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Timing Sensitive Games?
Posted: Tue Feb 04, 2020 2:39 am
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I've been discussing a concept with some friends for a few months now - mostly casually, rather than any serious intention for implementation - about what if SEGA went for a Game Gear successor in 1995-96, maybe based on the Mega Drive VDP.
One sticking point is how the CPU would be upgraded. The Z380 strikes me as a potential CPU that could be an incredibly powerful thing, whilst still being power efficient. The difficulty is that whilst it retains a Z80-compatible ISA, and a mode for it - the actual timings still run on the Z380 optimizations. This would mean that a custom Z380 variant would have to be ordered by SEGA so that there's two sets of microcode ROMs, one for proper Z80 timings in Game Gear/Master System mode. By this point, the topic is somewhat academic, but I am also still rather curious - outside of timing sensitive PCM loops, just what software is surprisingly timing sensitive for SEGA's pair of 8-bit machines? |
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Posted: Tue Feb 04, 2020 7:53 am |
Any peripheral needing timer between writes - particularly the VDP - will have cycle counting delays. Of course you probably have a compatible VDP.
I think it's more likely they'd have done something similar to the Game Boy Color where the CPU is custom anyway and it has a 100% compatibility mode and a high speed mode. More practically, they could have made Nomad use carts in the GG form factor and have backwards compatibility. I guess they were more concerned with taking standard Genesis cartridges - which was probably a good idea, although not great for portability. |
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Posted: Tue Feb 04, 2020 1:32 pm |
I was actually more asking what games for the Master System and the Game Gear were actually quite timing sensitive.
The things that could break are well known - PCM, non-interrupt based raster effects, other cycle dependent stuff - I was just quite curious how prevalent those things are. Given how cleanly the hardware is designed, it feels like very little games - outside of anything VDP writing whilst screen drawing, and PCM related - would have actually been all that timing delicate. Quite unlike the situation with the NES! With regards to any sort of actual concept for a 95-96ish Game Gear successor. I've had ideas, but nothing solid. It's just been a purely academic exercise between friends. I'd not even really want to make a thread on SMSPower! about the idea - it's not really worth draining peoples energy into the concept. |
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Posted: Tue Feb 04, 2020 3:06 pm |
There was some discussion in https://www.smspower.org/forums/6485-CPUReplacement particularly for the idea of running at the lower rate when needed. The VDP access is by far the most important, but there's also cycle counting for:
- sample playback (as you mentioned) - 50/60Hz detection - line interrupt palette updates (to make the noise appear in hblank) - FM sound - some peripherals like the paddle and sports pad Some games might also have some implicit timing assumptions but it's hard to know what they are because they always hold. |
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