Fix register typo: rpb -> rbp

Fixes #746
Add a yield_now function and use it in idle thread
2025-12-16 22:37:49 +00:00 · 2020-02-16 18:02:24 +01:00 · 2020-01-28 12:28:50 +01:00 · 2020-01-28 12:22:37 +01:00 · 2020-01-28 11:29:21 +01:00 · 2020-01-23 14:24:13 +01:00
25 changed files with 1347 additions and 242 deletions
--- a/.cargo/config.toml
+++ b/.cargo/config.toml
@@ -1,7 +1,3 @@
-[unstable]
-build-std = ["core", "compiler_builtins"]
-build-std-features = ["compiler-builtins-mem"]
-
 [build]
 target = "x86_64-blog_os.json"

--- a/.github/workflows/build-code.yml
+++ b/.github/workflows/build-code.yml
@@ -0,0 +1,96 @@
+name: Build Code
+
+on:
+  push:
+    branches:
+      - '*'
+      - '!staging.tmp'
+    tags:
+      - '*'
+  schedule:
+    - cron: '40 3 * * *'   # every day at 3:40
+  pull_request:
+
+jobs:
+  test:
+    name: "Test"
+
+    strategy:
+      matrix:
+        platform: [
+          ubuntu-latest,
+          macos-latest,
+          windows-latest
+        ]
+
+    runs-on: ${{ matrix.platform }}
+    timeout-minutes: 15
+
+    steps:
+    - name: "Checkout Repository"
+      uses: actions/checkout@v1
+
+    - name: Install Rustup
+      run: |
+        curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain nightly
+        echo ::add-path::$HOME/.cargo/bin
+      if: runner.os == 'macOS'
+
+    - name: "Print Rust Version"
+      run: |
+        rustc -Vv
+        cargo -Vv
+
+    - name: "Install Rustup Components"
+      run: rustup component add rust-src llvm-tools-preview
+    - name: "Install cargo-xbuild"
+      run: cargo install cargo-xbuild --debug
+    - name: "Install bootimage"
+      run: cargo install bootimage --debug
+
+    - name: "Run cargo xbuild"
+      run: cargo xbuild
+    - name: "Create Bootimage"
+      run: cargo bootimage
+
+     # install QEMU
+    - name: Install QEMU (Linux)
+      run: sudo apt update && sudo apt install qemu-system-x86
+      if: runner.os == 'Linux'
+    - name: Install QEMU (macOS)
+      run: brew install qemu
+      if: runner.os == 'macOS'
+      env:
+        HOMEBREW_NO_AUTO_UPDATE: 1
+        HOMEBREW_NO_BOTTLE_SOURCE_FALLBACK: 1
+        HOMEBREW_NO_INSTALL_CLEANUP: 1
+    - name: Install Scoop (Windows)
+      run: |
+        Invoke-Expression (New-Object System.Net.WebClient).DownloadString('https://get.scoop.sh')
+        echo ::add-path::$HOME\scoop\shims
+      if: runner.os == 'Windows'
+      shell: pwsh
+    - name: Install QEMU (Windows)
+      run: scoop install qemu
+      if: runner.os == 'Windows'
+      shell: pwsh
+    - name: "Print QEMU Version"
+      run: qemu-system-x86_64 --version
+
+    - name: "Run cargo xtest"
+      run: cargo xtest
+
+  check_formatting:
+    name: "Check Formatting"
+    runs-on: ubuntu-latest
+    timeout-minutes: 2
+    steps:
+    - uses: actions/checkout@v1
+    - name: "Use the latest Rust nightly with rustfmt"
+      uses: actions-rs/toolchain@v1
+      with:
+          toolchain: nightly
+          profile: minimal
+          components: rustfmt
+          override: true
+    - run: cargo fmt -- --check
--- a/.github/workflows/code.yml
+++ b/.github/workflows/code.yml
@@ -1,130 +0,0 @@
-name: Code
-
-on:
-  push:
-    branches:
-      - '*'
-      - '!staging.tmp'
-    tags:
-      - '*'
-  schedule:
-    - cron: '40 3 * * *'   # every day at 3:40
-  pull_request:
-  workflow_dispatch:
-
-jobs:
-  check:
-    name: Check
-    strategy:
-      fail-fast: false
-      matrix:
-        platform: [
-          ubuntu-latest,
-          macos-latest,
-          windows-latest
-        ]
-    runs-on: ${{ matrix.platform }}
-    steps:
-      - name: Checkout Repository
-        uses: actions/checkout@v2
-      - name: Install Rust Toolchain
-        uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: nightly
-      - name: Install `rust-src` Rustup Component
-        run: rustup component add rust-src
-      - name: Run `cargo check`
-        uses: actions-rs/cargo@v1
-        with:
-          command: check
-
-  test:
-    name: Test
-    strategy:
-      fail-fast: false
-      matrix:
-        platform: [
-          ubuntu-latest,
-          macos-latest,
-          windows-latest
-        ]
-    runs-on: ${{ matrix.platform }}
-    steps:
-      - name: Install Rust Toolchain
-        uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: nightly
-      - name: Install bootimage
-        run: cargo install bootimage --debug
-      - name: Checkout Repository
-        uses: actions/checkout@v2
-      - name: Install Rustup Components
-        run: rustup component add rust-src llvm-tools-preview
-      - name: Run `cargo bootimage`
-        uses: actions-rs/cargo@v1
-        with:
-          command: bootimage
-
-      # install QEMU
-      - name: Install QEMU (Linux)
-        run: sudo apt update && sudo apt install qemu-system-x86
-        if: runner.os == 'Linux'
-      - name: Install QEMU (macOS)
-        run: brew install qemu
-        if: runner.os == 'macOS'
-        env:
-          HOMEBREW_NO_AUTO_UPDATE: 1
-          HOMEBREW_NO_BOTTLE_SOURCE_FALLBACK: 1
-          HOMEBREW_NO_INSTALL_CLEANUP: 1
-      - name: Install QEMU (Windows)
-        run: |
-          choco install qemu --version 2021.5.5
-          echo "$Env:Programfiles\qemu" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
-        if: runner.os == 'Windows'
-        shell: pwsh
-      - name: "Print QEMU Version"
-        run: qemu-system-x86_64 --version
-
-      - name: Run `cargo test`
-        uses: actions-rs/cargo@v1
-        with:
-          command: test
-
-  check_formatting:
-    name: Check Formatting
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout Repository
-        uses: actions/checkout@v2
-      - name: Install Rust Toolchain
-        uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: nightly
-          components: rustfmt
-          override: true
-      - name: Run `cargo fmt`
-        uses: actions-rs/cargo@v1
-        with:
-          command: fmt
-          args: --all -- --check
-
-  clippy:
-    name: Clippy
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout Repository
-        uses: actions/checkout@v2
-      - name: Install Rust Toolchain
-        uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: nightly
-          components: clippy, rust-src
-          override: true
-      - name: Run `cargo clippy`
-        uses: actions-rs/cargo@v1
-        with:
-          command: clippy
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1,12 +1,19 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
-version = 4
+[[package]]
+name = "array-init"
+version = "0.0.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "23589ecb866b460d3a0f1278834750268c607e8e28a1b982c907219f3178cd72"
+dependencies = [
+ "nodrop",
+]

