09211: x86 CPU cores: REP opcodes are too slow with tight scheduler intervals.

ID	Category [?]	Severity [?]	Reproducibility	Date Submitted	Last Update
09211	Timing	Minor	Always	2 days ago	1 day ago

Tester	hap	View Status	Public	Platform	MAME (Official Binary)
Assigned To		Resolution	Open	OS	Windows 10/11 (64-bit)
Status [?]	Acknowledged			Driver
Version	0.278	Fixed in Version		Build	64-bit
		Fixed in Git Commit		Github Pull Request #

Summary	09211: x86 CPU cores: REP opcodes are too slow with tight scheduler intervals.
Description	x86 CPU cores: REP opcodes are too slow with tight scheduler intervals. REP opcodes are interruptable, and therefore, MAME checks for icount>0 while doing the REP loop. However, this also means that if icount<=0 due to another reason than an interrupt, it will redo the opcode from the start, instead of continuing the internal loop. This adds a couple of extra cycles and memory accesses that don't happen on the real CPU. If for example so-called perfect quantum is used, REP opcodes will be much slower (in other words, counter-intuitively, perfect quantum will lower timing accuracy for these opcodes). Applies to: cpu/i86 (i86, i186, i286) cpu/i386, probably cpu/nec - bug: REP opcodes interruptability unemulated - fixed for 0.279 and also fixed the issue described in this report cpu/v30mz - bug: REP opcodes interruptability unemulated
Steps To Reproduce	Unknown if there currently are any symptoms due to this bug.
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