Making a C128 act like a Sega Master System

[nathanallan]

Has this ever been thought of? It has a slightly faster Z80, better sound chip and more memory. I guess the games would have to be 'ported' over, but not sure why it's not feasible.

I'm thinking a boot disk, similar to kick it into CP/M, but into a mode that can run the Sega roms. But how about this (brainstorming here), have a hardware adapter that can use actual SMS cartridges?

If this could be done, it would be the best of both worlds in a huge way. I love both of the systems, but loved the 128 first.

By the way, hello, I'm Nathan in El Paso, I think I introduced myself already but it's been a while.

Nathan

[naI]

Well, the Master System audio has 4 channels to the SID's 3.  And just forget about FM.  I'm sure the (64k?) VDC could handle Master System graphics if they were ported.  But emulation will not be achievable unless your idea for an adapter includes components from the actual Master System.  You would basically need to make an expansion module similar to SuperCPU or the 2600 expansion for Colecovision - and if you need to do that, you might as well just use a real Sega Master System.

...A combo system would be an interesting mod, though.

[nathanallan]

A 'master system adapter' sounds like an interesting thing. I have a feeling that most of the audio, while not authentic, could be faked out. This definitely sounds like a neat project!!

[commodorejohn]

Well, the Master System audio has 4 channels to the SID's 3.

Only sort of; it uses an MSX-style PSG that provides only three channels of tonal audio (square waves.) However, it does also have a white noise channel, which can be mixed in with any of the three channels, which would be difficult to accomplish on the SID.

I'm sure the (64k?) VDC could handle Master System graphics if they were ported.

Not really; for one, the Master System provides paletted 9-bit RGB video, which is a far cry from the simple hard-wired 16-color mode of the VDC. In order to pull off anything like that kind of color you'd have to use a bunch of specialty tricks, which would be prohibitively slow, as if the emulation and translation of a video chip based around 4-bit tiles and sprites to a 1-bit bitmap format wouldn't already be.

But you're right in that the whole idea of running even a partial emulator for a 3.5MHz system on a 4MHz system is essentially impractical. The closest thing you could really get would be some kind of adapter cartridge containing (at a minimum) a Master System-compatible VDP chip. Of course, this is theoretically possible, if one is sufficiently motivated. However, if you're going to do that, you may as well just go ahead and include the PSG on there as well and save yourself the trouble of trying to redirect and convert PSG port writes to corresponding SID writes.

The biggest question is the flexibility of memory addressing; I'm not entirely familiar with the Z80 as used in the Commodore 128, but I was under the impression that port writes weren't even valid and everything was memory-mapped. If you can't send port writes out on the expansion connector, then the whole idea is essentially useless; everything on the SMS is done via port writes.

Also, it would have to be possible to mimic the SMS memory-mapping scheme of three switchable 16KB ROM banks from $0000-BFFF, with the area from $0000-0400 being fixed to bank 0, and 8KB of RAM mirrored at $C000-DFFF and $E000-FFFF. Of course, even if this were possible, you'd still need the SMS cartridges, unless you managed to map the "ROM" banks from the 128's RAM and have a loader program to copy in cartridge images from disk, which would, unfortunately, limit you to 128KB cartridges.

So...it may be possible, but it'd be a novelty project at best. But hey, everyone loves a good novelty project!

[airship]

Because of interrupt handling and other system stuff, the Z80 in the C128 runs closer to 2MHz.

The old quote is "You can emulate any computer on any other computer, as long as speed isn't an issue."

[nathanallan]

I wasn't really thinking emulation, but what would it take to run carts with an adapter, and see what would happen. The memory issue would be my main concern, also. Still thinking, lots to make work. I do have a spare MS, so I can use it for parts if needed.