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8255A at 80H: addresses, mode 0/1 words, BSR

Short answer

Port A 80H, B 81H, C 82H, control 83H. Control word for PA mode 0 out, PB mode 1 in, PC upper out, PC lower in is 87H. To enable Port B interrupt, set PC2 using BSR control word 05H (PC2 set, INTE_B=1).

An 8255A PPI connected to 8085 has a system base address of 80H. (i) What are the addresses assigned for Port A, Port B, Port C and control register? (ii) Write down the control word to initialize this card as follows: Port A mode 0 output, Port B handshake input, Port C upper output and remaining pin of Port C lower input. (iii) For above case, write down bit set/reset control word to initialize Port B interrupt request.

(i) Addresses

Base address 80H, so A1 A0 select the register: Port A = 80H, Port B = 81H, Port C = 82H, Control register = 83H.

(ii) Control word

  • Port A mode 0 output: D6D5 = 00, D4 = 0.
  • Port B handshake input: mode 1 (D2 = 1), input (D1 = 1).
  • Port C upper output: D3 = 0.
  • Remaining pins of port C lower input: D0 = 1. (In mode 1, PC0-PC2 are used for handshake, and the remaining pin PC3 is set as input.)
  • D7 = 1 (mode set).
 D7 D6 D5 D4 D3 D2 D1 D0
 1  0  0  0  0  1  1  1   = 87H

Control word = 87H.

(iii) BSR control word for Port B interrupt

In mode 1 the interrupt enable of port B (INTE_B) is bit PC2 for both input and output. To set PC2: D7 = 0 (BSR), D6-D4 = 000, D3D2D1 = 010 (bit 2), D0 = 1 (set) = 0000 0101 = 05H.

        MVI A,87H
        OUT 83H
        MVI A,05H
        OUT 83H     ; INTE_B = 1

Now INTR_B (PC0) is raised when data arrives at port B.

✓ Answer checked by Claude Sonnet 5.5

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