Chapter 5: OS Development - WIA-RUST-ADVANCED

Introduction to OS Development

Building an operating system is the ultimate systems programming challenge. Rust's safety guarantees make it an excellent choice for OS development, helping prevent many common kernel bugs while maintaining the low-level control needed for systems programming.

ℹ️ Prerequisites: This chapter assumes you're familiar with x86/x86-64 architecture, bootloaders, and basic OS concepts. We'll be building a minimal kernel following the excellent "Writing an OS in Rust" blog series by Philipp Oppermann.

1. Bootloader and Bare Bones Kernel

Project Setup

📝 Cargo.toml

[package]
name = "rust_os"
version = "0.1.0"
edition = "2021"

[dependencies]
bootloader = "0.9"
volatile = "0.2.6"
spin = "0.9"
x86_64 = "0.14"
uart_16550 = "0.2"
pic8259 = "0.10"
pc-keyboard = "0.7"

[profile.dev]
panic = "abort"

[profile.release]
panic = "abort"

Minimal Kernel

📝 main.rs - Bare Bones Kernel

#![no_std]
#![no_main]

use core::panic::PanicInfo;

#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {
    loop {}
}

static HELLO: &[u8] = b"Hello World!";

#[no_mangle]
pub extern "C" fn _start() -> ! {
    let vga_buffer = 0xb8000 as *mut u8;

    for (i, &byte) in HELLO.iter().enumerate() {
        unsafe {
            *vga_buffer.offset(i as isize * 2) = byte;
            *vga_buffer.offset(i as isize * 2 + 1) = 0xb;
        }
    }

    loop {}
}

VGA Text Mode

📝 VGA Buffer Implementation

use volatile::Volatile;
use core::fmt;

#[allow(dead_code)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u8)]
pub enum Color {
    Black = 0,
    Blue = 1,
    Green = 2,
    Cyan = 3,
    Red = 4,
    Magenta = 5,
    Brown = 6,
    LightGray = 7,
    DarkGray = 8,
    LightBlue = 9,
    LightGreen = 10,
    LightCyan = 11,
    LightRed = 12,
    Pink = 13,
    Yellow = 14,
    White = 15,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(transparent)]
struct ColorCode(u8);

impl ColorCode {
    fn new(foreground: Color, background: Color) -> ColorCode {
        ColorCode((background as u8) << 4 | (foreground as u8))
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(C)]
struct ScreenChar {
    ascii_character: u8,
    color_code: ColorCode,
}

const BUFFER_HEIGHT: usize = 25;
const BUFFER_WIDTH: usize = 80;

#[repr(transparent)]
struct Buffer {
    chars: [[Volatile; BUFFER_WIDTH]; BUFFER_HEIGHT],
}

pub struct Writer {
    column_position: usize,
    color_code: ColorCode,
    buffer: &'static mut Buffer,
}

impl Writer {
    pub fn write_byte(&mut self, byte: u8) {
        match byte {
            b'\n' => self.new_line(),
            byte => {
                if self.column_position >= BUFFER_WIDTH {
                    self.new_line();
                }

                let row = BUFFER_HEIGHT - 1;
                let col = self.column_position;

                let color_code = self.color_code;
                self.buffer.chars[row][col].write(ScreenChar {
                    ascii_character: byte,
                    color_code,
                });
                self.column_position += 1;
            }
        }
    }

    pub fn write_string(&mut self, s: &str) {
        for byte in s.bytes() {
            match byte {
                0x20..=0x7e | b'\n' => self.write_byte(byte),
                _ => self.write_byte(0xfe),
            }
        }
    }

    fn new_line(&mut self) {
        for row in 1..BUFFER_HEIGHT {
            for col in 0..BUFFER_WIDTH {
                let character = self.buffer.chars[row][col].read();
                self.buffer.chars[row - 1][col].write(character);
            }
        }
        self.clear_row(BUFFER_HEIGHT - 1);
        self.column_position = 0;
    }

    fn clear_row(&mut self, row: usize) {
        let blank = ScreenChar {
            ascii_character: b' ',
            color_code: self.color_code,
        };
        for col in 0..BUFFER_WIDTH {
            self.buffer.chars[row][col].write(blank);
        }
    }
}

impl fmt::Write for Writer {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        self.write_string(s);
        Ok(())
    }
}

use spin::Mutex;

lazy_static::lazy_static! {
    pub static ref WRITER: Mutex = Mutex::new(Writer {
        column_position: 0,
        color_code: ColorCode::new(Color::Yellow, Color::Black),
        buffer: unsafe { &mut *(0xb8000 as *mut Buffer) },
    });
}

