Loading ROMs

Wednesday, 20th February 2008

OutRunEuropa1.png OutRunEuropa3.png
OutRun Europa

I've been adding a series of new features to Cogwheel. I wrote a basic zip file class a while back (only supporting store and deflate compression - the majority of zip files use deflate), and have added a Utility namespace with methods for help with loading ROMs.

For example, there is a method that is passed a filename by reference, and it'll return an array of bytes of the loaded file. If the passed filename was a zip file, by any chance, it'll search for ROMs inside the zip, and modify the filename (so it might end up as C:\Path\SomeFile.zip\Game.sms).

Once this is done, the CRC-32 checksum of the file is calculated then looked up against a database of known ROM dumps. I'm currently using the SMS Checker .romdata files as the database source. These contain information about known bad dumps, and can be used to strip headers and footers, patch bytes and remove redundant overdump data to end up with a valid ROM image.

Finally, the mapper is detected. Currently, I have three mappers - RAM (which is just 64KB of RAM), Standard (the standard Sega Master System ROM mapper) and Codemasters. I use the RAM mapper for all SG-1000 and SC-3000 games (I'm not sure what they're meant to use, here, but some games - such as The Castle - don't work with the standard SMS mapper).

The Codemasters mapper uses a different method to swap pages, which means that the conventional SMS BIOS checksumming code fails to swap in pages correctly and thus doesn't check the entire cartridge. The games therefore have their own checksumming routines and store their own checksum in a Codemasters-specific header; this makes detecting Codemasters games fairly easy, as all you need to do is check for the extra Codemasters checksum.

As the BIOS doesn't check the checksum, the cartridge provides code to do so itself. You can run it by holding 1 and 2 down as the game boots.

The Excellent Dizzy Collection
Micro Machines

I seem to have broken interrupts somewhere along the line; most noticable are non-maskable interrupts (as generated by the Pause button) which crash the emulator (it appears that once they fire, interrupts are never re-enabled). I'm not sure what's causing this, but hopefully it won't be too unpleasant to fix!

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