kernel/src/main.rs

// src/main.rs
#![no_std]
#![no_main]
#![feature(naked_functions)]

use core::panic::PanicInfo;

mod heap;
mod uart;

#[naked]
#[no_mangle]
#[link_section = ".text.init"]
unsafe extern "C" fn _start() -> ! {
    use core::arch::asm;
    asm!(
        // before we use the `la` pseudo-instruction for the first time,
        //  we need to set `gp` (google linker relaxation)
        ".option push",
        ".option norelax",
        "la gp, _global_pointer",
        ".option pop",

        // set the stack pointer
        "la sp, _init_stack_top",

        // clear the BSS
        "la t0, _bss_start",
        "la t1, _bss_end",
        "bgeu t0, t1, 2f",
    "1:",
        "sb zero, 0(t0)",
        "addi t0, t0, 1",
        "bne t0, t1, 1b",
    "2:",
        // BSS is clear!

        // "tail-call" to {entry} (call without saving a return address)
        "tail {entry}",
        entry = sym entry, // {entry} refers to the function [entry] below
        options(noreturn) // we must handle "returning" from assembly
    );
}

extern "C" fn entry() -> ! {
    // UNSAFE: Called exactly once, right here.
    unsafe { heap::init() };

    // Now we're free to use dynamic allocation!
    {
        extern crate alloc;
        use alloc::boxed::Box;
        let _my_heap_pointer = Box::new(10);
        // _my_heap_pointer lives on the heap!
    }

    println!("This should not print because the console is not initialised.");
    unsafe { uart::init_console(0x1000_0000) };
    println!("Hello, world!");

    loop {
        let c = uart::CONSOLE.lock().as_mut().and_then(uart::Device::get);
        if let Some(c) = c {
            print!("{}", c as char);
        }
    }
}

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

// TODO unit testing