Example of printing a string on the serial with a custom 32bits BIOS - P1kachu/qemu-hvf GitHub Wiki

p1kachu@GreyLabOfSteel:custom-32bits-bios$ cat Makefile 
all:
	$(CC) -m32 -nostartfiles bios.S
	objcopy a.out custom_bios -O binary --only-section=.text
	rm a.out
	objdump -D -b binary -mi386 -Maddr32,data32 custom_bios
p1kachu@GreyLabOfSteel:custom-32bits-bios$ cat bios.S 
.intel_syntax noprefix

#define ROM_SIZE (1 << 16)

fw_start:

main:
    xor eax, eax
    mov dx, 0x3f8
    xor ecx, ecx
    not ecx

    mov edi, string - fw_start

    cld
    repne scasb

    mov edi, string - fw_start

    not ecx
    dec ecx

.Lprint_loop:
    mov al, [edi]
    out dx, al
    inc edi
    dec ecx
    jnz .Lprint_loop

.Lwait:
    hlt
    jmp .Lwait

string:
    .string "Custom BIOS succesfully launched\n"

.space ROM_SIZE - (. - fw_start + fw_end - _start)

.global _start
// fw_end - 16
_start:
    jmp main
    .space 16 - (. - _start)
fw_end:

p1kachu@GreyLabOfSteel:qemu-hvf$ ./configure --extra-cflags="-D CONFIG_HVF" --extra-ldflags="-framework Hypervisor" --target-list=x86_64-softmmu
p1kachu@GreyLabOfSteel:qemu-hvf$ make -j8
p1kachu@GreyLabOfSteel:qemu-hvf$ ./x86_64-softmmu/qemu-system-x86_64 --machine accel=hvf -bios ../custom_bios