In the 8085 microprocessor, what will be the value of the Program Counter after execution of the instruction RST 3?
0018H
The RST n (Restart) instructions of the Intel 8085 are single-byte software interrupt / call instructions. When executed, the processor pushes the current contents of the Program Counter (PC) onto the stack (so the program can return later) and then loads the PC with a fixed restart vector address, causing execution to jump there.
The vector address is computed by a simple rule:
Restart address = n × 8 = n × 0008H
Applying this to the standard restart set:
| Instruction | Vector address |
|---|---|
| RST 0 | 0000H |
| RST 1 | 0008H |
| RST 2 | 0010H |
| RST 3 | 0018H |
| RST 4 | 0020H |
| RST 5 | 0028H |
| RST 6 | 0030H |
| RST 7 | 0038H |
For RST 3: 3 × 8 = 24 decimal = 18 in hexadecimal, so the PC is loaded with 0018H. This is the correct answer.
The value 0030H is the vector for RST 6, not RST 3. The value 0024H (36 decimal) does not correspond to any RST vector, since valid vectors are always multiples of 8. The value 0003H comes from mistakenly reading the operand 3 directly as an address instead of multiplying by 8, which ignores how the restart mechanism computes its vector.
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