Explain the functions of the following Intel 8085 microprocessor signals: RESET-IN, IO/m, and RD. Also, state three Intel 8085 microprocessor software interrupts and describe SIM and RIM.

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SECTION A: MICROPROCESSOR SYSTEMS

(a) Explain the functions of each of the following Intel 8085 microprocessor signals:

• (i) RESET-IN: This is an active-low input signal. When the RESET-IN pin is held low, the microprocessor resets its Program Counter (PC) to $0000\text{H}$, clears all internal registers and flags, and restarts program execution from memory location $0000\text{H}$.
• (ii) IO/M: This is a status output signal. When this pin is high, it indicates that the CPU is performing an Input/Output (I/O) operation. When it is low, it indicates that the CPU is performing a memory operation. This signal helps external devices distinguish between memory and I/O access.
• (iii) RD: This is an active-low output signal. When the RD pin is asserted (pulled low), it indicates that the CPU is performing a read operation, meaning it is fetching data from either memory or an I/O device.

(b) Complete the table by stating the addressing mode and size in bytes for each instruction.

| Instruction | Addressing mode | Size in bytes | | :---------- | :-------------- | :------------ | | LXI H, 20FFH | Immediate | 3 | | ADD M | Register Indirect | 1 | | STAX B | Register Indirect | 1 | | DAA | Implied | 1 |

(c) Write an assembly language program to perform each of the following operation:

• (i) logically AND (10101101)$_2$ with (01110010)$_2$;

  First, convert the binary numbers to hexadecimal: $(10101101)_2 = AD_{16}$ $(01110010)_2 = 72_{16}$

  MVI A, ADH   ; Load the first operand (ADh) into the Accumulator
  MVI B, 72H   ; Load the second operand (72h) into Register B
  ANA B        ; Perform logical AND between Accumulator and Register B. Result is stored in Accumulator.
  HLT          ; Halt the program
  

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• (ii) multiply F2C$_{16}$ by 8.

  Multiplication by 8 is equivalent to three left shifts. In the 8085, a 16-bit left shift can be achieved by adding the 16-bit number to itself (DAD H instruction).

  LXI H, 0F2CH ; Load the 16-bit number F2C (0F2CH) into the H-L register pair
  DAD H        ; H-L = H-L + H-L (multiplies by 2)
  DAD H        ; H-L = H-L + H-L (multiplies by 4)
  DAD H        ; H-L = H-L + H-L (multiplies by 8)
  HLT          ; Halt the program. The result (F2C * 8) is in the H-L pair.
  

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(d) State three Intel 8085 microprocessor software interrupts.

1. RST 0
2. RST 1
3. RST 2 (Other valid answers include RST 3, RST 4, RST 5, RST 6, RST 7)

(e) Describe each of the interrupt instructions:

• (i) SIM (Set Interrupt Mask): This instruction is used to set the interrupt mask for the RST 7.5, RST 6.5, and RST 5.5 hardware