I recently ran across a bunch of Atari arcade source code that was added to the Historical Source Github site, including the source for one of my favorite games, Crystal Castles.
https://github.com/historicalsource/crystal-castles
I thought it would be cool to be able to re-assemble this back into the original ROMs. I started by trying to find an existing assembler that could do this, but the assembler used for this source had a lot of syntactical differences and features I couldn't find in any other 6502 assembler. I assume Atari used a VAX based assembler and I did even find documentation for a VAX assembler with similar features, but still not an exact match.
I then thought about manually modifying the source to work with an existing assembler. I quickly realized that this wasn't feasible due to some of the odd syntax in this assembler. For example the Atari assembler has an different syntax for addressing modes. You can write Zero Page X like this: "LDA ZX,$PSTSL", you couldn't fix this with a simple search and replace. This would also have to be done for every game I wanted to re-assemble.
So I decided the best option was to build my own assembler that could handle these files with little changes to the original source. The assembler I build is now able to reassemble the main source from the historical source repository and reproduce the MAME ccastles1 ROM set.
I have setup a GitHub repository to hold the C# source code for my assembler as well as tool to split the output into separate ROMs and handle the checksum process. There is also a folder there with the files and instructions needed to rebuild the source. There are some small changes needed to the source which I have documented in the instructions, and there was one file missing from the Historical Source archive which I re-created from the original ROMs.
https://github.com/danlb2000/AT6502
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