Forums

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?

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.

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.

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.

Author	Message
Flygon Joined: 30 Jun 2016 Posts: 196 Location: Melbourne, Australia	Timing Sensitive Games? 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?

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14735 Location: London	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.

Flygon Joined: 30 Jun 2016 Posts: 196 Location: Melbourne, Australia	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.

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14735 Location: London	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.

Forums

View topic - Timing Sensitive Games?