Preventing CGA revisionist history
trixter at oldskool.org
Wed Jan 25 21:39:03 CST 2006
Eric J Korpela wrote:
> I suppose it might be possible to get 160x200 on a CGA clone with 32K
> of RAM rather than 16K. Most "extended CGA" clones came with their
> own special graphics modes.
Nope, I've tried :-) On my AT&T PC 6300, which uses an MC6845 but with
32K RAM (to support the 640x400x2 mode), trying to set one scanline per
character cell does nothing, and that is what would be required.
I'm not in front of my CGA right now, but I seem to recall that trying
to set character cell height to anything other than 8 (the default) 4,
or 2 caused loss of screen sync and led to garbled display. It might be
possible to tweak other stuff to compensite, like vertical_start and
vertical_total, but I haven't tried.
I have gotten two graphics-mode video pages to work, but it's odd:
vertical_start and vertical_total adjustment don't take long to garble,
but horizontal start and horizontal total can take a ton of punishment.
The most usuable mode I got was 160x200 4 colors, two video pages, but
displayed as a COLUMN that was CENTERED in the middle of the screen.
Odd, but at least it was stable :-)
> Here's some screen shots of a game that uses 160x100 mode.
For even more interesting effects, look at:
Same technique, but using *all* characters instead of just the half-bar
characters. Starts to resemble Spectrum graphics at that point.
Jim Leonard (trixter at oldskool.org)
World's largest electronic gaming project: http://www.MobyGames.com/
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Various oldskool PC rants and ramblings: http://www.oldskool.org/
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