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 be safe. 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 -tony