Using it

Let’s write a small program using our driver to write to the serial console, and echo incoming bytes.


mod exceptions;
mod pl011;

use crate::pl011::Uart;
use core::fmt::Write;
use core::panic::PanicInfo;
use log::error;
use smccc::psci::system_off;
use smccc::Hvc;

/// Base address of the primary PL011 UART.
const PL011_BASE_ADDRESS: *mut u32 = 0x900_0000 as _;

extern "C" fn main(x0: u64, x1: u64, x2: u64, x3: u64) {
    // Safe because `PL011_BASE_ADDRESS` is the base address of a PL011 device,
    // and nothing else accesses that address range.
    let mut uart = unsafe { Uart::new(PL011_BASE_ADDRESS) };

    writeln!(uart, "main({x0:#x}, {x1:#x}, {x2:#x}, {x3:#x})").unwrap();

    loop {
        if let Some(byte) = uart.read_byte() {
            match byte {
                b'\r' => {
                b'q' => break,
                _ => {}

    writeln!(uart, "Bye!").unwrap();
  • As in the inline assembly example, this main function is called from our entry point code in entry.S. See the speaker notes there for details.
  • Run the example in QEMU with make qemu under src/bare-metal/aps/examples.