The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 6
Figure 6-8. With bit 7=0, writing to the 8255’s control register will set or clear
individual bits in Port C.
Bit Control
Another useful feature of the 8255 is the ability to set and clear individual bits on Port C.
You do so again by writing to the control register, as Figure 6-8 shows. Bit 7 must be 0. Bit
0 selects set or reset (clear) for the bit, and bits 1-3 select the bit to set or clear. For example,
this BASIC-52 statement sets bit 7 of Port C:
XBY(0FC03h)=0Fh
To clear bit 7, use this statement:
XBY(0FC03h)=0Eh
106 The Microcontroller Idea Book