One Byte at a Time

2024

Doing client work, sometimes means working way outside my normal cocoon of webdev and infrastructure.

Client: Can you reprogram an Atari 2600 game?

Me: Probably?!

My first Angelfire website (~1998) was dedicated to ROMs— how hard can this really be?

Research

Tools

  • DASM, a macro assembler that works like a compiler for my purposes.
  • Distella, a disassembler that "decompiles" original ROMs for the Atari 2600/7800
  • Stella is an incredible Emulator with a built in debugger
  • SprEd A pixel/sprite editor that exports to a number of different formats makes designing new graphics a lot less tedious.

Hack the Frog

The client wanted to change the original 1982 Frogger for the Atari 2600. The plan was to change a number of the sprites to spell out H-O-M-E; it will be featured in a performance art piece on an original Atari 2600.

The original ROM is easy to find, but the original source code is likely lost to time. The original code though is likely not too different from the disassembled dump that Distella produced.

## let's disassemble, assemble again and make sure we can still play the game.

 $ distella -paf ./Frogger.bin > ./Frogger.asm
 $ dasm Frogger.asm -f3 -oFrogger.bin
 $ stella ./Frogger.bin

Opening the ASM file is a little daunting at first. Luckily I wasn't planning on changing any logic in the game, so we'll ignore those stanzas of code and focus on the sprites. By identifying the sections of data loading code, I was able to isolate the graphics.

; https://gist.github.com/erik-nilcoast/cc55644c916c729e6a82ae07b681ac2b#file-frogger-asm-L2299-L2332
;
.byte $82 ; |X     X | Here's our frog. I know he doesn't look like much.
.byte $92 ; |X  X  X |
.byte $FE ; |XXXXXXX |
.byte $38 ; |  XXX   |
.byte $7C ; | XXXXX  |
.byte $BA ; |X XXX X |
.byte $92 ; |X  X  X |

By passing a cfg file, I was able to instruct the disassembler to draw an ASCII representation of of the bits that make up each byte. You pass the start and end address for the instructions you want to treat as graphics.

# frogger.cfg
GFX F1F5 F212
GFX FCFD FFF9

Now we can combine all of our tools to hand edit the graphics in the assembly file. Using the SprEd editor I was able to design new graphics for each one of the sprites. Editing the ASM file is incredibly delicate. I recommend finding a good workflow and doing a few tests. If like me, you can't "see" most of the graphics via the ASCII representation. I started with changing one byte every 8 instruction to $FF (11111111 in binary). This allowed me to identify most of the graphics in the game.

End Result

After a lot of trial and error, we got the result we were looking for:

The game is still completely playable and after mucking with a bunch of different hardware options, we were able to get the result loaded onto an original Atari 2600.

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