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Control Word

The functional description of the bits of the control word is as follows:
Bit D0 The D0 bit is used to set the Port Clower. When this bit is set to 1, Port Clower is an input port. If the bit is set to 0, Port Clower is an output port.
Bit D1 This bit is used for Port B. When this bit is set to 1, Port B is an input port. If the bit is set to 0, Port B is an output port.
Bit D2 The bit D2 is used for the selection of the mode operation of Port B. If this bit is set to 0, the Port B can be operating in Mode 0. For Mode 1 operation, D2 is set 1.
Bit D3 It is used for the Port Cupper. If the bit is set to 1, Port Cupper is an input port. When the bit is set to 0, Port Cupper is an output port.
Bit D4 The bit D4 sets the port A for input or output operation. When this bit is 1, the port A can be used as input port. When it is 0, the port A becomes the output port.
Bit D5 and D6 These bits are used to select the operating mode of Port A. The Port A can be operated in modes 0, 1 and 2. The mode of operation is selected by D5 and D6 as given below:

For Mode 2, Bit 5 is set to either 0 or 1; it is immaterial
Bit D7 This bit select the I/O mode or bit set/reset mode. When it is 1, Port A, B and C are defined as input/output ports. If it is set to 0, bit set/reset mode is selected.

Table: Control words of 8255 for Mode 0 operation

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