Viewing Issue Advanced Details
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 coco3.cpp
Version 0.190 Fixed in Version 0.198 Build 64-bit
Fixed in Git Commit Github Pull Request #
Summary MESS-specific 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
Regression Version
Affected Sets / Systems coco3 [megabug]
Attached Files
jpg file icon megabug-issue.jpg (109,693 bytes) Oct 10, 2017, 02:10 Uploaded by MrGW
Screenshot of issue
There are no relationship linked to this issue.
User avatar
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.
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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.
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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.
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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.
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May 19, 2021, 03:08
I just tested this (Mega-Bug running on the coco3 driver) in 0.234. And it is working fine.
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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.