Real mode assembly I
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WARNING: Please see the Real Mode OS Warning page before continuing.
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 28th, 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...
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!
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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