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View topic - Converting SG-1000 games with embedded ram to SMS

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  • Joined: 02 Aug 2015
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Converting SG-1000 games with embedded ram to SMS
Post Posted: Sun Jan 05, 2020 1:53 pm
The games The Castle and Othello use embedded extra ram located on the cartridge (https://www.smspower.org/Tags/EmbedExtraRAM). This extra ram was added because the SG-1000 only has 1 KB of ram which would not have been enough to run these games. The SMS, however, has 8 KB of ram and in theory would be able to run these games without the embedded extra ram.

Is there somebody interested to modify the aforementioned games, so that they can be enjoyed on a regular cartridge without embedded ram (for example on flash cartridges or FPGA consoles where embedded ram is not implemented)? From my understanding as a layman this should not be too difficult as only the memory calls would need to be replaced.

The Castle has 8 KB and Othello has 2 KB of RAM mapped at 0x8000 onwards (cf. https://github.com/ocornut/meka/blob/master/meka/meka.nam). Memory calls to 0x8000 and above would have to be remapped to 0xCFFF onwards. This is where SG-1000 memory ends and the remaining SMS memory is mapped (cf. https://www.smspower.org/Development/MemoryMap).

From a technical level it would be very interesting if this could be done. Unfortunately I don't have enough knowledge of assembler to accomplish this myself.
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Post Posted: Sun Jan 05, 2020 2:41 pm
One way to do it is to setup a breakpoint in the 0x8000..0xBFFF region using a debugger and then fix all the opcodes in the ROM. Rince and repeat until nothing hits that region.
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Post Posted: Sun Jan 05, 2020 7:23 pm
pcfreak324 wrote
The games The Castle and Othello use embedded extra ram located on the cartridge (https://www.smspower.org/Tags/EmbedExtraRAM). This extra ram was added because the SG-1000 only has 1 KB of ram which would not have been enough to run these games. The SMS, however, has 8 KB of ram and in theory would be able to run these games without the embedded extra ram.

Is there somebody interested to modify the aforementioned games, so that they can be enjoyed on a regular cartridge without embedded ram (for example on flash cartridges or FPGA consoles where embedded ram is not implemented)? From my understanding as a layman this should not be too difficult as only the memory calls would need to be replaced.

The Castle has 8 KB and Othello has 2 KB of RAM mapped at 0x8000 onwards (cf. https://github.com/ocornut/meka/blob/master/meka/meka.nam). Memory calls to 0x8000 and above would have to be remapped to 0xCFFF onwards. This is where SG-1000 memory ends and the remaining SMS memory is mapped (cf. https://www.smspower.org/Development/MemoryMap).

From a technical level it would be very interesting if this could be done. Unfortunately I don't have enough knowledge of assembler to accomplish this myself.


Nice idea! I hope someone can figure this out.

Right now both games are playable on older non-"X7" Master EverDrive models in a real SMS using the SG2GG patches from https://www.smspower.org/forums/16912-SG1000SC3000MarkIIIJPSMSRArrExportSMSRArrGameGearConverter#103058 but this still relies on EverDrive for SRAM, and at least in The Castle it's somehow incompatible with the newer X7 Everdrive. The patches just insert Sega Mapper initialization code and some (ignored by SMS) palette configuration.

Patches in this ZIP file: https://www.smspower.org/forums/download.php?id=14127

Edit: another consideration is that both games occupy 32k and don't include the "TMR SEGA" header or checksum, so systems with export BIOS likely won't boot them unmodified
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