rust-kernel

# rust-kernel

A tiny hobby x86_64 kernel written in Rust, following the 2nd edition of
Philipp Oppermann's [Writing an OS in Rust](https://os.phil-opp.com/).
Right now it boots in QEMU, prints to the VGA text screen and the serial port
(COM1), handles CPU exceptions (breakpoint, page fault, double fault on its own
stack), runs the timer interrupt (uptime clock top-right), echoes what you
type on the PS/2 keyboard, walks and edits its own page tables, and has a
kernel heap (`Box`, `Vec`, `Rc` work) backed by a fixed-size-block allocator.
It has a `cargo test` suite that runs inside QEMU.

Most code follows the blog_os sources (MIT / Apache-2.0) closely, with extra
comments; small additions are the serial mirroring, uptime clock, demo
features and scripts.

![boot screenshot](boot.png)

## What's installed (on this box)

| Tool | Version | Why |
|---|---|---|
| rustup | 1.29.2 | toolchain manager (in `~/.cargo/bin`, ahead of Debian's `/usr/bin/cargo`) |
| Rust nightly | rustc 1.101.0-nightly (2026-10-29) | custom JSON targets + `build-std` are nightly-only |
| `rust-src` component | – | source of `core`, recompiled for our bare-metal target |
| `llvm-tools-preview` component | – | `llvm-objcopy` etc., used by `bootimage` |
| `bootimage` (cargo subcommand) | 0.10.5 | links kernel + bootloader into a bootable disk image |
| QEMU (`qemu-system-x86`, `qemu-system-gui`) | 10.0.13 | the emulator we boot in (`-gui` adds the GTK/SDL window) |

`rust-toolchain.toml` pins the project to nightly with the two components, so
`cargo` inside this directory picks them up automatically.

Crates: `bootloader` 0.9.35 (BIOS bootloader used by the 2nd edition),
`volatile` 0.2, `spin` 0.5, `x86_64` 0.15, `uart_16550` 0.3, `lazy_static` 1.5,
`pic8259` 0.11 (interrupt controller), `pc-keyboard` 0.7 (scancode decoding),
`linked_list_allocator` 0.9 (fallback heap allocator).

Fresh setup on another machine:

```sh
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh   # rustup
rustup toolchain install nightly -c rust-src -c llvm-tools-preview
cargo install bootimage
sudo apt install qemu-system-x86 qemu-system-gui
```

## Build and run

```sh
cargo bootimage                     # build -> target/x86_64-rust_kernel/debug/bootimage-rust_kernel.bin
cargo run                           # build + boot in a QEMU window; serial output also prints in the terminal
```

**Trying the keyboard:** with `cargo run`, click into the QEMU window and type;
characters appear on the bottom line of the screen *and* in your terminal (via
serial). Shift works; Enter starts a new line; arrows/F-keys show as e.g.
`<ArrowUp>`. Backspace isn't handled yet (shows a block) -- a good first
exercise. To get the mouse/keyboard back from QEMU press Ctrl+Alt+G; close the
window or press Ctrl+C in the terminal to quit. (Over SSH without X, use
`cargo run -- -display curses` to see the VGA screen right in your terminal;
quit from another terminal with `pkill qemu-system`.)

Headless (no window, good for SSH / scripts):

```sh
./scripts/run-headless.sh                 # boot, breakpoint, ~3 s of timer ticks, then QEMU exits (script exit 0)
./scripts/run-headless.sh stack-overflow  # deliberately overflow the stack; passes if the double-fault handler catches it
./scripts/run-headless.sh page-fault      # write to unmapped 0xdeadbeaf; passes if the page-fault handler catches it
cargo run --features headless -- -display none                 # same as the first, via cargo
cargo run --features headless,stack-overflow -- -display none  # same as the second
                                          # (QEMU's exit status is 33 = success; cargo just reports it as non-zero)
cargo run -- -display none                # headless but kernel keeps running; Ctrl+C to stop
./scripts/screenshot.sh [out.png] [text]  # boot, "type" text via QEMU monitor `sendkey`, check it echoes
                                          # on serial, save the VGA screen to boot.png
```

