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 image which can fit in a tweet (140 characters):
perl -E"say'sgFoAKAHvqB9uBMAzRC/wPi5gAjzqqqBxz4Bc/jkYDxIchE8UHcNN5hISH IFLAJrwLCJwgHWtQa0C80Q4vwmMCR72uv','B'x589,'Vao='"|base64 -D>boot.img
* 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 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.
Here is my code with some of the original comments as well as some of my own.
; 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] ; This pragma tells the assembler that we ; are in 16 bit mode (which is the state ; of x86 when it boots from a floppy). [org 0x7C00] ; Program to tell the assembler where the ; code will be loaded. mov dl, 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. 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 60h. 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 AX 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 dx, ax kb_handle_end: add si, dx ; The original code used set pallete command (10h/0bh) to wait for ; the vertical retrace. Today's computers are however too fast, so ; we wrap things in a loop ~1500 times. mov ch, 0x6 mov ah, 0x0b wait_loop: int 10h loop wait_loop ; Draw worm and check for collision with parity ; (even parity=collision). 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
boot.img. You can boot the code in qemu, virtual box or your favorite virtualization software and play the game with the arrow keys.