Mail Archives: djgpp/1996/04/09/17:05:26
Xref: | news2.mv.net comp.os.msdos.djgpp:2561
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From: | bram AT gig DOT nl (Bram Stolk)
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Newsgroups: | comp.os.msdos.djgpp
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Subject: | Re: [Q] Homemade 16-bits-per-pixel gfx mode?
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Date: | 9 Apr 1996 08:32:34 GMT
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Organization: | Euronet Internet
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Lines: | 44
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Message-ID: | <4kd7b2$ba3@news.euro.net>
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References: | <4jtkgr$mfv AT news DOT euro DOT net>
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NNTP-Posting-Host: | shadwell.gig.nl
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To: | djgpp AT delorie DOT com
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DJ-Gateway: | from newsgroup comp.os.msdos.djgpp
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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
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