7 Feb 2018

Conversions on Jaleco Mega System 32

A not too well known system with only few titles but with great graphics for some of them.
It can be considered as Jaleco's answer to SNK's MVS as this system is composed of a mainboard in which you plug "cartridges" (which are more or less romboards with a plastic shell). Connection between the 2 isn't as easy as it is on MVS cause Jaleco didn't use edge connectors for its cartridges but high density connectors instead (similar to Capcom's CPS1 and CPS2 for instance).

Anyway, to prevent conversions, Jaleco implemented a custom chip in the cartridges for graphics decryption (texts and background only) with different decryption "parameters" between games. Some games share the same custom chip, thus can be converted straight forward, some others don't and even different versions (romsets) of the same game can use different custom chips.

Decryption algorithm being in MAME I first decrypted the files for gfx3 (background) and gfx4 (texts) for romset gratia (custom chip ref. SS92047-01). Then I made a second small piece of code for re-encryption. I chose to rencrypt the files for custom chip ref. SS92046_01 as it's used on Tetris Plus (romset tetrisp) and this game is quite common and cheap.

Finally I modified MAME's source to force it to use a different custom chip with romset gratia.

I changed this:
GAME( 1996, gratia,   0,        ms32, gratia,   ms32_state, ss92047_01, ROT0,   "Jaleco", "Gratia - Second Earth (92047-01 version)",
MACHINE_IMPERFECT_GRAPHICS | MACHINE_IMPERFECT_SOUND | MACHINE_SUPPORTS_SAVE )

To this:
GAME( 1996, gratia,   0,        ms32, gratia,   ms32_state, ss92046_01, ROT0,   "Jaleco", "Gratia - Second Earth (92047-01 version)",
MACHINE_IMPERFECT_GRAPHICS | MACHINE_IMPERFECT_SOUND | MACHINE_SUPPORTS_SAVE )

And tested the conversion successfully:



Next step is to test it on real hardware and maybe find a way to use decrypted files instead of re-encrypted ones to simplify things (no need to re-encrypt files for each different custom chip).

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