On Wed, 7 Jul 1999, Patrick LAM wrote:
> I thought that address mode wrap only worked on the 80286; by default I
> believe that it emulates an 8086 and reflects requests for ffff:0000 through
> ffff:ffff to 0000:0000 through 0000:ffff. The whole deal is that this
> behaviour on a 286 can be switched off.
[...]
you're right. the 8086 has 20 address lines (a0-a19) - so it could access
1Mb of memory. the 80286 has 24 of them (a0-a23) - so it can access 16Mb.
the 'a20 gate' is really an AND gate, with one of the
inputs connected to the processor a20 gate, and the other connected to one
of the unused bits in the keyboard status register (or something similar)
in order to emulate the xt 'wraparound' behavior.
btw, you could also get 'memory expansion boards' (or something with a
similar name). these allowed someone to have >640k in their XT workstation
:). this is probably the reason people think, that the xt had an a20 gate.
actually the memory present on the card was accessed using bank-switching
(the visible bank was put somewhere in a000:0-e800:0).
jacek
ps: i have an unrelated question - can anyone think of any use for
the math coprocessor (other than doing math of course:) ?
like to copy memory faster, or something similar...
