20 Dec
2005
20 Dec
'05
8:45 p.m.
Chuck Guzis wrote:
Sorry, crude analogy on my part.
(I say that
and, looking at my CGA card in graphics mode, there are
single-bit errors twitching on and off -- RAM tests okay so I guess the
ramdac has
blown a gasket somewhere)
Ramdac? I The output of the CGA was plain RGBI digital, buffered through an LS244. There is a composite output, but it's sort
of a simple-minded
resistor adder network fet to the base of a 2n3904 to so some level
shifting. Best recheck your RAM--and remember that writes to display
memory on a CGA can occur only during the retrace interval or they produce
"snow".
RAM checks out okay after 65536 pattern writes and reads. So whatever is
making the single-bit errors I don't know -- it only happens in 320x200
graphics mode, and is fine elsewhere. This is an original IBM CGA, not a clone.
As for snow, I've learned to live with it :-) Most programs that work around
the snow problem don't handle it properly -- they either only pump out
characters during horizontal retrace (slow!) or they try to fire off an entire
screen in retrace (produces snow since even 4K is too long) or worse they blank
the screen while updating, which just leads to annoying flashes! Best bet is
move a single 16-bit word during horizontal retrace and move about 250 words
during vertical retrace -- that's about as fast as I've gotten without
producing snow.
--
Jim Leonard (trixter at oldskool.org) http://www.oldskool.org/
Want to help an ambitious games project? http://www.mobygames.com/
Or check out some trippy MindCandy at http://www.mindcandydvd.com/