Question

Write an assembly language instruction to program the 82C55 to get data from port A and send it to port B, if PA=IN, PB=OUT, PC0-PC3=IN, and PC4- PC7=OUT and operating in Mode 0. Use port addresses of 300H-303H for the 82C55 chip.

Answer 1

;labelling addresses for each port and control register

porta equ 300h

portb equ 301h

portc equ 302h

creg equ 303h

;initalising 8255

mov al,10010001b

out creg,al

;taking data from port A

in al, porta

;sending to port A

out portb, al

The bits sent to control register for initialisation are as follows (0 is LSB):

7 - 0 for BSR mode and 1 for regular input output mode

6, 5 - mode for port A (mode 0 will be 00)

4 - Port A, 1 for input and 0 for output

3 - Port C upper, 1 for input and 0 for output

2 - mode for port B (mode 0 will be 0)

1 - Port B, 1 for input and 0 for output

0 - Port C lower,1 for input and 0 for output