[GEM Development] Video mode - bitplane vs packed pixels

[John Elliott]

In RAM it's straightforward: Byte 0 is the first pixel, byte 1 is the
next pixel and so on. But when you're using my 256-colour drivers, what
vro_cpyfm() gives you isn't in the same order as it is in video RAM.

The 256-colour drivers can be found in the FreeGEM VESA driver pack at
<https://www.seasip.info/Gem/drivers3.html>. My initial release of the
256-colour driver was called SD2569.VGA but later releases are called
SDW109.VGA (1024x768), SDW869.VGA (800x600), SDW649.VGA (640x480) and
SDVLF9.VGA (320x200).

--
John Elliott

Cyprian Konador wrote:
> On Thu, 1 Feb 2024 at 14:34, John Elliott wrote:
>
>     You can get the number of planes using vq_extnd(), but I don't think
>     there's any way to determine the native pixel format used by the
>     screen
>     (short of drawing on the screen, using vro_cpyfm() to read the
>     bits, and
>     making deductions from what you get back). Back when GEM was being
>     sold
>     commercially, almost everything was either mono or 16-colour in 4
>     bitplanes; I think the one 256-colour driver was for the 8514/a, which
>     used 8 bitplanes.
>
>
> Fortunately GEM is bitplane based plus one (I think) 8bit packed-pixel
> format, It seems recognizing the screen format would not be a
> difficult task. I will try to use vro_cpyfm or vr_trnfm (to transform
> from standard to device format).
>
>
>
>     The 256-colour drivers I wrote use packed pixels on the display, but
>     still present video memory to vro_cpyfm() as eight planes. The older
>     ones do a lot of shifts and rotations to do this (so the first "plane"
>     maps to bit 0 of every byte, the second "plane" maps to bit 1 of every
>     byte and so on). The newer ones avoid this by saying plane 0 is bytes
>     0,8,16,24... of video RAM, plane 1 is bytes 1,9,17,25... and so on.
>
>
> Do you mean "sd256.VGA" - I downloaded it from somewhere but now I'm
> not able to find it in Google.
> Does this mean the screen data in RAM is in linear format?
> A)
>     1st byte - 1st pixel, 2nt byte - 2nd pixel, 3rd byte - third
> pixel, and so on.
> Or 'interleaved':
> B)
>     1st byte - 1st pixel, 2nd byte - 9pixel, 3rd byte - 17pixel
>     .....
>     n byte - 2nd pixel, n+1 - 10pixel, n+2  - 18pixel
>     .....
>
> Thanks
>
>
> Regards
>
> Cyprian
> https://260ste.atari.org
>
>
> --
>
>
>
>
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