 [[package]]
 name = "bit_field"
-version = "0.10.1"
+version = "0.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "dcb6dd1c2376d2e096796e234a70e17e94cc2d5d54ff8ce42b28cef1d0d359a4"
+checksum = "ed8765909f9009617974ab6b7d332625b320b33c326b1e9321382ef1999b5d56"

 [[package]]
 name = "bitflags"
@@ -20,19 +27,35 @@ version = "0.1.0"
 dependencies = [
 "bootloader",
 "lazy_static",
+ "linked_list_allocator",
 "pc-keyboard",
- "pic8259",
+ "pic8259_simple",
 "spin",
 "uart_16550",
- "volatile 0.2.7",
- "x86_64",
+ "volatile",
+ "x86_64 0.8.1",
 ]

 [[package]]
 name = "bootloader"
-version = "0.9.31"
+version = "0.8.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "974e79cf1b0b737839f01330fb5393095daf1124d52693696494e32523ae9ef5"
+checksum = "d596849a47f28abdea62d7a6a25c4f6e69c3d9b09b0a2877db6e9cda004ca993"
+
+[[package]]
+name = "cast"
+version = "0.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4b9434b9a5aa1450faa3f9cb14ea0e8c53bb5d2b3c1bfd1ab4fc03e9f33fbfb0"
+dependencies = [
+ "rustc_version",
+]
+
+[[package]]
+name = "cpuio"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "22b8e308ccfc5acf3b82f79c0eac444cf6114cb2ac67a230ca6c177210068daa"

 [[package]]
 name = "lazy_static"
@@ -44,20 +67,59 @@ dependencies = [
 ]

 [[package]]
-name = "pc-keyboard"
-version = "0.7.0"
+name = "linked_list_allocator"
+version = "0.6.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ed089a1fbffe3337a1a345501c981f1eb1e47e69de5a40e852433e12953c3174"
+checksum = "47314ec1d29aa869ee7cb5a5be57be9b1055c56567d59c3fb6689926743e0bea"
+dependencies = [
+ "spin",
+]

 [[package]]
-name = "pic8259"
-version = "0.10.1"
+name = "nodrop"
+version = "0.1.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "08cc920d83ee33c0f9b73aa441e75468bf2d10c959a3eb6260cf720b05ac91a1"
+checksum = "72ef4a56884ca558e5ddb05a1d1e7e1bfd9a68d9ed024c21704cc98872dae1bb"
+
+[[package]]
+name = "pc-keyboard"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fff50ab09ba31bcebc0669f4e64c0952fae1acdca9e6e0587e68e4e8443808ac"
+
+[[package]]
+name = "pic8259_simple"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dc64b2fd10828da8521b6cdabe0679385d7d2a3a6d4c336b819d1fa31ba35c72"
 dependencies = [
- "x86_64",
+ "cpuio",
 ]

