Radio Shack CCR-81 Reference Handbook

For example, if your coffee pot circuit is port
addressed at FE Hex and 02 Hex will turn it on,
an assembler routine that generates the machine
code to load 02 into port FE is as follows:
OUT 0FE,02H ; Turn on coffee pot
In Level II language you could say:
OUT 254, 2
(This is in BASIC and does the same thing.)
The port instruction uses OUT* and IN* and the
8 lower order address lines.
Should you memory-map or should you use
ports? This is entirely up to you. If you memory
map, you should select your address lines high
enough so that you will not interfere with your
in-system RAM. If you have maximum memory
you may not be able to go high enough to be
free from RAM. But if you have only 16K or
less of RAM, you have thousands of potential
addresses to use. If you use ports, you on Iy have
255 available (you'll remember from our earlier
discussion that port FF is used for the cassette
recorder).
In terms of hardware, using ports will be slightly
better than memory mapping. Why? Address
decoding. In a port-based system you will only
need to decode 1 out of 256 possibilities. Only 8
lines. In a memory-mapped system you will have
to decode 1 out of 65,536 possibilities - that's
16 address lines. We'll show it both ways and let
you make up your own mind.
85

Radio Shack CCR-81 Specifications

General

Brand	Radio Shack
Model	CCR-81
Category	Desktop
Language	English