kernel/src/main.rs

#![no_std]
#![no_main]
#![feature(naked_functions)]

use core::panic::PanicInfo;

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",

    "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() -> ! {
    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 {}
}