+[[package]]
+name = "rustc_version"
+version = "0.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "138e3e0acb6c9fb258b19b67cb8abd63c00679d2851805ea151465464fe9030a"
+dependencies = [
+ "semver",
+]
+
+[[package]]
+name = "semver"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1d7eb9ef2c18661902cc47e535f9bc51b78acd254da71d375c2f6720d9a40403"
+dependencies = [
+ "semver-parser",
+]
+
+[[package]]
+name = "semver-parser"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "388a1df253eca08550bef6c72392cfe7c30914bf41df5269b68cbd6ff8f570a3"
+
 [[package]]
 name = "spin"
 version = "0.5.2"
@@ -66,33 +128,47 @@ checksum = "6e63cff320ae2c57904679ba7cb63280a3dc4613885beafb148ee7bf9aa9042d"

 [[package]]
 name = "uart_16550"
-version = "0.2.14"
+version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "503a6c0e6d82daa87985e662d120c0176b09587c92a68db22781b28ae95405dd"
+checksum = "803ea8cb602dbb32c1a657a866d2dd79fe7dbeab0fb2ac667cb4dcc7de12a58b"
 dependencies = [
 "bitflags",
- "x86_64",
+ "x86_64 0.7.7",
 ]

 [[package]]
-name = "volatile"
-version = "0.2.7"
+name = "ux"
+version = "0.1.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f6b06ad3ed06fef1713569d547cdbdb439eafed76341820fb0e0344f29a41945"
+checksum = "88dfeb711b61ce620c0cb6fd9f8e3e678622f0c971da2a63c4b3e25e88ed012f"

 [[package]]
 name = "volatile"
-version = "0.4.4"
+version = "0.2.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e4c2dbd44eb8b53973357e6e207e370f0c1059990df850aca1eca8947cf464f0"
+checksum = "6af0edf5b4faacc31fc51159244d78d65ec580f021afcef7bd53c04aeabc7f29"

 [[package]]
 name = "x86_64"
-version = "0.14.7"
+version = "0.7.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "fb611915c917c6296d11e23f71ff1ecfe49c5766daba92cd3df52df6b58285b6"
+checksum = "1f27d9168654aee1b0c1b73746caeb4aa33248f8b8c8f6e100e697fcc2a794b2"
 dependencies = [
+ "array-init",
 "bit_field",
 "bitflags",
- "volatile 0.4.4",
+ "cast",
+ "ux",
+]
+
+[[package]]
+name = "x86_64"
+version = "0.8.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f21672dcbed52bc09eea030d189600c0189c66c97bc5b31779eb780e064a201f"
+dependencies = [
+ "array-init",
+ "bit_field",
+ "bitflags",
+ "cast",
 ]
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -2,7 +2,7 @@
 name = "blog_os"
 version = "0.1.0"
 authors = ["Philipp Oppermann <dev@phil-opp.com>"]
-edition = "2024"
+edition = "2018"

 [[test]]
 name = "should_panic"
@@ -13,13 +13,14 @@ name = "stack_overflow"
 harness = false

 [dependencies]
-bootloader = "0.9"
+bootloader = { version = "0.8.0", features = ["map_physical_memory"]}
 volatile = "0.2.6"
 spin = "0.5.2"
-x86_64 = "0.14.2"
+x86_64 = "0.8.1"
 uart_16550 = "0.2.0"
-pic8259 = "0.10.1"
-pc-keyboard = "0.7.0"
+pic8259_simple = "0.1.1"
+pc-keyboard = "0.3.1"
+linked_list_allocator = "0.6.4"

 [dependencies.lazy_static]
 version = "1.0"
@@ -32,3 +33,6 @@ test-args = [
    "-display", "none"
 ]
 test-success-exit-code = 33         # (0x10 << 1) | 1
+
+[profile.release]
+lto = true
--- a/README.md
+++ b/README.md
@@ -1,32 +1,28 @@
-# Blog OS (Introduction to Paging)
+# Blog OS (Heap Allocation)

-[![Build Status](https://github.com/phil-opp/blog_os/workflows/Code/badge.svg?branch=post-08)](https://github.com/phil-opp/blog_os/actions?query=workflow%3A%22Code%22+branch%3Apost-08)
+[![Build Status](https://github.com/phil-opp/blog_os/workflows/Build%20Code/badge.svg?branch=post-10)](https://github.com/phil-opp/blog_os/actions?query=workflow%3A%22Build+Code%22+branch%3Apost-10)

-This repository contains the source code for the [Introduction to Paging][post] post of the [Writing an OS in Rust](https://os.phil-opp.com) series.
+This repository contains the source code for the [Heap Allocation][post] post of the [Writing an OS in Rust](https://os.phil-opp.com) series.

