09-27-1997,Frank Sexton wrote to Murray Lesser:
FS> Therefore the fact that some video cards interfere with
FS> 02e8 (COM4) is not recognized by the video card
FS> manufactures as a bad thing on their part.
The video cards are not really at fault.  The XT bus allowed the use
of 0x100 - 0x3ff for add-in ports (and 640K was enough memory for
anybody!).  Those 8 bit cards only decoded 10 bits of the address since
that was all that was needed.  The video cards use ports in a higher
range that requires decoding of (I think) 16 bits which means that the
addresses will alias into the 10 bit space.  The AT - and the XT, for
that matter - was fully capable of addressing 64k of i/o port
addresses;  they just didn't use it.  The 8514 display from IBM used a
port that aliased to 0x2E8 and that's where the problem started!  If
you follow the trail back, all the SVGA cards that have problems also
have the 8514 capability.  Cheap Tridents, for example, have no
problem.
There was no spec for COM3/4 but the problem is really a few cents
saved in decoding a few bits, not the spec or lack thereof.  That, and
IBM's definition of the 8514.  I always said that of the
IBM-compatibles, IBM had some of the least compatible :}
Will Honea
whonea@codenet.net
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