05789: Wrong vblank IP_ACTIVE_HIGH value.

ID	Category [?]	Severity [?]	Reproducibility	Date Submitted	Last Update
05789	DIP/Input	Minor	Always	Dec 8, 2014, 12:21	Dec 11, 2014, 16:46

Tester	darq	View Status	Public	Platform	MAME (Official Binary)
Assigned To	AWJ	Resolution	Fixed	OS	Windows Vista/7/8 (64-bit)
Status [?]	Resolved			Driver	snk/prehisle.cpp
Version	0.156	Fixed in Version	0.157	Build	Normal
		Fixed in Git Commit		Github Pull Request #

Summary	05789: Wrong vblank IP_ACTIVE_HIGH value.
Description	The IP_ACTIVE_HIGH in PORT_BIT 0x80 is wrong, the correct value should be IP_ACTIVE_LOW. To prove this, just implement a more realistic vblank read (with vpos usage) and without PORT_VBLANK. If you let IP_ACTIVE_HIGH instead of IP_ACTIVE_LOW, the game stucks after the memory check. I didn't go too deep into vblank implementation code in screen.c, but I really don't understand MCFG_SCREEN_VBLANK_TIME.
Steps To Reproduce
Additional Information
Github Commit

Flags	Noted in Source, Verified with Code
Regression Version
Affected Sets / Systems

Attached Files

No.11287 AWJ Developer Dec 10, 2014, 02:24	Agreed, if you look at the vblank interrupt handler, the first thing it does is copy the palette from work RAM to palette RAM (this has to be done in vblank or you'd get snow, proving that the interrupt does fire at the start of vblank), then it writes to the scroll registers, reads the inputs, and finally waits for $e0044 bit 7 to be high before it copies the sprite list from work RAM. This only makes sense if bit 7 transitions from low to high some time after vblank.
No.11290 darq Tester Dec 11, 2014, 16:46	Yes, you are right, in fact I obtained snow, heavy slowdowns or total freeze.

No.11287

AWJ

Developer

Dec 10, 2014, 02:24

Agreed, if you look at the vblank interrupt handler, the first thing it does is copy the palette from work RAM to palette RAM (this has to be done in vblank or you'd get snow, proving that the interrupt does fire at the start of vblank), then it writes to the scroll registers, reads the inputs, and finally waits for $e0044 bit 7 to be *high* before it copies the sprite list from work RAM. This only makes sense if bit 7 transitions from low to high some time after vblank.

No.11290

darq

Tester

Dec 11, 2014, 16:46

Yes, you are right, in fact I obtained snow, heavy slowdowns or total freeze.