-[post]: https://os.phil-opp.com/paging-introduction/
+[post]: https://os.phil-opp.com/heap-allocation/

 **Check out the [master branch](https://github.com/phil-opp/blog_os) for more information.**

 ## Building

-This project requires a nightly version of Rust because it uses some unstable features. At least nightly _2020-07-15_ is required for building. You might need to run `rustup update nightly --force` to update to the latest nightly even if some components such as `rustfmt` are missing it.
-
-You can build the project by running:
+You need a nightly Rust compiler. First you need to install the `cargo-xbuild` and `bootimage` tools:

 ```
-cargo build
+cargo install cargo-xbuild bootimage
 ```

-To create a bootable disk image from the compiled kernel, you need to install the [`bootimage`] tool:
-
-[`bootimage`]: https://github.com/rust-osdev/bootimage
+Then you can build the project by running:

 ```
-cargo install bootimage
+cargo xbuild
 ```

-After installing, you can create the bootable disk image by running:
+To create a bootable disk image, run:

 ```
 cargo bootimage
@@ -43,10 +39,10 @@ You can run the disk image in [QEMU] through:
 [QEMU]: https://www.qemu.org/

 ```
-cargo run
+cargo xrun
 ```

-[QEMU] and the [`bootimage`] tool need to be installed for this.
+Of course [QEMU] needs to be installed for this.

 You can also write the image to an USB stick for booting it on a real machine. On Linux, the command for this is:

--- a/src/allocator/bump.rs
+++ b/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;
+        }
+    }
+}
--- a/src/allocator/fixed_size_block.rs
+++ b/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);
+            }
+        }
+    }
+}
--- a/src/allocator/linked_list.rs
+++ b/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)
+    }
+}
--- a/src/allocator/mod.rs
+++ b/src/allocator/mod.rs
@@ -0,0 +1,84 @@
+use alloc::alloc::{GlobalAlloc, Layout};
+use core::ptr::null_mut;
+use fixed_size_block::FixedSizeBlockAllocator;
+use x86_64::{
+    structures::paging::{
+        mapper::MapToError, FrameAllocator, Mapper, Page, PageTableFlags, Size4KiB,
+    },
+    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: Locked<FixedSizeBlockAllocator> = Locked::new(FixedSizeBlockAllocator::new());
+
+pub fn init_heap(
+    mapper: &mut impl Mapper<Size4KiB>,
+    frame_allocator: &mut impl FrameAllocator<Size4KiB>,
+) -> Result<(), MapToError> {
+    let page_range = {
+        let heap_start = VirtAddr::new(HEAP_START as u64);
+        let heap_end = heap_start + HEAP_SIZE - 1u64;
+        let heap_start_page = Page::containing_address(heap_start);
+        let heap_end_page = Page::containing_address(heap_end);
+        Page::range_inclusive(heap_start_page, heap_end_page)
+    };
+
+    for page in page_range {
+        let frame = frame_allocator
+            .allocate_frame()
+            .ok_or(MapToError::FrameAllocationFailed)?;
+        let flags = PageTableFlags::PRESENT | PageTableFlags::WRITABLE;
+        mapper.map_to(page, frame, flags, frame_allocator)?.flush();
+    }
+
+    unsafe {
+        ALLOCATOR.lock().init(HEAP_START, HEAP_SIZE);
+    }
+
+    Ok(())
+}
+
+pub struct Dummy;
+
+unsafe impl GlobalAlloc for Dummy {
+    unsafe fn alloc(&self, _layout: Layout) -> *mut u8 {
+        null_mut()
+    }
+
+    unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {
+        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
+    }
+}
--- a/src/gdt.rs
+++ b/src/gdt.rs
@@ -1,7 +1,7 @@
 use lazy_static::lazy_static;
-use x86_64::VirtAddr;
 use x86_64::structures::gdt::{Descriptor, GlobalDescriptorTable, SegmentSelector};
 use x86_64::structures::tss::TaskStateSegment;
+use x86_64::VirtAddr;

 pub const DOUBLE_FAULT_IST_INDEX: u16 = 0;

@@ -9,10 +9,10 @@ lazy_static! {
    static ref TSS: TaskStateSegment = {
        let mut tss = TaskStateSegment::new();
        tss.interrupt_stack_table[DOUBLE_FAULT_IST_INDEX as usize] = {
-            const STACK_SIZE: usize = 4096 * 5;
+            const STACK_SIZE: usize = 4096;
            static mut STACK: [u8; STACK_SIZE] = [0; STACK_SIZE];

-            let stack_start = VirtAddr::from_ptr(&raw const STACK);
+            let stack_start = VirtAddr::from_ptr(unsafe { &STACK });
            let stack_end = stack_start + STACK_SIZE;
            stack_end
        };