Cargo features (in `Cargo.toml`):

| Feature | Effect |
|---|---|
| `headless` | print a `.` per timer tick to serial and exit QEMU after 54 ticks (~3 s); panics exit with failure |
| `stack-overflow` | after boot, recurse forever to test the double-fault handler (with `headless`: exit success when caught) |
| `page-fault` | after reading CR3, write to unmapped address `0xdeadbeaf` (with `headless`: exit success when caught) |

Expected serial output of `./scripts/run-headless.sh` (stack frame dump trimmed):

```
Hello from Tyler's Rust kernel!
EXCEPTION: BREAKPOINT
InterruptStackFrame { instruction_pointer: ..., ... }
Survived the breakpoint, still running.
[paging] CR3 (level 4 table) at phys 0x1000
[paging] physical memory mapped at virt 0x18000000000
[memory] 16 memory map regions, 123732 KiB usable RAM
[paging] translate VirtAddr(0xb8000) -> Some(PhysAddr(0xb8000))
[paging] translate VirtAddr(0x201008) -> Some(PhysAddr(0x401008))
[paging] translate VirtAddr(0x10000201a10) -> Some(PhysAddr(0x27ea10))
[paging] translate VirtAddr(0x18000000000) -> Some(PhysAddr(0x0))
[paging] mapped page 0xdeadbeaf000 -> VGA frame, wrote "New!" through it
[heap] 100 KiB heap at 0x444444440000 (fixed-size-block allocator)
[heap] Box::new(41) = 41 at 0x444444440000
[heap] Vec of 500 numbers at 0x444444441000, sum = 124750
[heap] Rc count after clone = 2
[heap] Rc count after drop  = 1
Type something! (keyboard echo)
Timer + keyboard interrupts enabled.
.................. 18 ticks
.................. 36 ticks
.................. 54 ticks
[ok] timer works, exiting
```

## Tests

```sh
cargo test                        # everything (lib unit tests + tests/*.rs), each booted in QEMU headless
cargo test --lib                  # just the unit tests inside src/
cargo test --test heap_allocation # one integration test
```

Each test binary is its own little kernel: `bootimage runner` boots it with the
`test-args` from `Cargo.toml` (no display, serial to stdout, isa-debug-exit) and
treats QEMU exit code 33 as a pass. Output looks like:

```
Running 5 tests
heap_allocation::large_allocation_uses_fallback...  [ok]
heap_allocation::large_vec...                       [ok]
heap_allocation::many_boxes...                      [ok]
heap_allocation::many_boxes_long_lived...           [ok]
heap_allocation::simple_allocation...               [ok]
```

| Test | What it checks |
|---|---|
| lib (`src/*.rs` `#[test_case]`s) | trivial assertion, breakpoint handler returns, VGA `println!` writes the right cells |
| `tests/basic_boot.rs` | printing works straight after boot, before any `init()` |
| `tests/heap_allocation.rs` | Box, big Vec, 100 K short-lived boxes (memory reuse), same with one long-lived box, >2 KiB fallback allocation |
| `tests/should_panic.rs` | a failing `assert_eq!` reaches the panic handler (`harness = false`) |
| `tests/stack_overflow.rs` | stack overflow lands in the double-fault handler on the IST stack (`harness = false`) |

How it works: `#![feature(custom_test_frameworks)]` collects every `#[test_case]`
function and passes them to `rust_kernel::test_runner` (in `src/lib.rs`), which
prints each name to serial, runs it, prints `[ok]`, then exits QEMU with success.
A panic goes to `test_panic_handler`, which prints `[failed]` and exits with failure.

Extra args after `--` in `cargo run -- ...` are appended to the QEMU command
line, e.g. `cargo run -- -d int -no-reboot` to log every interrupt and stop on
triple faults, or `cargo run -- -s -S` to wait for `gdb` on port 1234.

