What is a control word and how it write ?

As i mansion my earlier post that is send by programer to make understand 8255 that which port is Input and which one is Output. Now how we are do that ?

Format of Control Word




Mode Definition Control Byte. Indicate by bit b7=1.
Port A and upper half of Port C (Group A)
Bit b3 to bit b6 control the mode and direction of Group A

b6, b5            mode                operation
0                      0                        mode 0
0                      1                         mode 1
1                      0                         mode 2

b4 Port A direction, 0=output, 1=input

b3 upper half of Port C direction, 0=output, 1=input

Port B and lower half of Port C (Group B)
Bit b0 to bit b2 control the mode and direction of Group B

b2  mode operation, 0=mod 0, 1=mod 1
b1  Port B direction, 0=output, 1=input
 lower half of Port C direction, 0=output, 1=input

Now take a Example that we need Port A as a input Port and Port B as a Out put Port What will be the Control Word ?

For that we need  Bit  D7  =1 ( For Input/Output Operation )
                                  D6  =0
                                  D5  =0
                                  D4  =1 ( For Port A as Input)
                                  D3  =0
                                  D2  =0
                                  D1  =0  (For Port B as Output)
                                  D0  =0

So finally we got the number 10010000 or 90H.