Duss blabbed Mr. Hart to Mr. Hoyt ...

 BH> Hi, does anyone know how to safely enable the A20 (address line 20)?
 CH> It's not so easy. Try the book "The Undoccumented PC" for
more info,
 CH> but basicly you can do it the '286 way - issue commads to the keyboard
 CH> controller;
Ok commands to the keyboard controller.. but which port etc and what
commands?

 CH> the PS/2 way - Toggle a bit on port 92h (sometimes called
 CH> fastgate A20) or other proprietory means on specific chipsets.
Yeah I think I've figured out that way from some source I have but
it doesn't work on my Pc :)

 CH> Prgramming the keyboard controller isn't easy, nor is it 100%
 CH> reliable, and the fastgate method isn't implemented in every
 CH> chipset... Some chipets also fail to update both areas with the
 CH> current data when one method is used.. nasty!
Hmmm.. pity.

.. about my big/bad/ugly bug ..
 CH> Sounds like you are failing to properly setup the descriptors for the
 CH> segments before returning to realmode. The easiest method is to
 CH> restore all segment descriptors to base=0k, limit=64k, before exiting
 CH> protected mode, then to reload them with your realmode values. Leaving
 CH> the descriptors set "randomly" will cause all sorts of obscrure
 CH> problems which manifest themselves in unusual ways.

Thank you very very very very very very very very very very very very
very very very very very very very very get-the-idea? much. That is
_Precisely_ what was happening. I don't know exactly how, but some
weird programs were counting on 64k segment wraparound or something
and I still had all my segment limits 4gigs big! Oooooooooooooooops.
But thanx very much I've fixed my bug now and can get on with my pmode
header PROPERLY ;)

l8r, and Thanx again, JaKe

... "I toÛd you°not ²o touch t°e floppy diskÛs suÛface!"
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