## Project layout

```
.cargo/config.toml        build-std (core, alloc) + custom target + scripts/runner.sh as the cargo runner
rust-toolchain.toml       pin nightly + rust-src + llvm-tools-preview
x86_64-rust_kernel.json   custom target: no OS, no red zone, soft-float, abort on panic, rust-lld
Cargo.toml                deps, features, test settings, QEMU run/test args
src/lib.rs                the kernel library: module list, init(), kprintln!, test framework, exit_qemu
src/main.rs               the bootable binary: entry_point!(kernel_main), paging + heap demos, panic handler
src/gdt.rs                GDT + TSS with a separate IST stack for the double-fault handler
src/interrupts.rs         IDT; breakpoint / page fault / double fault handlers; 8259 PIC;
                          timer (uptime + ticks) and keyboard (echo) interrupt handlers
src/memory.rs             OffsetPageTable setup from CR3, example mapping, BootInfoFrameAllocator
src/allocator.rs          heap location/size, init_heap (maps heap pages), #[global_allocator], Locked<A>
src/allocator/bump.rs     bump allocator (not active, for learning)
src/allocator/fixed_size_block.rs  fixed-size-block allocator (ACTIVE), falls back to linked_list_allocator
src/vga_buffer.rs         VGA text-mode writer, print!/println!, uptime display
src/serial.rs             COM1 UART writer + serial_print!/serial_println!
tests/                    integration tests, one QEMU-booted kernel each
scripts/runner.sh         `cargo run`/`cargo test` runner: `bootimage runner` + stale-image fix
scripts/run-headless.sh   headless boot with timeout + exit-code check
scripts/screenshot.sh     headless boot + typed keys + PNG screenshot via QEMU monitor
boot.png                  screenshot of the VGA screen
```

## What the interrupt code does (chapters 5-7)

- **CPU exceptions / IDT** (`interrupts.rs`): the IDT maps vector numbers to handler
  functions written with `extern "x86-interrupt"` (nightly `abi_x86_interrupt`), so
  the compiler saves all registers and returns with `iretq`. `_start` fires an
  `int3` on purpose; the breakpoint handler prints the stack frame and execution
  continues. A page-fault handler prints the faulting address from CR2.
- **Double faults** (`gdt.rs`): a stack overflow hits the guard page -> page fault ->
  the CPU can't push the exception frame on the broken stack -> double fault ->
  same again -> *triple fault* = machine reset. We load our own GDT containing a
  TSS whose Interrupt Stack Table entry 0 points at a separate 20 KiB static stack,
  and mark the double-fault IDT entry to use it. Try `./scripts/run-headless.sh stack-overflow`.
- **Hardware interrupts**: the two 8259 PICs are remapped to vectors 32-47 (0-31 are
  CPU exceptions) and then `sti` enables interrupts. The timer (IRQ 0, ~18.2 Hz)
  bumps an atomic tick counter and redraws the uptime clock; the keyboard (IRQ 1)
  reads the scancode from port 0x60, decodes it with `pc-keyboard` (US layout,
  scancode set 1) and echoes it. Each handler must send End-Of-Interrupt to the
  PIC or no further interrupts arrive. `print!`/`serial_print!` disable interrupts
  while holding their lock, otherwise an interrupt handler printing while
  `_start` holds the lock would deadlock. The idle loop uses `hlt`, so QEMU
  doesn't burn a host CPU core.

## Memory (chapters 8-11)

**Paging.** Every address the kernel uses is *virtual*. The CPU translates it with
a 4-level page table tree (level 4 -> 3 -> 2 -> 1 -> 4 KiB page); 9 bits of the
address index each level, the low 12 bits are the offset in the page. CR3 holds the
*physical* address of the level-4 table (here `0x1000`). The `page-fault` feature
writes to `0xdeadbeaf`, which no table maps, and the page-fault handler reports
the address (from CR2) and the reason (`CAUSED_BY_WRITE`, not `PROTECTION_VIOLATION`
-> the page simply isn't present).

**Reaching page tables.** Page tables contain physical addresses, but we can only
dereference virtual ones. With the bootloader's `map_physical_memory` feature, all of
physical RAM is also mapped at `physical_memory_offset` (`0x18000000000` here), so
`virt = offset + phys`. `memory::init` wraps that in the x86_64 crate's
`OffsetPageTable`, which can `translate_addr` (walk the tables) and `map_to` (add entries).

**Virtual memory layout** (as set up by bootloader 0.9 in this build):

| Virtual address | What |
|---|---|
| `0xb8000` | VGA text buffer (identity-mapped: virt = phys) |
| `0x200000`... | kernel code + data (the ELF; maps to phys around `0x400000`) |
| `0x0100_0000_0000`... | kernel stack (80 pages, guard page below it) |
| `0x0180_0000_0000`... | all physical memory (`physical_memory_offset`) |
| `0x4444_4444_0000` | kernel heap, 100 KiB (we map it in `allocator::init_heap`) |
| `0xdead_beaf_000` | demo page we map to the VGA frame (writes "New!" on screen) |

The "New!" demo writes to screen row 20 through the new mapping; by the time you look,
later lines have scrolled it up, so in `boot.png` it overwrote the start of the first line.

