The Maplin Electronic Circuits Handbook, 2nd Edition

ADDING A 'USER PORT' 181

Table 8.1

Port address

Input selected

9F

A

BF

Β

DF

C

FF

D

6502 based systems

Since the 6502 does not have the IN and OUT instructions possessed by

the Z80, it is necessary to 'memory map' the I/O rather than treat it as a

port. This simply means finding a memory address which is unpopulated

with RAM, ROM or any other I/O device. Assuming that such an address

can be found, suitable decoding logic can be devised. Unless one can make

use of partial address decoding which may already be present within the

microcomputer system, this well generally involve the full width of the

address bus (i.e. all sixteen lines, AO to A15) ...