Xref: news2.mv.net comp.os.msdos.djgpp:2816
From: Robert.Wagner
Newsgroups: comp.os.msdos.djgpp
Subject: Re: [Q] Homemade 16-bits-per-pixel gfx mode?
Date: 16 Apr 1996 21:40:09 -0500
Organization: Louisiana State University
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Sender: pswagn AT unix1 DOT sncc DOT lsu DOT edu
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

>I tried this out on my Cirrus VGA, and indeed something happened.. but
>it doesn't seem to be exactly like that. After some experiments, I
>found out that my VGA had entered a mode like this:
>
>- resolution: unchanged (320x200)
>- bpp:        unchanged (8bpp)
>- write mode: 'direct', that is, what you write into video ram goes
>  directly to the DAC, regardless of the palette settings. Actually,
>  the bits in each video ram byte are organized as follows:
>
>  IRRBBBGG
>  ^      ^
> msb    lsb
>
  
This seemed  really fun thing to play around with, and I got the following
results:

after reading the port $3c6 four or more times, the mode you enter 
depends apon what value you send to the port.  Here are the unique ones I 
found:

value written | resulting bitpattern | notes
--------------|-------------------------------------------
 0xc0         | IRRBBBGG             | same as 0x80
 0xc1         | RRRBBGGG             | looks nicer, more gradients
 0xc8         | greyscale            | this is a 256 shade greyscale!
 0xc5         | strange greyscale    | " with first column redscale(strange)
 0xc9         | IRRRGGBB             | same as 0x80 but red has 8 shades
 0x90 - 0x9f  | *RRBBBGG             | as 0x80, but entries 128-255 are 
              |                      | the same as for normal mode 13h
--------------|----------------------|--------------------------------------

(for all of the "modes" that are 0xc?, c can be replaced with d or e).

Tweaking modes is fun :)

-Rob