**Physical frames.** `BootInfoFrameAllocator` hands out 4 KiB frames from the regions
the BIOS memory map marks `Usable` (~121 MiB of QEMU's default 128 MiB). It never frees;
fine for now.

**Heap.** `allocator::init_heap` maps the 25 pages of `HEAP_START..+HEAP_SIZE` to fresh
frames, then gives the range to the `#[global_allocator]`. From then on the `alloc`
crate (`Box`, `Vec`, `Rc`, `String`, `BTreeMap`...) works. `alloc` is compiled for our
target by `build-std` in `.cargo/config.toml`.

**Allocator designs** (`src/allocator/`): the *bump* allocator just moves a pointer
forward and can only reclaim memory when every allocation is freed (it fails the
`many_boxes_long_lived` test). The `linked_list_allocator` crate keeps a list of free
regions (first fit, slower, but reuses any freed memory). The active
*fixed-size-block* allocator rounds each request up to 8/16/.../2048 bytes and keeps
one free list per size, so alloc/free are O(1) pops/pushes; bigger requests and empty
lists fall back to the linked-list allocator. Switch allocators by editing one line in
`src/allocator.rs`.

## Roadmap

Done: chapters 1-11 of the 2nd edition (everything up to Async/Await).

- [x] Freestanding binary — [phil-opp](https://os.phil-opp.com/freestanding-rust-binary/)
- [x] Minimal kernel booting in QEMU — [phil-opp](https://os.phil-opp.com/minimal-rust-kernel/), [OSDev: Bare Bones](https://wiki.osdev.org/Bare_Bones)
- [x] VGA text mode printing — [phil-opp](https://os.phil-opp.com/vga-text-mode/), [OSDev: Printing to Screen](https://wiki.osdev.org/Printing_To_Screen)
- [x] Serial output + QEMU exit — [phil-opp: Testing](https://os.phil-opp.com/testing/), [OSDev: Serial Ports](https://wiki.osdev.org/Serial_Ports)
- [x] CPU exceptions / IDT — breakpoint & page fault handlers. [phil-opp: CPU Exceptions](https://os.phil-opp.com/cpu-exceptions/), [OSDev: IDT](https://wiki.osdev.org/Interrupt_Descriptor_Table), [OSDev: Exceptions](https://wiki.osdev.org/Exceptions)
- [x] GDT / TSS / double faults — separate IST stack so stack overflows don't triple-fault. [phil-opp: Double Faults](https://os.phil-opp.com/double-fault-exceptions/), [OSDev: GDT](https://wiki.osdev.org/Global_Descriptor_Table), [OSDev: TSS](https://wiki.osdev.org/Task_State_Segment)
- [x] Testing — `cargo test` in QEMU, custom test framework, integration tests. [phil-opp: Testing](https://os.phil-opp.com/testing/)
- [x] Hardware interrupts — 8259 PIC, timer, PS/2 keyboard echo, `hlt` idle loop. [phil-opp: Hardware Interrupts](https://os.phil-opp.com/hardware-interrupts/), [OSDev: 8259 PIC](https://wiki.osdev.org/8259_PIC), [OSDev: PIT](https://wiki.osdev.org/Programmable_Interval_Timer), [OSDev: PS/2 Keyboard](https://wiki.osdev.org/PS/2_Keyboard) (later: [APIC](https://wiki.osdev.org/APIC))

- [x] Introduction to paging — CR3, page fault demo. [phil-opp](https://os.phil-opp.com/paging-introduction/), [OSDev: Paging](https://wiki.osdev.org/Paging)
- [x] Paging implementation — OffsetPageTable, translation, frame allocator, example mapping. [phil-opp](https://os.phil-opp.com/paging-implementation/), [OSDev: Page Frame Allocation](https://wiki.osdev.org/Page_Frame_Allocation)
- [x] Heap allocation — `alloc`, heap mapping, global allocator. [phil-opp](https://os.phil-opp.com/heap-allocation/), [OSDev: Memory Allocation](https://wiki.osdev.org/Memory_Allocation)
- [x] Allocator designs — bump, linked list (crate), fixed-size block (active). [phil-opp](https://os.phil-opp.com/allocator-designs/)