@@ -41,12 +41,12 @@ struct Selectors {
 }

 pub fn init() {
-    use x86_64::instructions::segmentation::{CS, Segment};
+    use x86_64::instructions::segmentation::set_cs;
    use x86_64::instructions::tables::load_tss;

    GDT.0.load();
    unsafe {
-        CS::set_reg(GDT.1.code_selector);
+        set_cs(GDT.1.code_selector);
        load_tss(GDT.1.tss_selector);
    }
 }
--- a/src/interrupts.rs
+++ b/src/interrupts.rs
@@ -1,6 +1,6 @@
 use crate::{gdt, hlt_loop, print, println};
 use lazy_static::lazy_static;
-use pic8259::ChainedPics;
+use pic8259_simple::ChainedPics;
 use spin;
 use x86_64::structures::idt::{InterruptDescriptorTable, InterruptStackFrame, PageFaultErrorCode};

@@ -47,12 +47,12 @@ pub fn init_idt() {
    IDT.load();
 }

-extern "x86-interrupt" fn breakpoint_handler(stack_frame: InterruptStackFrame) {
+extern "x86-interrupt" fn breakpoint_handler(stack_frame: &mut InterruptStackFrame) {
    println!("EXCEPTION: BREAKPOINT\n{:#?}", stack_frame);
 }

 extern "x86-interrupt" fn page_fault_handler(
-    stack_frame: InterruptStackFrame,
+    stack_frame: &mut InterruptStackFrame,
    error_code: PageFaultErrorCode,
 ) {
    use x86_64::registers::control::Cr2;
@@ -65,32 +65,29 @@ extern "x86-interrupt" fn page_fault_handler(
 }

 extern "x86-interrupt" fn double_fault_handler(
-    stack_frame: InterruptStackFrame,
+    stack_frame: &mut InterruptStackFrame,
    _error_code: u64,
 ) -> ! {
    panic!("EXCEPTION: DOUBLE FAULT\n{:#?}", stack_frame);
 }

-extern "x86-interrupt" fn timer_interrupt_handler(_stack_frame: InterruptStackFrame) {
+extern "x86-interrupt" fn timer_interrupt_handler(_stack_frame: &mut InterruptStackFrame) {
    print!(".");
    unsafe {
        PICS.lock()
            .notify_end_of_interrupt(InterruptIndex::Timer.as_u8());
    }
+    crate::multitasking::invoke_scheduler();
 }

-extern "x86-interrupt" fn keyboard_interrupt_handler(_stack_frame: InterruptStackFrame) {
-    use pc_keyboard::{DecodedKey, HandleControl, Keyboard, ScancodeSet1, layouts};
+extern "x86-interrupt" fn keyboard_interrupt_handler(_stack_frame: &mut InterruptStackFrame) {
+    use pc_keyboard::{layouts, DecodedKey, Keyboard, ScancodeSet1};
    use spin::Mutex;
    use x86_64::instructions::port::Port;

    lazy_static! {
        static ref KEYBOARD: Mutex<Keyboard<layouts::Us104Key, ScancodeSet1>> =
-            Mutex::new(Keyboard::new(
-                ScancodeSet1::new(),
-                layouts::Us104Key,
-                HandleControl::Ignore
-            ));
+            Mutex::new(Keyboard::new(layouts::Us104Key, ScancodeSet1));
    }

    let mut keyboard = KEYBOARD.lock();
@@ -112,8 +109,13 @@ extern "x86-interrupt" fn keyboard_interrupt_handler(_stack_frame: InterruptStac
    }
 }

+#[cfg(test)]
+use crate::{serial_print, serial_println};
+
 #[test_case]
 fn test_breakpoint_exception() {
+    serial_print!("test_breakpoint_exception...");
    // invoke a breakpoint exception
    x86_64::instructions::interrupts::int3();
+    serial_println!("[ok]");
 }
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2,13 +2,28 @@
 #![cfg_attr(test, no_main)]
 #![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"]

+extern crate alloc;
+
 use core::panic::PanicInfo;

+pub mod allocator;
 pub mod gdt;
 pub mod interrupts;
+pub mod memory;
+pub mod multitasking;
 pub mod serial;
 pub mod vga_buffer;

@@ -18,25 +33,11 @@ pub fn init() {
    unsafe { interrupts::PICS.lock().initialize() };
    x86_64::instructions::interrupts::enable();
 }
-pub trait Testable {
-    fn run(&self) -> ();
-}

-impl<T> Testable for T
-where
-    T: Fn(),
-{
-    fn run(&self) {
-        serial_print!("{}...\t", core::any::type_name::<T>());
-        self();
-        serial_println!("[ok]");
-    }
-}
-
-pub fn test_runner(tests: &[&dyn Testable]) {
+pub fn test_runner(tests: &[&dyn Fn()]) {
    serial_println!("Running {} tests", tests.len());
    for test in tests {
-        test.run();
+        test();
    }
    exit_qemu(QemuExitCode::Success);
 }
@@ -70,10 +71,15 @@ pub fn hlt_loop() -> ! {
    }
 }

+#[cfg(test)]
+use bootloader::{entry_point, BootInfo};
+
+#[cfg(test)]
+entry_point!(test_kernel_main);
+
 /// Entry point for `cargo xtest`
 #[cfg(test)]
-#[unsafe(no_mangle)]
-pub extern "C" fn _start() -> ! {
+fn test_kernel_main(_boot_info: &'static BootInfo) -> ! {
    init();
    test_main();
