changing PC floppy port address
ard at p850ug1.demon.co.uk
Mon Feb 20 17:25:00 CST 2006
> the 8088 can support 64k ports, so logically the lower
> 16 address lines would be utilized to address them.
> A16-19 were multiplexed with other pin functions. IIRC
> the pc only allows for 256. Probably a good book on
The PC hardware outputs all of the address lines onto the slot, and an
ISA card can decode A0-A15 during I/O operaitons.
The thing is that, in general, IBM only decoded the bottom 10 lines
(A0-A9). So most cards _and the I/O ports in chips on the motherboard_
appear 64 times in the I/O map, once for every combination of the A10-A15.
But there's nothing to stop _you_ decoding all 16 lines, provided you
make sure your card avoids all the ghosts of other cards. If you stick
to the space reserved for prototype cards (and it's ghosts), you should
A few IBM cards decoded more of the lines. I forget which ones, but I
think it's one of the 'lab I/O' cards, either the GPIB card or the data
acquisition adapter. And IIRC, the addresses for the second, third, etc
card of the same type differed in the values of some of A10-A15
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