On 16 Apr 98 at 8:14, [log in to unmask] wrote:

>         First PC designs only considered 10 bits for decoding ports
> on the ISA bus, and this limit was on the mobo design, and perhaps
> on the ISA cards. So reading or writing on port xx or xx+400H was
> irrelevant. Above 6EAH=2EAH+400H, but this is only one of thousands
> examples of the aliasing (any or many bits 11 up).

  You're right, of course, but that's not -- so far as any source I
can find indicates -- what's happening in this particular case.
  In 1987, IBM had a plan of base addresses for COM3-COM8. [*]
Unfortunately, they kept it a secret, and the rest of the industry
went out and built equipment that put COM3 at 3E8 and COM4 at 2E8.
IBM, though, considered 2E8 unallocated, and used it in their design
of the 8514/A adapter.  Not, so far as I can find, 6E8 or 22E8 or
anything else that could get aliased to 02E8...

[*] You may recall the era when most BIOSes wouldn't automatically
detect ports beyond COM2....

David G