07965: gamegear [ddragon, ddragonp]: Corrupt background tiles

ID	Category [?]	Severity [?]	Reproducibility	Date Submitted	Last Update
07965	Media Support	Major	Always	May 4, 2021, 00:03	May 23, 2021, 09:10

Tester	kmg	View Status	Public	Platform	MAME (Self-compiled)
Assigned To		Resolution	Fixed	OS	MacOS X
Status [?]	Resolved			Driver	sega/sms.cpp
Version	0.231	Fixed in Version	0.232	Build	64-bit
		Fixed in Git Commit	f2392ff	Github Pull Request #

Summary	07965: gamegear [ddragon, ddragonp]: Corrupt background tiles
Description	Double Dragon (and prototype clone) start with working graphics. As soon as you start a game from the title screen the background graphics become completely corrupt. Foreground sprites seem to be fine. See images.
Steps To Reproduce	mame gamegear ddragon, and then simply start the game
Additional Information
Github Commit

Flags
Regression Version
Affected Sets / Systems	gamegear [ddragon, ddragonp]

Attached Files	intertitle1.png (3,364 bytes) May 4, 2021, 00:03 Uploaded by kmg
Attached Files	level1.png (6,706 bytes) May 4, 2021, 00:04 Uploaded by kmg

No.18799 Tafoid Administrator May 4, 2021, 03:42	This is an odd one. It work/loads graphics fine if loaded picker style-cart c:\location\of\cart\ddragon.zip as far back as I could test. I guess the issue would be finding the difference in what happens from softlist load vs. picker loading.
No.18801 kmg Senior Tester May 4, 2021, 07:57 edited on: May 4, 2021, 07:57	It wouldn't even have occurred to me to test it outside the software list, but this was the perfect clue. I figured it was just some VDP issue, but it turned out to be a simple bank switching problem. Thanks Tafoid! When loading with -cart the sega8 slot code allocates 32K of RAM because it has no way of knowing in advance if a game has any on chip. The softlist allocates none. I verified in the debugger that the game writes to $FFFC (RAM bank select) when you start a new game (but not before, hence the working title screen). Fix is in https://github.com/mamedev/mame/pull/8027/commits/f2392ffcf666f7707310bea22628e9d787c17a06 though I'd like it if someone could confirm these carts actually have 8K of RAM (and not more)? I think this guess is correct. One, it seems only GG carts with battery backup ever had more than 8K? And two, MAME only had data in the first 8K of the 32K nvram files it created when running the games via -cart.
No.18894 kmg Senior Tester May 23, 2021, 08:27	The fix above was merged into master. I believe this bug can be resolved now.