2015-07-01-bootloader_retro_game_tweet.html

---
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">
    </video>

    <p>
      In 1998, people were competing to write the smallest possible DOS
      games, a type of progamming competition later coined as
      "code golfing".
    </p>
    <p>
      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):
    </p>
    <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>
  </div>
  <br/>
  <p>
    * 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.
  </p>
</div>

<section>
  <h3>hugi.de</h3>
  <p>
    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.
  </p>
  <p>
    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.
  </p>
  <p>
    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>
</section>

<section>
  <h3>floppy.asm</h3>
  <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

restart_game:
          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
          rep
draw_loop:
          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.

game_loop:
          ; 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.
          aaa
          cbw
          dec       ax
          dec       ax
          jc        kb_handle
          sub       al, 2
          imul      ax, ax, byte -0x50
kb_handle:
          mov       bx, ax

kb_handle_end:
          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
</pre>
</section>

<section>
  <h3>Screenshot</h3>
  <img src="/files/2015/bootloader_retro_game_tweet/screenshot.png">
  <div style="width: 75%; margin: auto">
    <small>
    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.
    </small>
  </div>
</section>

<section>
  <h3>Links</h3>
  <ul>
    <li><a href="http://www.hugi.scene.org/compo/compoold.htm">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="http://viralpatel.net/taj/tutorial/hello_world_bootloader.php">Hello world bootloader</a></li>
    <li><a href="http://wiki.osdev.org/OSKit">OSKit wiki</a></li>
  </ul>
</section>