#[macro_export]
macro_rules! print {
    ($($arg:tt)*) => ($crate::vga_buffer::_print(format_args!($($arg)*)));
}

#[macro_export]
macro_rules! println {
    () => ($crate::print!("\n"));
    ($($arg:tt)*) => ($crate::print!("{}\n", format_args!($($arg)*)));
}

#[doc(hidden)]
pub fn _print(args: fmt::Arguments) {
    use core::fmt::Write;
    WRITER.lock().write_fmt(args).unwrap();
}

2. Interrupt Handling

Interrupt Descriptor Table (IDT)

CPU Exception/Interrupt
         ↓
    IDT Lookup
         ↓
  Handler Function
         ↓
  Process Interrupt
         ↓
      IRET

📝 IDT Implementation

use x86_64::structures::idt::{InterruptDescriptorTable, InterruptStackFrame};

lazy_static::lazy_static! {
    static ref IDT: InterruptDescriptorTable = {
        let mut idt = InterruptDescriptorTable::new();
        idt.breakpoint.set_handler_fn(breakpoint_handler);
        idt.double_fault.set_handler_fn(double_fault_handler);
        idt[InterruptIndex::Timer.as_usize()]
            .set_handler_fn(timer_interrupt_handler);
        idt[InterruptIndex::Keyboard.as_usize()]
            .set_handler_fn(keyboard_interrupt_handler);
        idt
    };
}

pub fn init_idt() {
    IDT.load();
}

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

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

extern "x86-interrupt" fn timer_interrupt_handler(
    _stack_frame: InterruptStackFrame
) {
    print!(".");

    unsafe {
        PICS.lock()
            .notify_end_of_interrupt(InterruptIndex::Timer.as_u8());
    }
}

extern "x86-interrupt" fn keyboard_interrupt_handler(
    _stack_frame: InterruptStackFrame
) {
    use x86_64::instructions::port::Port;

    let mut port = Port::new(0x60);
    let scancode: u8 = unsafe { port.read() };

    crate::task::keyboard::add_scancode(scancode);

    unsafe {
        PICS.lock()
            .notify_end_of_interrupt(InterruptIndex::Keyboard.as_u8());
    }
}

Programmable Interrupt Controller (PIC)

📝 PIC Configuration

use pic8259::ChainedPics;
use spin;

pub const PIC_1_OFFSET: u8 = 32;
pub const PIC_2_OFFSET: u8 = 40;

pub static PICS: spin::Mutex =
    spin::Mutex::new(unsafe { ChainedPics::new(PIC_1_OFFSET, PIC_2_OFFSET) });

#[derive(Debug, Clone, Copy)]
#[repr(u8)]
pub enum InterruptIndex {
    Timer = PIC_1_OFFSET,
    Keyboard,
}

impl InterruptIndex {
    fn as_u8(self) -> u8 {
        self as u8
    }

    fn as_usize(self) -> usize {
        usize::from(self.as_u8())
    }
}

pub fn init() {
    unsafe { PICS.lock().initialize() };
    x86_64::instructions::interrupts::enable();
}

3. Memory Management

Paging

Virtual Address (48-bit)
 ┌────────┬────────┬────────┬────────┬──────────┐
 │ Level 4│ Level 3│ Level 2│ Level 1│   Offset │
 └────────┴────────┴────────┴────────┴──────────┘
     ↓        ↓        ↓        ↓         ↓
    PML4 →   PDP →    PD  →   PT   →  Physical
                                         Address

📝 Page Table Implementation

use x86_64::{
    structures::paging::{
        Page, PageTable, PageTableFlags, PhysFrame, Mapper, Size4KiB,
        FrameAllocator,
    },
    VirtAddr, PhysAddr,
};

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)
}

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
}

pub fn create_example_mapping(
    page: Page,
    mapper: &mut OffsetPageTable,
    frame_allocator: &mut impl FrameAllocator,
) {
    use x86_64::structures::paging::PageTableFlags as Flags;

    let frame = PhysFrame::containing_address(PhysAddr::new(0xb8000));
    let flags = Flags::PRESENT | Flags::WRITABLE;

    let map_to_result = unsafe {
        mapper.map_to(page, frame, flags, frame_allocator)
    };
    map_to_result.expect("map_to failed").flush();
}

