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I find it hard to believe that it would be truly random (or random all the time). I'd expect there to be some special case that's triggering this.

Is the test done using the same screen parameters (possibly even contents) as the place of the bug? Or maybe sound? Are irqs enabled?

(fyi, I know nothing about snes hardware)

Yeah, this has got to be some kind of DMA/IRQ/who knows what interaction. Lots of other games would break if that readback really did randomly screw up.

ETA: does this game work on the later "slimline" SNES with the revised less-buggy chipset?

I can provide the .nv-file here. Just walk to the red and white tent and enter Swanky's show.
The ROM used is the NTSC version verified with no-intro.
NVRAM:
http://www.megaupload.com/?d=5Y7V67R2

thanks. it saves me a lot of time (especially because my eeepc cannot run snes at 100% speed)

I hope to find the problem (and a fix)

EDIT: I figured it out. Solution is here:
http://board.byuu.org/viewtopic.php?p=12986#p12986

Basically, when NMI triggers (on the bus cycle edge), it clears the S-CPU Memory Data Register.

As the game loops while reading 16-bits, eventually an NMI triggers on the opcode edge after lda $185d,x [x=181a, effective=3077]; which turns $1818 into $0018. And that value magically gets the loop to break.

By observing the number of loop iterations needed to break out, it appears to match hardware very closely. Obviously it's not exact since the counters aren't aligned when you hit the button.

I will write a proper proof ROM later tonight to verify it 100%, but I'm pretty confident that we have this one cracked now.

Unfortunately this test is going to require clock-perfect S-CPU timing, but at least seeing it pass/fail on hardware, you can implement it in MESS

ah, gotta love game bugs like this which accidentally abuse hardware.

LN

Just noticed an odd issue with the KO countdown timer in Super Punch-Out and 0.137. The left half of the timer number is diplayed on the far left side; while the right half is on the far right. The number should appear in the center of the screen (as it does in bsnes and real hw).

It's easy enough to replicate - just start the game and wait about 3-5 minutes through the attract mode/demo. Eventually the timer will appear.

Side note: verified that the Speedy Gonzales thing is actually DMA and HDMA updating the MDR, not NMI as first suspected.

http://byuu.org/temp/test_mdrhdma.zip

It brings up entirely new problems by exposing that HDMA is asserting MDR after an opcode's read cycle from which the HDMA triggers; which is going to require a ground-up rethink of the MDR and HDMA sync subsystems. But anyway for right now, even without that, just let H/DMA update the MDR and Speedy will work.

Also the crap hack "Sonic on SNES" game that was based on Speedy. (That was way more interesting back before Sega started releasing 4 games a year featuring Sonic on Nintendo hardware).

Is this "ground up rethink" going to require major changes to your S-CPU? I want to know when it's safe to port, because I was gonna do it this weekend. That obviously would be dumb if it's gonna be in serious flux though.

I guess it's impossible to say it in advance:

http://board.byuu.org/viewtopic.php?f=16&t=614