    hlt_loop();
@@ -84,3 +90,8 @@ pub extern "C" fn _start() -> ! {
 fn panic(info: &PanicInfo) -> ! {
    test_panic_handler(info)
 }
+
+#[alloc_error_handler]
+fn alloc_error_handler(layout: alloc::alloc::Layout) -> ! {
+    panic!("allocation error: {:?}", layout)
+}
--- a/src/main.rs
+++ b/src/main.rs
@@ -4,33 +4,96 @@
 #![test_runner(blog_os::test_runner)]
 #![reexport_test_harness_main = "test_main"]

-use blog_os::println;
+extern crate alloc;
+
+use alloc::{boxed::Box, rc::Rc, vec, vec::Vec};
+use blog_os::multitasking::{self, thread::Thread, with_scheduler};
+use blog_os::{print, println};
+use bootloader::{entry_point, BootInfo};
 use core::panic::PanicInfo;

-#[unsafe(no_mangle)]
-pub extern "C" fn _start() -> ! {
-    use x86_64::registers::control::Cr3;
-    println!("Hello World{}", "!");
+entry_point!(kernel_main);

+fn kernel_main(boot_info: &'static BootInfo) -> ! {
+    use blog_os::allocator;
+    use blog_os::memory::{self, BootInfoFrameAllocator};
+    use x86_64::VirtAddr;
+
+    println!("Hello World{}", "!");
    blog_os::init();

-    let (level_4_page_table, _) = Cr3::read();
+    let phys_mem_offset = VirtAddr::new(boot_info.physical_memory_offset);
+    let mut mapper = unsafe { memory::init(phys_mem_offset) };
+    let mut frame_allocator = unsafe { BootInfoFrameAllocator::init(&boot_info.memory_map) };
+
+    allocator::init_heap(&mut mapper, &mut frame_allocator).expect("heap initialization failed");
+
+    // allocate a number on the heap
+    let heap_value = Box::new(41);
+    println!("heap_value at {:p}", heap_value);
+
+    // create a dynamically sized vector
+    let mut vec = Vec::new();
+    for i in 0..500 {
+        vec.push(i);
+    }
+    println!("vec at {:p}", vec.as_slice());
+
+    // create a reference counted vector -> will be freed when count reaches 0
+    let reference_counted = Rc::new(vec![1, 2, 3]);
+    let cloned_reference = reference_counted.clone();
    println!(
-        "Level 4 page table at: {:?}",
-        level_4_page_table.start_address()
+        "current reference count is {}",
+        Rc::strong_count(&cloned_reference)
+    );
+    core::mem::drop(reference_counted);
+    println!(
+        "reference count is {} now",
+        Rc::strong_count(&cloned_reference)
    );

    #[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();
 }
@@ -40,8 +103,3 @@ fn panic(info: &PanicInfo) -> ! {
 fn panic(info: &PanicInfo) -> ! {
    blog_os::test_panic_handler(info)
 }
-
-#[test_case]
-fn trivial_assertion() {
-    assert_eq!(1, 1);
-}
--- a/src/memory.rs
+++ b/src/memory.rs
@@ -0,0 +1,154 @@
+use bootloader::bootinfo::{MemoryMap, MemoryRegionType};
+use x86_64::{
+    structures::paging::{
+        mapper, FrameAllocator, Mapper, OffsetPageTable, Page, PageTable, PhysFrame, Size4KiB,
+        UnusedPhysFrame,
+    },
+    PhysAddr, VirtAddr,
+};
+
+/// Initialize a new OffsetPageTable.
+///
+/// This function is unsafe because the caller must guarantee that the
+/// complete physical memory is mapped to virtual memory at the passed
+/// `physical_memory_offset`. Also, this function must be only called once
+/// to avoid aliasing `&mut` references (which is undefined behavior).
+pub unsafe fn init(physical_memory_offset: VirtAddr) -> OffsetPageTable<'static> {
+    let level_4_table = active_level_4_table(physical_memory_offset);
+    OffsetPageTable::new(level_4_table, physical_memory_offset)
+}
+
+/// Returns a mutable reference to the active level 4 table.
+///
+/// This function is unsafe because the caller must guarantee that the
+/// complete physical memory is mapped to virtual memory at the passed
+/// `physical_memory_offset`. Also, this function must be only called once
+/// to avoid aliasing `&mut` references (which is undefined behavior).
+unsafe fn active_level_4_table(physical_memory_offset: VirtAddr) -> &'static mut PageTable {
+    use x86_64::registers::control::Cr3;
+
+    let (level_4_table_frame, _) = Cr3::read();
+
+    let phys = level_4_table_frame.start_address();
+    let virt = physical_memory_offset + phys.as_u64();
+    let page_table_ptr: *mut PageTable = virt.as_mut_ptr();
+
+    &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,
+    mapper: &mut OffsetPageTable,
+    frame_allocator: &mut impl FrameAllocator<Size4KiB>,
+) {
+    use x86_64::structures::paging::PageTableFlags as Flags;
