The registers presented here demonstrate the following registers that may be used for microcomputer interfacing:

PIPO:    parallel-in, parallel-out   (used to expand either input or output ports, are faster, and require more interfacing pins)
PISO:    parallel-in, serial-out      (used to expand input ports, are slower, and require fewer interfacing pins)
SISO:    serial-in, serial-out         (used for time-delay purposes)
SIPO:    serial-in, parallel-out      (used to expand output ports, are slower, and require fewer interfacing pins)
SIPO1:  serial-in, parallel-out      (same as SIPO, except with added output hold register)
SPIO:    serial or parallel in/out   (combined features of all of the above)