Julia Evans

Writing an OS in Rust in tiny steps (Steps 1-5)

I’m giving a talk tomorrow on writing a kernel in Rust.

My experience of writing a kernel that it was like jumping in puddles: it’s a lot of fun, and there are a lot of mishaps:

Here are a few of the tiny steps I took. There are more, but those will have to wait for the evening.

Step 1: copy some code from the internet

I didn’t know what I was doing, so I didn’t want to start from scratch! So I started with something that already existed! Behold rustboot, a tiny 32-bit kernel written in Rust.

Rustboot does only two things, but it does them well!

  1. Turn the screen red
  2. Hang

Of course what it actually does is a bit more complicated – there’s

  • a loader written in assembly
  • a Makefile that lets you run it with qemu
  • Some Rust code to clear the screen

Here’s the code that clears the screen:

unsafe fn clear_screen(background: Color) {
    range(0, 80*25, |i| {
        *((0xb8000 + i * 2) as *mut u16) = (background as u16) << 12;

What does this mean? The key part here is that the address of the VGA buffer is 0xb8000, so we’re setting some bytes there. And there’s a loop.

Step 2: Turn the screen blue instead.

The first thing I did was:

  1. Make sure I could run rustboot.
  2. Change ‘red’ to ‘blue’ and run it again

This sounds silly, but psychologically it’s an important step! It forced me to look at the code and understand how it worked, and it was really exciting that it worked right away.

Step 3: Start writing I/O functions

The next obvious step now that I had a blue screen was to try to write a print function.

Here’s what it looked like!

pub fn putchar(x: u16, y: u16, c: u8) {
    let idx : uint =  (y * VGA_WIDTH * 2 + x * 2) as uint;
    unsafe {
        *((VGA_ADDRESS + idx) as *mut u16) = make_vgaentry(c, fg_color, bg_color);

I didn’t explain the unsafe block before. Everything inside unsafe{} is unsafe code. This particular code is unsafe because it accesses a memory address directly. Wrapping it in an unsafe block tells Rust “okay, I checked and I promise this code is actually doing the right thing and won’t blow anything up”.

We can also look at make_vgaentry:

fn make_vgaentry(c: u8, fg: Color, bg: Color) -> u16 {
    let color = fg as u16 | (bg as u16 << 4);
    return c as u16 | (color << 8);

In the VGA buffer, each character is represented by 2 bytes (so a u16). The lower 8 bits are the ASCII character, and the upper 8 bits are the foreground and background colour (4 bits each). Color here is an enum so that I can refer to Red or Green directly.

I found this part pretty approachable and it didn’t take too long. Which isn’t to say that I didn’t have problems! I had SO MANY PROBLEMS. Most of my problems were to do with arrays and string and iterating over strings. Here’s some code that caused me much pain:

pub fn write(s: &str) {
    let bytes : &[u8] = as_bytes(s);
    for b in super::core::slice::iter(bytes) {

This code looks simple! It is a lie. Friends. Here were some questions that I needed to ask to write this code.

  • How do I turn a string into a byte array? (as_bytes())
  • What is the type of a byte array? (&[u8])
  • How do I iterate over a byte array? (+ “it still doesn’t work!”, 4 times)

Also, what is this super::core::slice::iter business? This brings us to a fairly long digression, and an important point

Why you can’t write a kernel in Python

So you want to write an operating system, let’s say for x86. You need to write this in a programming language!

Can you write your operating system in Python (using CPython, say)? You cannot. This is not being curmudgeonly! It is actually just not possible.

What happens when you write print "Hello!" in Python?

Well, many things happen. But the last thing that happens is that the CPython interpreter will do something like printf("Hello"). And you might think, well, maybe I could link against the code for printf somehow!

But what printf does is it calls the write() system call. The write() system call is implemented IN YOUR KERNEL.


This also means that you can’t write a kernel as a “normal” C program which includes C libraries. Any C libraries. All C libraries for Linux are built on top of some version of libc, which makes calls to the Linux kernel! So if you’re writing a kernel, this doesn’t work.

Why you can write a kernel in Rust

Writing Rust code has many of the same problems, of course! By default, if you compile a Rust program with a print statement, it will call your kernel’s equivalent to write.

But! Unlike with Python, you can put #[no_std] at the beginning of your Rust program.

You lose a lot! You can no longer

  • allocate memory
  • do threading
  • print anything
  • many many more things

It’s still totally fine to define functions and make calculations, though. And you can of course define your own functions to allocate memory.

You also lose things like Rust’s iterators, which is sad!


rust-core is “a standard library for Rust with freestanding support”. What this means is that if you’re writing an OS, rust-core will provide you with all kinds of helpful data structures and functions that you lost when you wrote #[no_std].

I found using this library pretty confusing, but the author hangs out in IRC all the time and was really friendly to me, so it wasn’t a huge problem.

So back to super::core::slice::iter! This says “iterate over this using an iteration function from rust-core

Step 4: keyboard interrupts!

So it took me a few days to learn how to print because I needed to learn about freestanding mode and get confused about rust-core and at the same time I didn’t really understand Rust’s types very well.

Once that was done, I wanted to be able to do the following:

  1. Press a key (‘j’ for example)
  2. Have that letter appear on the screen.

I thought this wouldn’t be too hard. I was pretty wrong.

I wrote about what went wrong in After 5 days, my OS doesn’t crash when I press a key.

It lists all my traumas in excruciating detail and I won’t repeat them here. Go read it. It’s kinda worth it. I’ll wait.

Step 5: malloc!

After I’d done that, I thought it might be fun to be able to allocate memory.

You may be surprised at this point. We have printed strings! We have made our keyboard work! Didn’t we need to allocate memory? Isn’t that… important?

It turns out that you can get away without doing it pretty easily! Rust would automatically create variables on the stack for me, so I could use local variables. And for anything else I could use global variables, and the space for those was laid out at compile time.

But allocating memory seemed like a fun exercise. To allocate something on the heap in Rust, you can do

let a = ~2

This creates a pointer to a 2 on the heap. Of course, we talked before about how there is no malloc! So I wrote one, and then made sure that Rust knew about it.

You can see the malloc function I wrote in Writing malloc wrong, for fun

The hardest parts of this were not writing the function, but

  • getting the type right
  • Understanding how Rust’s language features can be turned on and off.


So in rust-core, if you go to heap.rs, you’ll see this code:

#[lang = "exchange_malloc"]
pub unsafe fn alloc(size: uint) -> *mut u8 {
    if size == 0 {
        0 as *mut u8
    } else {
        let ptr = malloc(size);
        if ptr == 0 as *mut u8 {

This weird-looking #[lang = "exchange_malloc"] bit means “Code like let x = ~2 is now allowed to work”. It requires there to be an implementation of malloc, which I wrote. It also needs implements of realloc and free, but I left those blank :)

Before seeing that, Rust would not compile code that allocated memory.

I think this language feature gating is really cool: it means that you can write Rust programs that can allocate memory, but not do threading. Or that can do hardly anything at all!

I need to get up now.


Debugging shared library problems with strace My Rust OS will never be finished (and it's a success!)