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Sega G80 Raster games - MAMEDEV Forum
armis2000
Posts: 2
Joined: Wed Sep 22, 2021 3:16 pm

Sega G80 Raster games

Hi,

First time posting here, I hope I am posting such that the internet gods here don't pounce me.

I have long held that the G80 raster games have run too fast, verified running MAME side by side with the real machine. MAME runs this game at about the right speed if you slow CPU0 down to 70%. This is due to the video hardware generating wait states during video memory accesses.

I can comment about Astro Blaster, as its the machine I have access to, but would be willing to bet the change would be universal due to the architecture.

I have attempted to reach out to the owner of that driver to provide more information without any success. I am not a coder, but am into the hardware, so writing changes myself would not be a successful venture for me.

Currently, the emulation does not appear to utilize the PROM on the video board (I have dumped this) which is critical to memory timing. The schematics are actually MISSING jumpers at U12 that connect YRWT to MYWT and VRWT to RWT. Bouncing back and forth between the 2 pages indicates that WAIT is applied to the G80 bus much of the time VRAM is accessed. This was verified with a logic analyzer connected to a working boardset.

U52 PROM dump and notes for jumpers can be downloaded from:

https://coinoplove.com/AstroblasterU52.zip


Cheers - Chris

User avatar
cuavas
Posts: 99
Joined: Tue Nov 11, 2014 1:04 pm
Location: Sydney, Australia
Contact: Website

Re: Sega G80 Raster games

On the topic of speed, I believe it’s incomplete/inaccurate emulation of wait states or access time penalties. This is an area MAME doesn’t do so well in.

armis2000
Posts: 2
Joined: Wed Sep 22, 2021 3:16 pm

Re: Sega G80 Raster games

True, that would be hard to accommodate in real -time.

But, for a more accurate emulation, a pad on the CPU speed should be added. Those raster games run entirely too fast.

Haze
Posts: 11
Joined: Thu Nov 06, 2014 1:16 pm

Re: Sega G80 Raster games

that would be considered a hack, and less accurate emulation.

results aren't the deciding factor, how those results are achieved is the basis on which accuracy is determined in the project.

the waitstates on accesses need emulating, which is more difficult than doing things in a hacky way

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