---
title: "Computer functions and cycles"
subject: "Computer Organization and Architecture"
chapter: 1
key: "semester_5/computer_organization_and_architecture:computer-functions-cycles"
source: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/computer-functions-cycles
site: BCT Engineering
---

# Computer functions and cycles

Computer Organization and Architecture · Chapter 1: Introduction

- Asked in: 2072 Kartik, 3 marks
- Video: [Instruction Cycle in Computer Organization & Architecture: Detailed Steps | COA](https://www.youtube.com/watch?v=cO-H3tDqvXM) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/computer-functions-cycles
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_organization_and_architecture/chapter-1

## Question

Explain the computer functions with different cycles.

## Short answer

The instruction cycle consists of fetch (read instruction), decode, indirect (if needed), execute (perform operation), and interrupt (check and service). After execution, if an interrupt is pending and enabled, the CPU saves its state, executes the service routine, and resumes.

## Answer

A computer performs four basic functions: data processing, data storage, data movement and control. It carries them out by executing programs, and program execution is organised into cycles.

- **Fetch cycle:** the CPU reads the instruction from the memory location given by the PC into the IR, and the PC is incremented.
- **Decode:** the control unit finds the operation and the operands.
- **Indirect cycle:** if the instruction uses indirect addressing, the effective address is read from memory.
- **Execute cycle:** the operation is performed: a data transfer between the CPU and memory or I/O, an arithmetic or logic operation in the ALU, or a change in control flow (jump).
- **Interrupt cycle:** after execution, the CPU checks for interrupts; if one is pending, it saves the PC and loads the address of the interrupt service routine.

Instruction cycle = fetch + (indirect) + execute + (interrupt). The cycle repeats until the program halts.

## Check yourself

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

1. Which set lists the four basic computer functions?
   A. Data processing, data storage, data movement, control
   B. Fetch, decode, execute, store
   C. Input, output, memory, ALU
   D. Addressing, arbitration, timing, caching
   Answer: A. These are the functions every computer performs. (Key point: Four functions)

2. When is the indirect cycle used?
   A. When an interrupt is pending
   B. When the instruction is a jump
   C. When the instruction uses indirect addressing and the effective address must be read from memory
   D. When the program halts
   Answer: C. The effective address is fetched from memory before the operand is accessed. (Key point: Indirect cycle)

3. Which expression gives the instruction cycle?
   A. Fetch + (indirect) + execute + (interrupt)
   B. Execute + store only
   C. Interrupt + decode
   D. Fetch + store
   Answer: A. Indirect and interrupt cycles occur only when needed. (Key point: Instruction cycle composition)

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