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PostPosted: Fri Apr 13, 2018 8:22 am 
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I am only speculating because I am at work at the moment but the nestest probably ran the zero page addressing mode of ASL on a zero page address other than $00. My opcode handler would work just fine in that situation.


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PostPosted: Fri Apr 13, 2018 9:16 am 
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tdondich wrote:
@Rainwarrior , is there any documnentation on what addresses would have open bus behavior on the cpu data bus?

And let me clarify, the value that will be returned on $4016 should be based on how it was addressed. Since most control pad read loops use absolute addressing, that's why the bits 7 - 4 will be set with 010, since it's the bits in the high byte of val 0x4016. Correct? The only way it'd be different is if it was addressed differently.

Hmm, I think the open bus article could use some additional info about what regions may be open bus.

Yes, you can get a different result by using indexed addressing from a lower address, but I think a PowerPak or Everdrive (or other cartridge) might have the potential to alter the open bus behaviour in different regions as well. I don't know the specifics of this. I think tepples tested it a while back.

zeroone wrote:
Just out of curiosity, why didn't the instruction set test ROMs detect an issue with ASL ?

The test ROMs probably don't test every instruction on every possible address. ;)


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PostPosted: Fri Apr 13, 2018 9:58 am 
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Results from Riding the open bus are that PowerPak pull-ups modify the open bus behavior, but EverDrive does not.

But if you're still getting games to run, don't worry too much. That test ROM and derivatives thereof are probably the only thing that relies on any difference between open bus in bits 7-5 and just driving them to 010 (the upper bits of $40/$41). And even then, the only difference is probably that the NES controller graphic will or won't be turned into a dogbone or an RF Famicom controller I or II.


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PostPosted: Fri Apr 13, 2018 10:44 am 
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tepples wrote:
Results from Riding the open bus are that PowerPak pull-ups modify the open bus behavior, but EverDrive does not.

Was that only testing the overlap with registers?

I seem to recall that Everdrive's $6000-7FFF area is subject to a different behaviour (when RAM is not mapped) than the NES open bus?


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