---
title: "Software interrupts of 8085"
subject: "Microprocessor"
chapter: 5
key: "semester_4/microprocessor:8085-software-interrupts"
source: https://www.bctengineer.com/q/semester_4/microprocessor/8085-software-interrupts
site: BCT Engineering
---

# Software interrupts of 8085

Microprocessor · Chapter 5: Interrupt Operations

- Asked in: 2074 Bhadra, 3 marks
- Video: [Software Interrupts In 8085](https://www.youtube.com/watch?v=8dOhy00D6-U) — TutorialsPoint
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/microprocessor/8085-software-interrupts
- Whole chapter: https://www.bctengineer.com/md/semester_4/microprocessor/chapter-5

## Question

What are the software interrupts of 8085?

## Short answer

The 8085 software interrupts are RST 0–7, 1-byte instructions that call fixed addresses 0000H, 0008H, … 0038H (vector=8×n). They push PC, jump to ISR, end with RET, and are not maskable. Used for system calls and breakpoints.

## Answer

Software interrupts in the 8085 are the eight restart instructions **RST 0 to RST 7**. They are 1-byte instructions put in a program, and they act like a CALL to a fixed address. Each is executed in 12 T-states: the PC is pushed on the stack and the program jumps to the vector 8 × n.

| Instruction | Opcode | Vector address |
|---|---|---|
| RST 0 | C7H | 0000H |
| RST 1 | CFH | 0008H |
| RST 2 | D7H | 0010H |
| RST 3 | DFH | 0018H |
| RST 4 | E7H | 0020H |
| RST 5 | EFH | 0028H |
| RST 6 | F7H | 0030H |
| RST 7 | FFH | 0038H |

- They are not maskable and are not affected by DI/EI, because the CPU itself fetches them.
- The same opcodes are also used by an external device in response to INTR.
- RST 0 acts as a reset-like restart at 0000H. RST 1 to RST 7 are used for system calls and debugging breakpoints (RST 7 is often used as a breakpoint).
- The routine ends with RET.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which instructions are the software interrupts of the 8085?
   A. RST 0 to RST 7
   B. TRAP and INTR
   C. RST 5.5, 6.5 and 7.5
   D. EI, DI, SIM and RIM
   Answer: A. The eight restart instructions RST 0-7 are the software interrupts. (Key point: What are software interrupts)

2. Where does RST 3 jump to?
   A. 000CH
   B. 0003H
   C. 0018H (3 x 8)
   D. 0030H
   Answer: C. The vector is 8 x n; 8 x 3 = 24 = 18H. (Key point: Vector address)

3. Why are RST 0-7 not affected by DI?
   A. They use the TRAP pin
   B. They are instructions fetched by the CPU itself, so they cannot be masked
   C. They are executed only when IF = 1
   D. They are controlled by the SIM masks
   Answer: B. As instructions in the program they always execute; DI/EI only affect hardware interrupt pins. (Key point: Maskability)

## Related in Microprocessor

- [Hardware vs software interrupts](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ahardware-vs-software-interrupts) — Microprocessor, chapter 5
- [Interrupt vs polling; interrupts in 8085](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ainterrupt-vs-polling-8085) — Microprocessor, chapter 5