QEMU AArch64 Virt Bare Bones
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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!