NES: Fixed GetInternalOpenBus returning external bus value by mistake.#82
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zdg-kinlon wants to merge 1 commit into
Open
NES: Fixed GetInternalOpenBus returning external bus value by mistake.#82zdg-kinlon wants to merge 1 commit into
zdg-kinlon wants to merge 1 commit into
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It should return _internalOpenBus to correctly reflect CPU-internal open bus state, especially after reads like $ 4015.
Mokona
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May 17, 2026
When DMC is enabled via $4015, it starts a timer for when the resulting load DMA needs to occur. When the frame ends, this timer can be decremented an extra time due to a call to NesApu::EndFrame that wasn't expected to have execution side effects. Stepping back can also trigger this because EndFrame is called from NesApu::Serialize. Sour suggested moving the DMC ProcessClock call out of this function to fix it, which works. This fixes very rare failures in the AccuracyCoin Implicit DMA Abort test.
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Summary
Fix
OpenBusHandler::GetInternalOpenBus()returning external bus valueDescription
Fixed a bug in
OpenBusHandlerwhereGetInternalOpenBus()incorrectly returned_externalOpenBusinstead of_internalOpenBus.Technical Details
_externalOpenBuswhen it should return_internalOpenBus.$ 4015reads may appear unaffected in simple cases (sinceGetInternalOpenBus()is called before updating the bus), the bug breaks correctness whenever the internal and external bus values differ — such as after a$ 4015read or in advanced mappers/debuggers that rely on accurate internal bus state.$ 4015does not drive the external bus; only the CPU's internal bus is updated. Thus, internal and external open bus values must be tracked separately.