Question

An 8-bit microcontroller with 16 address lines has 3 fixed interrupts i.e., Int1, Int2 and Int3 with corresponding interrupt vector addresses as 0008H, 0010H and 0018H. To execute a 32-byte long Interrupt Service Subroutine for Int1 starting at the address ISS1, the location 0008H onwards should ideally contain
1. a CALL to ISS1
2. an unconditional JUMP to ISS1
3. a conditional JUMP to ISS1
4. Both (1) and (2)

Answer 1

Correct Answer - Option 4 : Both (1) and (2)

• A vectored interrupt is a processing technique in which the interrupting device directs the processor to the appropriate interrupt service routine.
• Vector address corresponding to Int1 is 0008 H
• Int1 interrupt the processor and send address 0008 H after getting acknowledgement from CPU.
• Now we are at 0008 H and to execute interrupt service routine (ISS1) we need to go to Interrupt Service Subroutine for Int1. This can be achieved by two ways:

1. CALL ISS1
2. JMP ISS1 (unconditional JMP)

 

Tips and Tricks:

Interrupting device gets the acknowledgment from the CPU because we are at 0008H (vector address)and it had to execute the interrupt service subroutine corresponding 1. Both CALL to ISS1 and an unconditional JUMP to ISS1 can take to the required Interrupt Service Subroutine 1 to execute. No need for conditional jump in such cases.