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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
80h
81h
82h
88h
89h
8Ah
8Bh
90h
92h
93h
98h
99h
9Ah
9Bh
Table 4-3. P I O Control Word
Port A
Out
Out
Out
Out
Out
Out
Out
Out
In
In
In
In
In
In
Port B
Out
O
u
t
Out
I
n
In
O
u
t
In
I
n
Out
O
u
t
Out
I
n
In
O
u
t
In
I
n
Out
O
u
t
In
In
O
u
t
In
I
n
Out
O
u
t
In
Out
In
I
n
Port C P o r t C
Lower 0 to 3 Upper 4 tp 7
Out
Out
Out
In
In
Out
Out
Out
In
In
In
In
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.
1
4-4
P
r
o
g
r
a
m
m
i
n
g
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