GO64 Background sprites show/hide incorrectly with multicolor characters enabled (compared to C64)

[jeffreyvanderschilden]

Test Environment (required)

• Platform: MEGA65 R3
• Core Commit: aaf4542
• ROM Release: M65 V920413

Describe the bug
Sprites on C64 with $D018 set to show sprites behind characters will produce a different result in GO64 as compared to a C64 when tested against multicolor characters. The C64 will have 2 background colors and 2 foreground colors with the latter hiding the sprite. The GO64 mode will have 1 background color (as in THE background color) and all other colors hiding the sprite.

To Reproduce
Steps to reproduce the behavior:

1. Start little basic program mctest.zip
2. Compare results on MEGA65 vs C64.
3. Dan suggested on Discord this could perhaps be tested on a real C65 to answer the question "What will the VIC3 do" before considering this a bug

Expected behavior
The same as a real C65 I guess. (I'm simply comparing results to C64)

Additional context
Youtube video on the journey (using Street Rod as the example)
https://youtu.be/nTxmMMBVXC0

[lydon42]

Having something be different in GO64 mode compared to the real C64 is to be expected, so this is not a bug in itself, but rather a feature.

The question is: is it the same in MEGA65 mode (I assume yes, as GO64 is still the MEGA65) and how is it on a real C65?

[ki-bo]

I can confirm the VIC-III behaves like the VIC-II when running your test program. This is a bug in the VIC-IV implementation.