Real mode assembly I

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WARNING: Please see the Real Mode OS Warning page before continuing.

Difficulty level
Beginner

In this tutorial we will assemble a small 16-bit assembly language kernel with NASM and boot it.

Overview

You're probably going to sigh and dismiss yet another tutorial on writing operating systems in x86 assembly language, especially since this one uses real mode. But there's a catch to this one; it actually does more than printing "Hello World" to the screen and halting.

For this, you'll need:

• the latest version of NASM (2.05.01 as of November 28^th, 2008)
• PARTCOPY on Windows or dd on Linux
• an emulator like QEMU, Bochs, or Microsoft Virtual PC

So what's it going to look like?

Well, there will be a single source file, the kernel. What about a bootloader? This is such a small kernel, we're not going to use a filesystem at all, we're just going to put the kernel into the first few sectors of the disk image!

The system will have a string printing call (of course), keyboard input, and a strcmp call similar to that of C, all packaged into less than a sector.

But I want a GUI and sound effects and all the Windows games to work on my OS...

Beginner Mistakes

So where's the code?

Here you go, go wild.

 org 0x7C00   ; add 0x7C00 to label addresses
 bits 16      ; tell the assembler we want 16 bit code

   mov ax, 0  ; set up segments
   mov ds, ax
   mov es, ax
   mov ss, ax     ; setup stack
   mov sp, 0x7C00 ; stack grows downwards from 0x7C00
 
   mov si, welcome
   call print_string
 
 mainloop:
   mov si, prompt
   call print_string
 
   mov di, buffer
   call get_string
 
   mov si, buffer
   cmp byte [si], 0  ; blank line?
   je mainloop       ; yes, ignore it
 
   mov si, buffer
   mov di, cmd_hi  ; "hi" command
   call strcmp
   jc .helloworld
 
   mov si, buffer
   mov di, cmd_help  ; "help" command
   call strcmp
   jc .help
 
   mov si,badcommand
   call print_string 
   jmp mainloop  
 
 .helloworld:
   mov si, msg_helloworld
   call print_string
 
   jmp mainloop
 
 .help:
   mov si, msg_help
   call print_string
 
   jmp mainloop
 
 welcome db 'Welcome to My OS!', 0x0D, 0x0A, 0
 msg_helloworld db 'Hello OSDev World!', 0x0D, 0x0A, 0
 badcommand db 'Bad command entered.', 0x0D, 0x0A, 0
 prompt db '>', 0
 cmd_hi db 'hi', 0
 cmd_help db 'help', 0
 msg_help db 'My OS: Commands: hi, help', 0x0D, 0x0A, 0
 buffer times 64 db 0
 
 ; ================
 ; calls start here
 ; ================
 
 print_string:
   lodsb        ; grab a byte from SI
 
   or al, al  ; logical or AL by itself
   jz .done   ; if the result is zero, get out
 
   mov ah, 0x0E
   int 0x10      ; otherwise, print out the character!
 
   jmp print_string
 
 .done:
   ret
 
 get_string:
   xor cl, cl
 
 .loop:
   mov ah, 0
   int 0x16   ; wait for keypress
 
   cmp al, 0x08    ; backspace pressed?
   je .backspace   ; yes, handle it
 
   cmp al, 0x0D  ; enter pressed?
   je .done      ; yes, we're done
 
   cmp cl, 0x3F  ; 63 chars inputted?
   je .loop      ; yes, only let in backspace and enter
 
   mov ah, 0x0E
   int 0x10      ; print out character
 
   stosb  ; put character in buffer
   inc cl
   jmp .loop
 
 .backspace:
   cmp cl, 0	; beginning of string?
   je .loop	; yes, ignore the key
 
   dec di
   mov byte [di], 0	; delete character
   dec cl		; decrement counter as well
 
   mov ah, 0x0E
   mov al, 0x08
   int 10h		; backspace on the screen
 
   mov al, ' '
   int 10h		; blank character out
 
   mov al, 0x08
   int 10h		; backspace again
 
   jmp .loop	; go to the main loop
 
 .done:
   mov al, 0	; null terminator
   stosb
 
   mov ah, 0x0E
   mov al, 0x0D
   int 0x10
   mov al, 0x0A
   int 0x10		; newline
 
   ret
 
 strcmp:
 .loop:
   mov al, [si]   ; grab a byte from SI
   mov bl, [di]   ; grab a byte from DI
   cmp al, bl     ; are they equal?
   jne .notequal  ; nope, we're done.
 
   cmp al, 0  ; are both bytes (they were equal before) null?
   je .done   ; yes, we're done.
 
   inc di     ; increment DI
   inc si     ; increment SI
   jmp .loop  ; loop!
 
 .notequal:
   clc  ; not equal, clear the carry flag
   ret
 
 .done: 	
   stc  ; equal, set the carry flag
   ret
 
   times 510-($-$$) db 0
   dw 0AA55h ; some BIOSes require this signature

To assemble on Windows:

nasm kernel.asm -f bin -o kernel.bin
partcopy kernel.bin 0 200 -f0

Or on Linux:

nasm kernel.asm -f bin -o kernel.bin
dd if=kernel.bin of=/dev/sda

Those commands assemble your kernel binary and write them to the first disk (sda might be your system disk, so use sdb, sdc etc. according to your configuration. USB-sticks also appear as one of the "sd" devices). Go ahead and test out your operating system now!

What next?

Why, that's up to you of course! You could add more commands, expand your OS to another sector and learn to use the BIOS sector load functions, add a stack and improve the calls, etc.

Have fun with your OS, however you decide to write it!

EDIT on December 12 2008: I've written a second part to this tutorial at Real mode assembly II. This and future parts will have less code to copy and paste and more theory!

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