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  • Joined: 30 Jun 2016
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Timing Sensitive Games?
Post Posted: Tue Feb 04, 2020 2:39 am
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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Post 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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Post 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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Post 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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