> {
+        &mut self.stack_pointer
+    }
+}

tests/heap_allocation.rs

@@ -58,6 +58,18 @@ fn many_boxes() {
     serial_println!("[ok]");
 }
 
+#[test_case]
+fn many_boxes_long_lived() {
+    serial_print!("many_boxes_long_lived... ");
+    let long_lived = Box::new(1); // new
+    for i in 0..HEAP_SIZE {
+        let x = Box::new(i);
+        assert_eq!(*x, i);
+    }
+    assert_eq!(*long_lived, 1); // new
+    serial_println!("[ok]");
+}
+
 #[panic_handler]
 fn panic(info: &PanicInfo) -> ! {
     blog_os::test_panic_handler(info)