06705: coco3 [megabug]: Graphic corruption when running MegaBug using the Coco 3 driver

ID	Category [?]	Severity [?]	Reproducibility	Date Submitted	Last Update
06705	Graphics	Major	Always	Oct 10, 2017, 02:10	May 19, 2021, 04:42

Tester	MrGW	View Status	Public	Platform	MAME (Official Binary)
Assigned To		Resolution	Fixed	OS	Linux (64-bit)
Status [?]	Resolved			Driver	trs/coco3.cpp
Version	0.190	Fixed in Version	0.198	Build	64-bit
		Fixed in Git Commit		Github Pull Request #

Summary	06705: coco3 [megabug]: Graphic corruption when running MegaBug using the Coco 3 driver
Description	William Schaub discovered a graphic corruption issue when playing MegaBug on the Coco 3. It loads as a "split screen" where the top half is the entire game with slight corruption at the very bottom. The bottom half is a duplicate copy of the top half with the exception that the bottom half screen will change back and forth from a copy of the top half to complete graphic corruption. This happens whether you are moving your player character or not. This behavior is exhibited when loading the game as a CART, DSK image or CASSETTE image. The game works correctly when using the Coco 2 drivers.
Steps To Reproduce	Start MAME with the following command line: mame coco3 -nomouse -window -debug Insert the Megabug rompak (CART) using MAME's built-in UI.
Additional Information	William was able to find a workaround (with a rather interesting palette) by entering b@ff22=CD in the debugger window. The same issue happens when trying this on the official MAME binary for Windows (William has Windows v8.1).
Github Commit

Flags
Regression Version
Affected Sets / Systems	coco3 [megabug]

Attached Files	megabug-issue.jpg (109,693 bytes) Oct 10, 2017, 02:10 Uploaded by MrGW Screenshot of issue

No.14245 MrGW Tester Oct 10, 2017, 21:44	Rick Adams has confirmed he sees this same behavior with the Temple of ROM game cart. Rick is the original author of the game.
No.14246 Tafoid Administrator Oct 10, 2017, 22:23	So, you are stating that megabug doesn't work on original hardware coco3? I have both but my set up is long packed away so I can't test readily.
No.14247 MrGW Tester Oct 11, 2017, 00:47	Hi Tafoid. Let me clarify by saying that Rick sees this same issue when running his game in MAME. Both games work correctly on real Coco 3 hardware.
No.14264 MrGW Tester Oct 12, 2017, 10:21	Steve Bjork wanted me to forward his comments for this issue: "Have the guys working on the bug in mane look at the Sam/VDG mode system. From that I can see in the pictures, the game is trying to display a 3k screen, but MAME is doing it as a 6k screen. Easy for 1/2 coco but hard for the CoCo 3 graphics system. Mega-Bug did not work on the first designs of the Gime for the CoCo 3." As an FYI, Steve is the original author of MegaBug.
No.18858 tlindner Moderator May 19, 2021, 03:08	I just tested this (Mega-Bug running on the coco3 driver) in 0.234. And it is working fine.
No.18860 Tafoid Administrator May 19, 2021, 04:42	Found resolution version. Also, the softlist method of loading "coco3 megabug" didn't start working until 0.209 - unrelated to this bug which had you load picker style.