05434: a800: [possible] INC WSYNC skips 3 scanlines while Acid800 expects one scanline to be skipped.

ID	Category [?]	Severity [?]	Reproducibility	Date Submitted	Last Update
05434	Timing	Minor	Always	Jan 23, 2014, 23:33	Jan 24, 2014, 18:43

Tester	Alegend45	View Status	Public	Platform	MESS (Self-compiled)
Assigned To		Resolution	Open	OS	Linux
Status [?]	Acknowledged			Driver	atari/atari400.cpp
Version	0.152	Fixed in Version		Build	64-bit
		Fixed in Git Commit		Github Pull Request #

Summary	05434: a800: [possible] INC WSYNC skips 3 scanlines while Acid800 expects one scanline to be skipped.
Description	This bug occurs in Acid800's CPU timing test, where it states "Incorrect DEX/BNE cycle count: 6." I investigated this, and the culprit, assuming the cycle counts in the 6502 core are correct, is an INC WSYNC instruction, which issues a read, and 2 writes to and from WSYNC, according to how the 6502 does it. This skips 3 scanlines in MESS when Acid800 only expects it to skip one. I have a partial fix for this that makes reads from WSYNC do nothing, as the Altirra Hardware Reference Manual says they do (or at least it says the WSYNC is write-only). Something that should fully fix this would be, according to the same document, making the 6502 core not respond to RDY during write cycles. However, this is beyond my ability.
Steps To Reproduce	1. Run Acid800 in the a800 driver. 2. Wait for the CPU timing test.
Additional Information
Github Commit

Flags	Possible
Regression Version
Affected Sets / Systems	a800

Attached Files	antic.diff (626 bytes) Jan 23, 2014, 23:34 Uploaded by Alegend45 Partial fix patch [Show Content]