Heap Allocation

📝 Heap Allocator

use alloc::alloc::{GlobalAlloc, Layout};
use core::ptr::null_mut;
use x86_64::{
    structures::paging::{
        mapper::MapToError, FrameAllocator, Mapper, Page, PageTableFlags, Size4KiB,
    },
    VirtAddr,
};

pub const HEAP_START: usize = 0x_4444_4444_0000;
pub const HEAP_SIZE: usize = 100 * 1024; // 100 KiB

pub fn init_heap(
    mapper: &mut impl Mapper,
    frame_allocator: &mut impl FrameAllocator,
) -> 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;
        unsafe {
            mapper.map_to(page, frame, flags, frame_allocator)?.flush()
        };
    }

    unsafe {
        ALLOCATOR.lock().init(HEAP_START, HEAP_SIZE);
    }

    Ok(())
}

// Simple bump allocator
pub struct BumpAllocator {
    heap_start: usize,
    heap_end: usize,
    next: usize,
    allocations: usize,
}

impl BumpAllocator {
    pub const fn new() -> Self {
        BumpAllocator {
            heap_start: 0,
            heap_end: 0,
            next: 0,
            allocations: 0,
        }
    }

    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 {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        let mut bump = self.lock();

        let alloc_start = align_up(bump.next, layout.align());
        let alloc_end = match alloc_start.checked_add(layout.size()) {
            Some(end) => end,
            None => return null_mut(),
        };

        if alloc_end > bump.heap_end {
            null_mut()
        } 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();

        bump.allocations -= 1;
        if bump.allocations == 0 {
            bump.next = bump.heap_start;
        }
    }
}

fn align_up(addr: usize, align: usize) -> usize {
    (addr + align - 1) & !(align - 1)
}

4. Multitasking

📝 Async Task Executor

use alloc::collections::VecDeque;
use core::task::{Context, Poll, Waker};

pub struct Task {
    id: TaskId,
    future: Pin>>,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct TaskId(u64);

impl TaskId {
    fn new() -> Self {
        static NEXT_ID: AtomicU64 = AtomicU64::new(0);
        TaskId(NEXT_ID.fetch_add(1, Ordering::Relaxed))
    }
}

pub struct Executor {
    tasks: VecDeque,
    waker_cache: BTreeMap,
}

impl Executor {
    pub fn new() -> Self {
        Executor {
            tasks: VecDeque::new(),
            waker_cache: BTreeMap::new(),
        }
    }

    pub fn spawn(&mut self, future: impl Future + 'static) {
        let task = Task {
            id: TaskId::new(),
            future: Box::pin(future),
        };
        self.tasks.push_back(task);
    }

    pub fn run(&mut self) -> ! {
        loop {
            self.run_ready_tasks();
            self.sleep_if_idle();
        }
    }

    fn run_ready_tasks(&mut self) {
        while let Some(mut task) = self.tasks.pop_front() {
            let task_id = task.id;
            let waker = self.waker_cache
                .entry(task_id)
                .or_insert_with(|| TaskWaker::new(task_id));

            let mut context = Context::from_waker(waker);
            match task.future.as_mut().poll(&mut context) {
                Poll::Ready(()) => {
                    self.waker_cache.remove(&task_id);
                }
                Poll::Pending => {
                    self.tasks.push_back(task);
                }
            }
        }
    }

    fn sleep_if_idle(&self) {
        use x86_64::instructions::interrupts;

        interrupts::disable();
        if self.tasks.is_empty() {
            interrupts::enable_and_hlt();
        } else {
            interrupts::enable();
        }
    }
}

5. Device Drivers

📝 Keyboard Driver

use pc_keyboard::{layouts, DecodedKey, HandleControl, Keyboard, ScancodeSet1};
use spin::Mutex;

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

pub fn add_scancode(scancode: u8) {
    let mut keyboard = KEYBOARD.lock();

    if let Ok(Some(key_event)) = keyboard.add_byte(scancode) {
        if let Some(key) = keyboard.process_keyevent(key_event) {
            match key {
                DecodedKey::Unicode(character) => print!("{}", character),
                DecodedKey::RawKey(key) => print!("{:?}", key),
            }
        }
    }
}

Best Practices

🔑 OS Development Guidelines

Test Thoroughly: Use QEMU and real hardware
Handle Edge Cases: OS code must be bulletproof
Document Everything: OS code is complex
Use Safe Abstractions: Encapsulate unsafe code
Optimize Carefully: Profile before optimizing
Follow Conventions: Learn from existing kernels

Summary

✅ You've learned:

Setting up a bare-bones kernel in Rust
VGA text mode and display management
Interrupt handling with IDT and PIC
Memory management with paging
Heap allocation implementation
Async multitasking and device drivers

Building an OS is a massive undertaking, but it's also incredibly rewarding. This chapter has given you the foundation to start your OS development journey. Continue learning by implementing more features like file systems, networking, and multi-core support!

🖥️ OS Development