+
+    let frame = PhysFrame::containing_address(PhysAddr::new(0xb8000));
+    // FIXME: ONLY FOR TEMPORARY TESTING
+    let unused_frame = unsafe { UnusedPhysFrame::new(frame) };
+    let flags = Flags::PRESENT | Flags::WRITABLE;
+
+    let map_to_result = mapper.map_to(page, unused_frame, flags, frame_allocator);
+    map_to_result.expect("map_to failed").flush();
+}
+
+/// A FrameAllocator that always returns `None`.
+pub struct EmptyFrameAllocator;
+
+unsafe impl FrameAllocator<Size4KiB> for EmptyFrameAllocator {
+    fn allocate_frame(&mut self) -> Option<UnusedPhysFrame> {
+        None
+    }
+}
+
+/// A FrameAllocator that returns usable frames from the bootloader's memory map.
+pub struct BootInfoFrameAllocator {
+    memory_map: &'static MemoryMap,
+    next: usize,
+}
+
+impl BootInfoFrameAllocator {
+    /// Create a FrameAllocator from the passed memory map.
+    ///
+    /// This function is unsafe because the caller must guarantee that the passed
+    /// memory map is valid. The main requirement is that all frames that are marked
+    /// as `USABLE` in it are really unused.
+    pub unsafe fn init(memory_map: &'static MemoryMap) -> Self {
+        BootInfoFrameAllocator {
+            memory_map,
+            next: 0,
+        }
+    }
+
+    /// Returns an iterator over the usable frames specified in the memory map.
+    fn usable_frames(&self) -> impl Iterator<Item = UnusedPhysFrame> {
+        // get usable regions from memory map
+        let regions = self.memory_map.iter();
+        let usable_regions = regions.filter(|r| r.region_type == MemoryRegionType::Usable);
+        // map each region to its address range
+        let addr_ranges = usable_regions.map(|r| r.range.start_addr()..r.range.end_addr());
+        // transform to an iterator of frame start addresses
+        let frame_addresses = addr_ranges.flat_map(|r| r.step_by(4096));
+        // create `PhysFrame` types from the start addresses
+        let frames = frame_addresses.map(|addr| PhysFrame::containing_address(PhysAddr::new(addr)));
+        // we know that the frames are really unused
+        frames.map(|f| unsafe { UnusedPhysFrame::new(f) })
+    }
+}
+
+unsafe impl FrameAllocator<Size4KiB> for BootInfoFrameAllocator {
+    fn allocate_frame(&mut self) -> Option<UnusedPhysFrame> {
+        let frame = self.usable_frames().nth(self.next);
+        self.next += 1;
+        frame
+    }
+}
--- a/src/multitasking/context_switch.rs
+++ b/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()
+}
--- a/src/multitasking/mod.rs
+++ b/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))
+}
--- a/src/multitasking/scheduler.rs
+++ b/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);
+        }
+    }
+}
--- a/src/multitasking/thread.rs
+++ b/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
+    }
+}
--- a/src/vga_buffer.rs
+++ b/src/vga_buffer.rs
@@ -3,6 +3,9 @@ use lazy_static::lazy_static;
 use spin::Mutex;
 use volatile::Volatile;

+#[cfg(test)]
+use crate::{serial_print, serial_println};
+
 lazy_static! {
    /// A global `Writer` instance that can be used for printing to the VGA text buffer.
    ///
@@ -177,14 +180,18 @@ pub fn _print(args: fmt::Arguments) {

 #[test_case]
 fn test_println_simple() {
+    serial_print!("test_println... ");
    println!("test_println_simple output");
+    serial_println!("[ok]");
 }

 #[test_case]
 fn test_println_many() {
+    serial_print!("test_println_many... ");
    for _ in 0..200 {
        println!("test_println_many output");
    }
+    serial_println!("[ok]");
 }

 #[test_case]
@@ -192,6 +199,8 @@ fn test_println_output() {
    use core::fmt::Write;
    use x86_64::instructions::interrupts;

+    serial_print!("test_println_output... ");
+
    let s = "Some test string that fits on a single line";
    interrupts::without_interrupts(|| {
        let mut writer = WRITER.lock();
@@ -201,4 +210,6 @@ fn test_println_output() {
            assert_eq!(char::from(screen_char.ascii_character), c);
        }
    });
+
+    serial_println!("[ok]");
 }
--- a/tests/basic_boot.rs
+++ b/tests/basic_boot.rs
@@ -4,10 +4,10 @@
 #![test_runner(blog_os::test_runner)]
 #![reexport_test_harness_main = "test_main"]

-use blog_os::println;
+use blog_os::{println, serial_print, serial_println};
 use core::panic::PanicInfo;

