A minimal boot sector tutorial

The natural first step of building an operating system is to find a way to run programs on “bare hardware”. The task turns out to be quite easy despite its daunting first impression. After researching on the net and trying out various tutorials, I found all of them overly complicated. Most of them assume interfacing with a C-like language and implementing a Unix-like system, but C and Unix are not the only story about operating systems design. A boot sector tutorial should teach nothing more than how to boot a computer.

After learning the good parts from the tutorials and applying my own simplifications, I arrived at my first boot sector. It is very simple and does very little — it just boots the machine and displays a colorful banner — but it illustrates the only things you need to know for booting a computer.

The code is very short — only 21 lines of code excluding comments and blank lines.

org 7C00H                      ; the program will be loaded at 7C00H

  mov eax, string_start
  mov ch, 1                    ; ch contains color of text
  mov ebx, 0B8000H + 718H      ; B8000H is VGA memory
                               ; 718H is offset to approx center

  mov cl, [eax]                ; load char into cl
  mov [ebx], cx                ; store [color:char] from cx into VGA
  add ch, 1                    ; change color to (ch+1) mod 16
  and ch, 0x0F
  add eax, 1                   ; advance string pointer
  add ebx, 2                   ; advance VGA pointer
  cmp eax, string_end          ; until the end of string
  jg stop
  jmp print

  jmp stop                     ; infinite loop after printing

  string_start db 'My colorful new OS!'
  string_end equ $

  times 510-($-$$) db 0        ; pad remainder of boot sector with 0s
  dw 0xAA55                    ; standard PC boot signature

It is in NASM syntax and needs nasm to be assembled into machine code. After that it can be booted by QEMU, a processor emulator. The only two necessary command lines are (assuming the code is stored in a file named myfirst.asm):

nasm -f bin -o myfirst.bin myfirst.asm
qemu -hda myfirst.bin

Of course, you can also burn the disk image (myfirst.bin) onto a CD and boot a real machine from it.