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Cycle accurate CPU broke FDS [resolved]
http://forums.nesdev.com/viewtopic.php?f=3&t=13068
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Author:  zeroone [ Wed Jul 29, 2015 11:57 am ]
Post subject:  Cycle accurate CPU broke FDS [resolved]

Recently, I rewrote my CPU based on this link. Each instruction now runs cycle-by-cycle on the microcode level. It passes all the CPU test roms that I am aware of and games run fine. But, the change affected FDS loading. Now, it always reports Error #20. Could the way that some CPU instructions behave on the microcode level (e.g. writing back to addresses that were just read from) affect the FDS registers differently than a CPU that treats each instruction atomically? Are there any test roms that may help me track this down. Thanks.

Edit: The rewritten CPU was not initialized correctly on start up. After a minor tweak, FDS is functional again. Thanks for your help anyway.

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