Next stages:

1. **Async/await multitasking** — a cooperative executor; turn the keyboard handler
   into a task that reads scancodes from a lock-free queue (also gives you a place to
   handle Backspace properly). [phil-opp: Async/Await](https://os.phil-opp.com/async-await/),
   [OSDev: Multitasking Systems](https://wiki.osdev.org/Multitasking_Systems)
2. **Preemptive threads** — timer-driven context switches between kernel threads with
   their own stacks. [OSDev: Brendan's Multi-tasking Tutorial](https://wiki.osdev.org/Brendan%27s_Multi-tasking_Tutorial),
   [OSDev: Context Switching](https://wiki.osdev.org/Context_Switching)
3. **User mode & syscalls** — per-process page tables, ring 3 code/data segments in
   the GDT, `syscall`/`sysret`, loading ELF programs. (Beyond the phil-opp series.)
   [OSDev: Getting to Ring 3](https://wiki.osdev.org/Getting_to_Ring_3),
   [OSDev: System Calls](https://wiki.osdev.org/System_Calls),
   [OSDev: SYSENTER/SYSCALL](https://wiki.osdev.org/SYSENTER), [OSDev: ELF](https://wiki.osdev.org/ELF)
4. **Filesystem** — a VFS layer, a RAM disk / initrd first, then FAT on an ATA/virtio disk.
   [OSDev: File Systems](https://wiki.osdev.org/File_Systems), [OSDev: VFS](https://wiki.osdev.org/VFS),
   [OSDev: Initrd](https://wiki.osdev.org/Initrd), [OSDev: FAT](https://wiki.osdev.org/FAT),
   [OSDev: ATA PIO Mode](https://wiki.osdev.org/ATA_PIO_Mode)
5. **Shell** — line editing on top of the keyboard task, a few built-in commands
   (`help`, `mem`, `uptime`, `ls`, `cat`), later launching user programs.
   [OSDev: Shell](https://wiki.osdev.org/Shell)

Small exercises along the way: handle Backspace, speed up the PIT
([OSDev: PIT](https://wiki.osdev.org/Programmable_Interval_Timer)), add handlers for
divide error / general protection fault, make `BootInfoFrameAllocator` O(1), add a
`mem` stat (bytes allocated) to the allocator.

Other good references: [OSDev wiki main page](https://wiki.osdev.org/Expanded_Main_Page),
[Intel SDM](https://www.intel.com/content/www/us/en/developer/articles/technical/intel-sdms.html),
the [blog_os repo](https://github.com/phil-opp/blog_os) (one branch per post, e.g. `post-04`),
and for a UEFI/framebuffer setup later, the newer [`bootloader` 0.11](https://github.com/rust-osdev/bootloader) crate.

## Notes / gotchas

- The target JSON changes occasionally with new nightlies (e.g. `target-pointer-width`
  became an integer, `rustc-abi` was added). If a nightly update breaks the build,
  compare with the latest `x86_64-blog_os.json` in the blog_os repo, or pin a date in
  `rust-toolchain.toml` (`channel = "nightly-2026-10-29"`).
- The "packages contain code that will be rejected by a future version of Rust: core"
  warning comes from building `core` with build-std and is harmless.
- `bootimage` prints "`CARGO_MANIFEST_DIR` env variable not set" when run outside
  `cargo run`; also harmless.
- `cargo test` with build-std breaks ("duplicate lang item in crate `core`: `sized`")
  if `Cargo.toml` sets `panic = "abort"` in `[profile.*]`. The target JSON already
  sets `"panic-strategy": "abort"`, so the profile settings were removed.
- Stale disk image when switching cargo features: cargo re-uses a previously built
  kernel ELF with an *old* mtime, so bootimage thinks its cached image is current
  and boots the wrong build. `scripts/runner.sh` and the scripts `touch` the ELF
  first to avoid this. If you call `cargo bootimage --features ...` by hand after
  switching features, `touch target/x86_64-rust_kernel/debug/rust_kernel` first.