Xref: news2.mv.net comp.os.msdos.djgpp:2561
From: bram AT gig DOT nl (Bram Stolk)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: [Q] Homemade 16-bits-per-pixel gfx mode?
Date: 9 Apr 1996 08:32:34 GMT
Organization: Euronet Internet
Lines: 44
Message-ID: <4kd7b2$ba3@news.euro.net>
References: <4jtkgr$mfv AT news DOT euro DOT net>
NNTP-Posting-Host: shadwell.gig.nl
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

Hello Coder,

In a previous posting I wrote:


: I want to use a 16 bit per pixel mode (or else 15 bpp).
: And I would like a resolution of 320x200. (or something close,
: in fact 320x240 would be better because of the square pixels).

: However, UNIVBE doesnt give me this mode.

: Is it possible to create your own mode, without having to support
: all chipsets? 
: So I mean: by just using standard VGA stuff?

: I already found out the following trick:

:   - read the port 3c6 four times
:   - write 3c6 once with value 128

: What it seems to do, is halve the horiz resolution, and make it
: 15 bpp.

: So if you do this in 320x200x8bpp,
: you end up in 160x200x15bpp.
: However, sometimes when I do this, the performance of screenwrites
: seems to come to a halt. (e.g. 50 times slower than regular modes)


Now I know something more about it.
3c6 is a regular VGA register: the DAC Pixel Mask 0.

However, on some vga cards, when you do a read 4 times, an internal
flipflop does a flip or a flop, and then another register is selected
for writing.

I think that this hack only works for SDACs.

Lot of people mailed me telling me it doesnt work,
so sorry for the confusion. Hope it's a bit more clear now.


  Bram Stolk