📝 Disassembly of Tetris for Game Boy.
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README.md

tetris

Disassembly of the original Tetris for Game Boy. See tetris.asm.

It doesn't build yet -- the GBDis assembly syntax is not quite the same as RGBDS's, which is the standard today (used in pokered, for instance).

You might be able to build it with some ancient 90s assembler on Jeff Frohwein's site or something, but so far I've only used it to see what's going on in Tetris, not to mod it, so I haven't tried.

Origin

This disassembly is basically Jeff Frohwein's; all I've done is regenerate it and put it online with instructions.

res/TETRIS.TAB is Jeff Frohwein's original table from June 8, 1996. I used it to generate the initial disassembly from the Tetris ROM with MD5 982ed5d2b12a0377eb14bcdc4123744e.

Generating tetris.asm from the table file

Get the tetris.gb ROM somewhere and put it alongside TETRIS.TAB in the res folder.

Download GBDis V1.3 (gbdis13.zip) from Jeff Frohwein's site. It's a DOS binary, but it comes with its Turbo Pascal source code in a file called DIS.PAS. Here's the header at the top:

{   Nintendo Gameboy Disassembler V1.3  }
{              by                       }
{         Jeff Frohwein                 }
{   Written in Turbo Pascal V6.0        }
{   Started: 25 Nov 93                  }
{   Last edited: 9 Jun 96               }

Kind of hard to get Turbo Pascal these days, but it turns out you can get the Free Pascal Compiler -- on macOS, brew install fpc -- and it's completely compatible with Turbo Pascal from 20+ years ago.

Then I just did MACOSX_DEPLOYMENT_TARGET=10.12 fpc DIS.PAS (had to get around some weird bug with FPC and Sierra) and got a Mac binary DIS.

$ unzip gbdis13.zip -d gbdis13
Archive:  gbdis13.zip
  inflating: gbdis13/GBDIS.EXE       
  inflating: gbdis13/GBDIS.TXT       
  inflating: gbdis13/DIS.PAS         
$ cd gbdis13
$ ls
DIS.PAS   GBDIS.EXE GBDIS.TXT
$ MACOSX_DEPLOYMENT_TARGET=10.12 fpc DIS.PAS
Free Pascal Compiler version 3.0.2 [2017/03/18] for x86_64
Copyright (c) 1993-2017 by Florian Klaempfl and others
Target OS: Darwin for x86_64
Compiling DIS.PAS
DIS.PAS(103,28) Warning: Local variable "z" does not seem to be initialized
DIS.PAS(129,6) Note: Local variable "b" not used
DIS.PAS(129,8) Note: Local variable "c" not used
DIS.PAS(290,6) Note: Local variable "a" not used
DIS.PAS(312,21) Warning: Comparison might be always true due to range of constant and expression
DIS.PAS(310,6) Note: Local variable "a" not used
DIS.PAS(27,6) Note: Local variable "count" is assigned but never used
DIS.PAS(27,39) Note: Local variable "pair" not used
DIS.PAS(28,2) Note: Local variable "cp" is assigned but never used
DIS.PAS(28,5) Note: Local variable "pc" not used
DIS.PAS(28,39) Note: Local variable "z" not used
Assembling (pipe) DIS.s
Linking DIS
[... a bunch of warnings ...]
1230 lines compiled, 0.2 sec
2 warning(s) issued
9 note(s) issued

Finally, I ran DIS res/tetris tetris.asm in this repo, which uses tetris.gb and tetris.tab to make the disassembly tetris.asm.

Twenty-year-old software, and it works out of the box!

From now on, however, we should probably just edit tetris.asm directly, like in pokered et al, so this section is for historical interest only.