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layout: post
title: 'Bootloader + retro game in a tweet'
permalink: '/bootloader_retro_game_tweet/'
tags: ['bootloader', 'operating systems', 'nibbles', 'code golf']
<div class="lead">
<video width="640" height="360" style="max-width: 100%;" autoplay muted loop playsinline>
<source src="/files/2015/bootloader_retro_game_tweet/video.mp4" type="video/mp4">
In 1998, people were competing to write the smallest possible DOS
games, a type of progamming competition later coined as
"code golfing".
I decided to revisit one such piece of code (nibbles.asm) but turn
it into a bootable floppy image which can fit in a tweet (140 characters):
<pre style="code">perl -E"say'swFoAKAHvqB9uBMAzRC/wPi5gAjzqqqBxz4Bc/jkYDxIchE8UHcNN5hISH
IFLAJrwLCJwwHetIa2780VtEUmMCR72uv','B'x589,'Vao='"|base64 -D>boot.img</pre>
<small>note: on some systems, e.g. Linux, you'll have to use <code>base64 -d</code>.</small>
* Technically, my code isn't a bootloader. A real bootloader does things
like setup the x86 machine in protected mode, load more data from the
disk (the BIOS only loads the first 512 bytes), etc. I skip all of that
and instead setup some registers and jump straight into the game.
A popular website which hosted such competitions was Hugi and one of my
favorite entries was "Nibbles by Altair/ODDS entertainment",
an implementation of nibbles (also known as Tron or Snakes) in only
48 bytes.
Unfortunately, running the original code today isn't very easy: you need
DOS and if you are running in an emulator, the assumptions the original
programmers made about their hardware (and often the speed of their
hardware) needs to hold up.
I therefore decided to take Altair's code and turn it into a bootable
floppy disk. I also made a few tweaks to make the game more playable.
<p>Here is my code with some of the original comments as well as some
of my own.</p>
<pre style="code">
; floppy.asm: a bootloader(*) + retro game which fits in a tweet:
; perl -E"say'sgFoAKAHvqB9uBMAzRC/wPi5gAjzqqqBxz4Bc/jkYDxIchE8UHcNN5hISH
; IFLAJrwLCJwgHWtQa0C80Q4vwmMCR72uv','B'x589,'Vao='"|base64 -D>boot.img
; to compile and run:
; nasm floppy.asm -o floppy.img
; qemu-system-i386 -fda floppy.img
[bits 16] ; Pragma, tells the assembler that we
; are in 16 bit mode (which is the state
; of x86 when booting from a floppy).
[org 0x7C00] ; Pragma, tell the assembler where the
; code will be loaded.
mov bl, 1 ; Starting direction for the worm.
push 0xa000 ; Load address of VRAM into es.
pop es
mov si, 320*100+160 ; worm's starting position, center of
; screen
; Set video mode. Mode 13h is VGA (1 byte per pixel with the actual
; color stored in a palette), 320x200 total size. When restarting,
; this also clears the screen.
mov ax, 0x0013
int 0x10
; Draw borders. We assume the default palette will work for us.
; We also assume that starting at the bottom and drawing 2176 pixels
; wraps around and ends up drawing the top + bottom borders.
mov di, 320*199
mov cx, 2176
stosb ; draw right border
stosb ; draw left border
add di, 318
jnc draw_loop ; notice the jump in the middle of the
; rep stosb instruction.
; We read the keyboard input from port 0x60. This also reads bytes from
; the mouse, so we need to only handle [up (0x48), left (0x4b),
; right (0x4d), down (0x50)]
in al, 0x60
cmp al, 0x48
jb kb_handle_end
cmp al, 0x50
ja kb_handle_end
; At the end bx contains offset displacement (+1, -1, +320, -320)
; based on pressed/released keypad key. I bet there are a few bytes
; to shave around here given the bounds check above.
dec ax
dec ax
jc kb_handle
sub al, 2
imul ax, ax, byte -0x50
mov bx, ax
add si, bx
; The original code used set pallete command (10h/0bh) to wait for
; the vertical retrace. Today's computers are however too fast, so
; we use int 15h 86h instead. This also shaves a few bytes.
; Note: you'll have to tweak cx+dx if you are running this on a virtual
; machine vs real hardware. Casual testing seems to show that virtual machines
; wait ~3-4x longer than physical hardware.
mov ah, 0x86
mov dh, 0xef
int 0x15
; Draw worm and check for collision with parity
; (even parity=collision).
mov ah, 0x45
xor [es:si], ah
; Go back to the main game loop.
jpo game_loop
; We hit a wall or the worm. Restart the game.
jmp restart_game
TIMES 510 - ($ - $$) db 0 ; Fill the rest of sector with 0
dw 0xaa55 ; Boot signature at the end of bootloader
<img src="/files/2015/bootloader_retro_game_tweet/screenshot.png">
<div style="width: 75%; margin: auto">
The code in the tweet creates a bootable file <code>boot.img</code>. You
can boot the code in qemu, virtual box or your favorite virtualization
software and play the game with the arrow keys.
You can also put the code on a floppy and boot it, as seen in the video at the top of this page.
<li><a href="">Hugi assembly size optimization competition</a></li>
<li><a href="/files/2015/bootloader_retro_game_tweet/NIBBLES.ASM">Altair's original source code</a></li>
<li><a href="">Hello world bootloader</a></li>
<li><a href="">OSKit wiki</a></li>