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To simplify your selection of a control word to match your choice of port
directions, use Table 4-3. This atble summarizes information from the
Intel peripheral data book on control words and their port directions for
mode 0.
Control Word
A noteworthy characteristic of the 8255 PPI is that all port registers are
cleared by a write to the control register. Consequently, if your program
intends to repeatedly change the port configuration, you may need to
make provisions for restoring data to cleared ports.
4-4
P
r
Table 4-3. P I O Control Word
Port A
80h
Out
81h
Out
82h
Out
Out
88h
Out
Out
89h
8Ah
Out
8Bh
Out
90h
In
92h
93h
In
98h
In
99h
In
9Ah
In
9Bh
In
o
g
r
Port C P o r t C
Port B
Lower 0 to 3 Upper 4 tp 7
Out
O
u
Out
I
n
In
O
u
In
I
n
Out
O
u
Out
I
n
In
O
u
In
I
n
Out
O
u
In
O
u
In
I
n
Out
O
u
In
I
n
a
m
m
t
Out
t
Out
Out
t
In
In
t
t
In
Out
t
Out
Out
t
In
In
In
Out
In
In
i
n
g
1
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Troubleshooting
