nesdev.org

You need to start execution at $FFFC, not $8000. In other words, the value stored at $FFFC tells you where to set your program counter to start execution. See http://wiki.nesdev.com/w/index.php/CPU_memory_map.

OK problem solved! In my case there was no issue with the illegal instructions or clocking the sweep registers etc. I was setting the pulse channel's timer period incorrectly when writing to $4002/$4006 and $4003/$4007. I had thought that the period should be set based on the current value of both s...

Great, thanks for the quick reply!

Streemerz apparently relies on some tricky APU emulation, as it claims "Sound problems may occur on old/inaccurate emulators, avoid them if you can." https://www.fauxgame.com/ On the title screen I hear some intermittent loud high-pitched screeching, so I'm wandering what sort of trickery ...

The wiki page for the APU noise channel has a "timer period" table (bits 0 - 3 of $400E): http://wiki.nesdev.com/w/index.php/APU_Noise Do the values in this table indicate the period for the associated timer, or the period reload value for the timer divider, as defined here: http://wiki.ne...

Bionic Commando uses the BRK instruction - this happens after destroying the "Main System" at the end of the first level.

Chester Field uses implied NOP $1A at $C97A (when you jump).

After some investigation, I found that the game tries to write to the pattern tables, even though it has no CHR-RAM. I was allowing the writes, which caused the CHR-ROM bank to be corrupted. I found a couple of other games that do this as well: - Baseball Stars 2 - Empire Strikes Back - Fantasy Zone...

Yes, I believe so. Though in this case it wouldn't matter, since Cowboy Kid has 256 1K CHR-ROM banks so there is no way it could ask for a bank value out of range. It's strange that no other games out of the hundreds I've tried exhibit similar behavior - it's just this one game, which is what makes ...

I'm having some trouble getting Cowboy Kid to work properly with my emulator. Whenever you're "outside", many of the background tiles appear to be black! However, when you go into a building, the background renders properly (see attached images). It appears to be a mapper 4 game. My emulat...

Ninja JaJaMaru-kun uses $04 (NOP). This happens when you touch an enemy.

rainwarrior wrote:
What does "bypassing" an opcode mean? Do you mean treating it as a 1-byte NOP instead?

Yes.

The Action 52 games "Alfredo" and "Jigsaw" both crash because they have $7C (3 byte NOP) at $AE96. Emulating as a 3 byte NOP causes both to go into an infinite loop, and the games never load. Simply bypassing the opcode (and subsequent unofficial opcodes) allows the games to boot...

thefox wrote:Couldn't reproduce either one of those.

Oops - my apologies - Turns out I had bad dumps of both Rollergames and Bart Vs. The World. There is no unofficial opcode use in either.

...and The Simpsons: Bart vs The World uses $0B (ANC) at $F126. You can see this happen on the China boat level, when Bart throws his ball.