nes6502 - issue with mirrored memory about processortests HOT 5 CLOSED

Hi @olduf - thanks for the detailed report. I'm new to the emulation space but as far as I can tell this is a real issue. addresses 308 (0x134) and 4404 (0x1134) map to the same physical CPU RAM address.

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Apologies for the long time with no input; I'm very wishy-washy on this.

On the one hand the tests in net are designed not to assume anything external to the CPU — including how addresses are decoded.

On the other, the Ricoh 2A03 (/'NES 6502'): (i) is used (almost) exclusively in the NES; and (ii) includes the APU and some other parts in the chip, so the "external to the CPU" test is fuzzier than it should be.

I think possibly a good compromise would be just to restrict all memory accesses in the NES-specific tests to the first 2kb of memory? Would that satisfy the problem adequately?

from processortests.

I think possibly a good compromise would be just to restrict all memory accesses in the NES-specific tests to the first 2kb of memory? Would that satisfy the problem adequately?

Now that I've finished my cycle-accurate CPU emulator of this chip for the NES I feel more confident weighing in here.

I think it could be an improvement to only use addresses that don't have special mapping behavior, but it invites a lot of complexity. I'm confused about why you're choosing the first 2KB (assuming this is 0x0000 - 0x1fff which is just 0x0000 - 0x07ff mirrored.

However if we do acknowledge the memory mapping, then that invites a lot of additional complexity where we need to support mappers.

I think the real issue is that the documentation could be more explicit (instead of implicit) about not choosing to respect memory mapping or any special cartridge mapping behavior, and I do like that this suite of tests allows me to properly isolate and unit test the CPU. Time and energy permitting, I'd like to find a suite of test roms and create some per-cycle truth data with a similar level of thoroughness found in this repo. (It's a fantastic repo!)

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I'm confused about why you're choosing the first 2KB

The first 2kb runs to address 0x07ff; up to 0x1fff would be the first 8kb.

However if we do acknowledge the memory mapping, then that invites a lot of additional complexity where we need to support mappers.

I'm suggesting I just get the generator silently to drop any test that happens to generate an access outside of the range [0x0000, 0x07ff] and let it keep going until there are the desired number of test cases.

... though more explicit documentation might also just be the correct fix.

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So I've gone with "more explicit documentation" as the immediate fix, as per #37 but if I can persuade myself that an appropriate test filter — such as the proposed first-2kb-only, but possibly something else — would render the issue moot then I'll return to the topic.

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