On 19 Aug 2001, at 15:11, [log in to unmask] wrote:

>   On the colour graphics adapter -- which is what most VGAs and more
> recent designs emulate for text mode -- the contents of this ROM was
> copied to RAM at boot time, and although you *could* change the ROM,
> it was simpler and cheaper for a program that needed a special font
> to rewrite that RAM area, and force a reload from ROM when it
> finished.  (Programs that know about VGA features can play some
> additional tricks with this, but I don't think that applies to the
> BIOS code or most DOS stuff.)
>
>   Since the problem is only appearing in text mode emulations, my
> belief is that it is this process of retrieving pixel patterns for
> each character that is failing.  Either the "ROM" (in a modern
> laptop, this may not be an identifiable single chip) which contains
> those patters is corrupted, or the RAM that they get loaded to has
> some problem.  (If both r's and s's show as s's, and both t's and u's
> show as u's, that sounds to me like the least-significant bit of the
> character code -- used as an "index" into the pixel-pattern RAM, is
> being ignored, perhaps due to a broken or shorted address line.)

  I've thought a bit more about this, and realized:

(a) Of course, it's actually a VGA text mode that's being emulated;
the pixel patterns required are larger than CGA.

(b) An old laptop running 3.11 *might* actually have a character set
ROM chip after all, and the problem could be with this chip rather
than with the RAM it gets copied to.  If there *is* such a chip, and
if it's socketed, rough handling over the years may have loosened it
in the socket.
  *IF* you can open the case, making sure all socketted chips are
fully seated might, just possibly, help.

David Gillett

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