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Forums > C64 Coding > REU and Color Memory
2023-10-06 07:57
Fungus

Registered: Sep 2002
Posts: 631
REU and Color Memory

Can the REU access color memory or is it a quirk that it cannot?

I've not been able to get color memory scrolling to work with an REU and don't know if it's just a VICE bug or if this is intended.
2023-10-06 08:14
tlr

Registered: Sep 2003
Posts: 1732
Quote: Can the REU access color memory or is it a quirk that it cannot?

I've not been able to get color memory scrolling to work with an REU and don't know if it's just a VICE bug or if this is intended.


Interesting question. The color memory is connected in a special fashion due to it being a separate bus on the VIC-II. The data bus to it depends on AEC for instance. I think it looks in the schematic like it should be accessable, but this should be tested on HW of course.

Do you have a small test program that someone with a REU could run?

Preferably a read/write color ram test with go/no-go if possible.
2023-10-06 08:22
6R6

Registered: Feb 2002
Posts: 244
Correct zp 01 value when transfer?
2023-10-06 10:54
Fungus

Registered: Sep 2002
Posts: 631
I don't have a test program atm, I was trying to fix a game with it but it didn't work so reverted the code in frustration. I just figured the REU can't access the color memory, but then I am using an older vice so thought it might be a bug in it (3.2).

I thought Sonic used it, so it was possible, but I can't get it work. I'll try again, maybe I just didn't code it correctly. Although I have been using the same code in my tape transfers without issue.
2023-10-06 11:25
Mr. SID

Registered: Jan 2003
Posts: 424
Sonic only writes to Screen and Color RAM, it never reads. Maybe reading doesn't work, I've never tried that...
2023-10-06 12:48
tlr

Registered: Sep 2003
Posts: 1732
No point in speculating here. I'll ask on #vice-dev if someone knows if this is tested, otherwise we need to see what it does on a real machine.
2023-10-06 13:57
chatGPZ

Registered: Dec 2001
Posts: 11154
it should work (ignoring that transfers from/to I/O are glitchy on some setups)

https://sourceforge.net/p/vice-emu/code/HEAD/tree/testprogs/REU..

And yes, for REU things use recent VICE :)

PS: make sure REU transfers do not start or end at the same cycle as VICII DMA, that causes weird behaviour/glitches. VICE does NOT emulate this correctly atm.

https://sourceforge.net/p/vice-emu/code/HEAD/tree/testprogs/REU..
https://sourceforge.net/p/vice-emu/code/HEAD/tree/testprogs/REU..
2023-10-06 20:03
Fungus

Registered: Sep 2002
Posts: 631
What do you mean by "some setups" ?
2023-10-06 21:31
chatGPZ

Registered: Dec 2001
Posts: 11154
Random C64s, reason unknown. It may or may not be related to the (known) glitches that happen when REU DMA starts (or stops) at the same time as a VICII DMA. In some of those demos that show 1 pixel wide color splits using REU, you can see glitches - on some C64s.
2023-10-06 22:22
Fungus

Registered: Sep 2002
Posts: 631
Okidoki, I'll check that REU is not dmaing at the same time then.
2023-10-08 05:01
Martin Piper

Registered: Nov 2007
Posts: 647
Do you need to avoid bad lines?
2023-10-08 12:49
Fungus

Registered: Sep 2002
Posts: 631
Yes, it can cause problems. Verified.
2023-10-10 21:46
Repose

Registered: Oct 2010
Posts: 222
Quote: Okidoki, I'll check that REU is not dmaing at the same time then.

1 cycle is 8 pixels - how is a 1 cycle colour split done?
2023-10-10 21:54
chatGPZ

Registered: Dec 2001
Posts: 11154
Simply DMA to a color register (with fixed target address)
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