CGA artefacts in unsupported modes \ VOGONS

CGA artefacts in unsupported modes

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First post, by Laaca

Posted on 2023-08-01, 21:32

Laaca Offline

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Rank: Newbie
Posts: 8
Joined: 2019-01-01, 16:44

In the past I had a czechoslovakian computer PP-O6 with soviet clone of the i8086 CPU and with modified CGA graphic card.
The CGA modifications meant the hardware czech/slovak font (in 895 coding) and absence of "snowing" while writting to videomemory during vertical retrace.
And it also did interresting effect when I ran a game using EGA or VGA mode.
The mode was not set, of course, but on monitor occured an artefact similar to "snowing" or rather "snow storm". It differed whether EGA or VGA mode was tried to set.
My question is whether such artefact occures also on original CGA cards and whether the "snow storm effect" can be somehow controled by programmer using direct port accessed to CGA card.

Reply 1 of 1, by reenigne

Posted on 2023-08-02, 08:59

reenigne Offline

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Posts: 610
Joined: 2006-11-30, 05:13
Location: Cornwall, UK

I'm not sure what the snow storm effect you're talking about could be, being unfamiliar with this modified CGA card. Something writing rapidly to VRAM in 80-column text mode might look like a snow storm. Or perhaps it could be the CRTC being set to values that the monitor can't sync to. I also once managed to accidentally create create a snow storm effect on an Amstrad PC1512 (which doesn't suffer from VRAM contention snow at all) by writing different values to the mode register in a tight loop. Perhaps if you could get some video footage of the effect we can see what's going on.

Depending on the BIOS involved, trying to set an EGA or VGA mode on a CGA machine could do any or all of these things. Anything that the BIOS code can do can also be done from a user program so whatever it's doing can also be controlled by a programmer.

Btw, original IBM CGA cards didn't exhibit snow writing to VRAM during horizontal or vertical overscan/retrace, only during the display enable (active image) period (I'm guessing that's what you meant, just wanted to clarify). I'm curious about what sort of mod would prevent snow. Does it make the CPU wait until display disable?

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