Good haul of old pc stuph

[Jim Leonard]

Chuck Guzis wrote:
>>(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

Sorry, crude analogy on my part.

> 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/
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