-#[unsafe(no_mangle)] // don't mangle the name of this function
+#[no_mangle] // don't mangle the name of this function
 pub extern "C" fn _start() -> ! {
    test_main();

@@ -21,5 +21,7 @@ fn panic(info: &PanicInfo) -> ! {

 #[test_case]
 fn test_println() {
+    serial_print!("test_println... ");
    println!("test_println output");
+    serial_println!("[ok]");
 }
--- a/tests/heap_allocation.rs
+++ b/tests/heap_allocation.rs
@@ -0,0 +1,76 @@
+#![no_std]
+#![no_main]
+#![feature(custom_test_frameworks)]
+#![test_runner(blog_os::test_runner)]
+#![reexport_test_harness_main = "test_main"]
+
+extern crate alloc;
+
+use alloc::{boxed::Box, vec::Vec};
+use blog_os::{allocator::HEAP_SIZE, serial_print, serial_println};
+use bootloader::{entry_point, BootInfo};
+use core::panic::PanicInfo;
+
+entry_point!(main);
+
+fn main(boot_info: &'static BootInfo) -> ! {
+    use blog_os::allocator;
+    use blog_os::memory::{self, BootInfoFrameAllocator};
+    use x86_64::VirtAddr;
+
+    blog_os::init();
+    let phys_mem_offset = VirtAddr::new(boot_info.physical_memory_offset);
+    let mut mapper = unsafe { memory::init(phys_mem_offset) };
+    let mut frame_allocator = unsafe { BootInfoFrameAllocator::init(&boot_info.memory_map) };
+    allocator::init_heap(&mut mapper, &mut frame_allocator).expect("heap initialization failed");
+
+    test_main();
+    loop {}
+}
+
+#[test_case]
+fn simple_allocation() {
+    serial_print!("simple_allocation... ");
+    let heap_value = Box::new(41);
+    assert_eq!(*heap_value, 41);
+    serial_println!("[ok]");
+}
+
+#[test_case]
+fn large_vec() {
+    serial_print!("large_vec... ");
+    let n = 1000;
+    let mut vec = Vec::new();
+    for i in 0..n {
+        vec.push(i);
+    }
+    assert_eq!(vec.iter().sum::<u64>(), (n - 1) * n / 2);
+    serial_println!("[ok]");
+}
+
+#[test_case]
+fn many_boxes() {
+    serial_print!("many_boxes... ");
+    for i in 0..HEAP_SIZE {
+        let x = Box::new(i);
+        assert_eq!(*x, i);
+    }
+    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)
+}
--- a/tests/should_panic.rs
+++ b/tests/should_panic.rs
@@ -1,10 +1,10 @@
 #![no_std]
 #![no_main]

-use blog_os::{QemuExitCode, exit_qemu, serial_print, serial_println};
+use blog_os::{exit_qemu, serial_print, serial_println, QemuExitCode};
 use core::panic::PanicInfo;

-#[unsafe(no_mangle)]
+#[no_mangle]
 pub extern "C" fn _start() -> ! {
    should_fail();
    serial_println!("[test did not panic]");
@@ -13,7 +13,7 @@ pub extern "C" fn _start() -> ! {
 }

 fn should_fail() {
-    serial_print!("should_panic::should_fail...\t");
+    serial_print!("should_fail... ");
    assert_eq!(0, 1);
 }

--- a/tests/stack_overflow.rs
+++ b/tests/stack_overflow.rs
@@ -2,14 +2,14 @@
 #![no_main]
 #![feature(abi_x86_interrupt)]

-use blog_os::{QemuExitCode, exit_qemu, serial_print, serial_println};
+use blog_os::{exit_qemu, serial_print, serial_println, QemuExitCode};
 use core::panic::PanicInfo;
 use lazy_static::lazy_static;
 use x86_64::structures::idt::{InterruptDescriptorTable, InterruptStackFrame};

-#[unsafe(no_mangle)]
+#[no_mangle]
 pub extern "C" fn _start() -> ! {
-    serial_print!("stack_overflow::stack_overflow...\t");
+    serial_print!("stack_overflow... ");

    blog_os::gdt::init();
    init_test_idt();
@@ -23,7 +23,6 @@ pub extern "C" fn _start() -> ! {
 #[allow(unconditional_recursion)]
 fn stack_overflow() {
    stack_overflow(); // for each recursion, the return address is pushed
-    volatile::Volatile::new(0).read(); // prevent tail recursion optimizations
 }

 lazy_static! {
@@ -44,7 +43,7 @@ pub fn init_test_idt() {
 }

 extern "x86-interrupt" fn test_double_fault_handler(
-    _stack_frame: InterruptStackFrame,
+    _stack_frame: &mut InterruptStackFrame,
    _error_code: u64,
 ) -> ! {
    serial_println!("[ok]");
--- a/x86_64-blog_os.json
+++ b/x86_64-blog_os.json
@@ -1,6 +1,6 @@
 {
    "llvm-target": "x86_64-unknown-none",
-    "data-layout": "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128",
+    "data-layout": "e-m:e-i64:64-f80:128-n8:16:32:64-S128",
    "arch": "x86_64",
    "target-endian": "little",
    "target-pointer-width": "64",
@@ -11,6 +11,5 @@
    "linker": "rust-lld",
    "panic-strategy": "abort",
    "disable-redzone": true,
-    "features": "-mmx,-sse,+soft-float",
-    "rustc-abi": "x86-softfloat"
+    "features": "-mmx,-sse,+soft-float"
  }