The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Inputs and Outputs
XBY(0FC03h)=9BH
On reset, the 8255 uses this mode, until you tell it differently.
To read Port A, use
PRINT XBY(0FC00H)
or
PH0. XBY(0FC00H)
To read Port B or C, use the same statement, but with 0FC01H or 0FC02H to select the
port you desire.
To configure all all bits as outputs in mode 0, use this statement:
XBY(0FC03h)=80H
Then, to write a value to Port A, use
XBY(0FC00h)=xx
where xx is the value to be written.
Use FC01h to write to Port B, and FC02h for Port C.
Combinations of Inputs and Outputs
Fourteen other combinations of inputs and outputs are possible by setting or clearing bits 0,
1, 3, and 4 in the control word as shown in Figure 6-7. To change the mode settings, determine
which bits to set and clear for the configuration you want, convert the value to decimal or
hexadecimal, and write the value to 0FC03h.
One handy feature of the 8255 is that you can read back the last value written to an output
port. With the ports configured as outputs, you can use the same statements you use to read
the input ports. At Port B or C, reading an output port gives the value in the output latch,
which contains the last value you wrote to the port. Port A works a little differently. Instead
of reading the output latch, you read the actual logic states on Port A’s pins. This means that
at Port A, if a bit is shorted to ground, you will read back a 0 for that bit, even if the last
value written to it was a 1.
The Microcontroller Idea Book 105