BIOS hello world

README.md

How to build?

as -32 -o ./main.o ./main.s
ld -m elf_i386 -T ld.script -o ./a.out ./main.o
objcopy -j .text -j .data -O binary ./a.out ./boot.text.bin

ld.script

SECTIONS
{
  . = 0x00007C00;
  .text : {
    _text_start = .;
    *(.text)
    _text_end = .;
  }
  .data : {
    _rodata_start = .;
    *(.rodata)
    _rodata_end = .;
    _data_start = .;
    *(.data)
    _data_end = .;
  }
}

main.s

  .file "main.s"
  
  .text
  .code16
  
  .globl _start
_start:
  cli
  
  cld
  xor  %ax, %ax
  
  movw %ax, %ss
  
  movw $_start + 0x400, %bp
  movw %bp, %sp
  
  movw %ax, %ds
  movw %ax, %es
  
_main:
  movb $0x00, %ah 
  movb $0x12, %al
  int  $0x10

  movb $0x02, %ah  
  movb $0x00, %bh
  movb $0x00, %dh
  movb $0x00, %dl
  int  $0x10

  movb $0x61, %al
  call _putc

  movb $0x62, %al
  call _putc

  movb $0x63, %al
  call _putc

  movb $0x0a, %al
  call _putc

  movb $0x0d, %al
  call _putc
  
  mov  $_mesg, %si
  call _puts

_finish:
  hlt
  jmp  _finish

_puts:
  pushw %bp
  movw %sp, %bp
_puts.func:
  lodsb
  testb %al, %al
  jz   _puts.finish
  call _putc
  jmp  _puts.func
_puts.finish:
  leave
  ret

_putc:
  pushw %bp
  movw %sp, %bp
  movb $0x0e, %ah
  movb $0x00, %bh
  movb $0xd7, %bl
  int  $0x10
  leave
  ret

  .section .rodata
_mesg:
  .asciz "Hello, world.\r\n"