QEMU AArch64 Virt Bare Bones

QEMU is a free and open source hardware emulator that supports multiple architecture, including ARM. QEMU has its own generic board mainly for virtualization usages, called virt. virt supports ARM and RISC-V architecture.

In this tutorial, we'll write a basic kernel for QEMU virt board, specifically Aarch64.

Prerequisites

For this tutorial, you'll need

• An ARM cross-compiler toolchain
• QEMU, of course you'll need QEMU because this tutorial designed for QEMU virt.

Code

Now we'll need to write some code for our little kernel.

boot.s

.global _start
_start:
    ldr x30, =stack_top
    mov sp, x30
    bl kmain
    b .

kernel.c

#include <stdint.h>
 
volatile uint8_t *uart = (uint8_t *) 0x09000000;
 
void putchar(char c) {
    *uart = c;
}
 
void print(const char *s) {
    while(*s != '\0') {
        putchar(*s);
        s++;
    }
}
 
void kmain(void) {
     print("Hello world!\n");
}

linker.ld

ENTRY(_start)
SECTIONS {
    . = 0x40100000; /* RAM starts at 0x40000000 but if we ask to load the kernel there, QEMU will not load a DTB */
    .startup . : { boot.o(.text) }
    .text : { *(.text) }
    .data : { *(.data) }
    .bss : { *(.bss COMMON) }
    . = ALIGN(8);
    . += 0x1000; /* 4kB of stack memory */
    stack_top = .;
}

Compiling the kernel

Now, we'll need to compile our kernel so we can boot it.

$ aarch64-elf-as boot.s -o boot.o
$ aarch64-elf-gcc -ffreestanding -c kernel.c -o kernel.o
$ aarch64-elf-ld -nostdlib -Tlinker.ld boot.o kernel.o -o kernel.elf

Booting our kernel

Since our kernel is now compiled, we'll boot it with QEMU

$ qemu-system-aarch64 -machine virt -cpu cortex-a57 -kernel kernel.elf -nographic
Hello world!

See also

• ARM Overview
• Raspberry Pi Bare Bones