Question

Sequence of steps in 8085 microprocessor interrupt execution :
• [A.] Interrupt acknowledged
• [B.] Program content saved
• [C.] Control transferred to ISR
• [D.] Interrupt vector is read Choose the correct answer from the options given below :

• A. D, A, B, C
• B. A, B, D, C
• C. A, B, C, D
• D. D, A, C, B

Hint:

8085 interrupt execution sequence: \[ \text{Interrupt acknowledge} \rightarrow \text{Save program state} \rightarrow \text{Read interrupt vector} \rightarrow \text{Transfer control to ISR} \]

Answer 1

The Correct Option is B

Concept: When an interrupt occurs in 8085 microprocessor, the processor temporarily stops the current program execution and executes an Interrupt Service Routine (ISR). The interrupt handling process follows a systematic sequence:
• Interrupt recognition
• Saving current execution state
• Fetching interrupt vector
• Branching to ISR

Step 1: Interrupt acknowledgement. When an interrupt request arrives:
• The processor first checks whether interrupts are enabled.
• If valid, the interrupt is acknowledged. This corresponds to: \[ A \] Thus: \[ A \text{ occurs first} \]

Step 2: Saving the current program execution state. Before servicing the interrupt:
• The processor must preserve the current execution information.
• Typically, the Program Counter (PC) contents are pushed onto the stack. This ensures the interrupted program can resume later. This corresponds to: \[ B \] Hence: \[ B \text{ occurs second} \]

Step 3: Reading the interrupt vector. After saving current status:
• The processor fetches the interrupt vector address.
• The vector specifies the starting location of ISR. This corresponds to: \[ D \] Thus: \[ D \text{ occurs third} \]

Step 4: Transfer of control to ISR. Finally:
• The processor jumps to ISR address.
• Execution of interrupt service routine begins. This corresponds to: \[ C \] Therefore: \[ C \text{ occurs last} \]

Step 5: Write the complete sequence. Hence the correct order is: \[ A \rightarrow B \rightarrow D \rightarrow C \]

Step 6: Write the final answer. Thus the correct option is: \[ \boxed{(B)\ A,\ B,\ D,\ C} \]