Multiboot Specification Sample

boot.asm

        bits    32
        section .text

        extern  osmain
        global  start
        global  _start
        global  hlt_loop

MULTIBOOT_HEADER_MAGIC  equ     0x1BADB002
MULTIBOOT_HEADER_FLAGS  equ     0x00000001

start:
_start:
        jmp     multiboot_entry
        align   4

multiboot_header:
        ;; MULTIBOOT HEADER
        dd      MULTIBOOT_HEADER_MAGIC
        dd      MULTIBOOT_HEADER_FLAGS
        dd      -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)

multiboot_entry:
        mov     esp, stack
        push    ebx             ; multiboot info structure
        push    eax             ; multiboot header magic
        call    osmain

hlt_loop:
        hlt
        jmp     hlt_loop

stack_top:
        section .bss
        resb    0x4000
stack:  nop

kernel.c

#define VRAM_TEXT_ADDR 0x000b8000
#define VRAM_TEXT_WIDTH 80
#define VRAM_TEXT_HEIGHT 25

typedef struct {
  char ch;
  char attr;
} char_vga;

void puts(char *str)
{
  char_vga *vram = (char_vga *)VRAM_TEXT_ADDR;
  
  while(*str != '\0') {
    vram->attr = 0x0f;
    vram->ch = *(str++);
    vram++;
  }
}

void osmain(void)
{
  puts("Hello world!");
}

Makefile

CFLAGS = -fno-builtin -fhosted -ffreestanding -nostdlib -nodefaultlibs

all:
        nasm -f elf boot.asm -o boot.o
        gcc $(CFLAGS) -o kernel.o -c kernel.c
        ld -Ttext=0x100000 --oformat elf32-i386 boot.o kernel.o -o os.bin