Instruction Cycle

Table of Contents
Basic Computing Systems Organization…………………………………………………………………3 Instruction Cycle…………………………………………………………………………………………..3

The Fetch-Decode-Execute Cycle……..………………………........…………………………………….4

Fetch Cycle………………………………………………………………………...………………………5

Decode Cycle………………………………………………………………………………………………6

Execute Cycle……………………………………………………………………………………..….……7

System Buses……………………………………………………………………………………….…..….8

Registers……………………………………………………………………………………………...……9

Clocks………………………………………………………….…………………………………………...9

Appendix………………………………………………………………………………….……………….10

References……………………………………………….………………………………………………...11

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BASIC COMPUTING SYSTEMS ORGANIZATION

INSTRUCTION CYCLE
An instruction cycle (fetch-and-execute cycle, fetch-decode-execute cycle, or FDX) is the basic operation cycle of a computer. It is the process by which a computer retrieves a program instruction from its memory, determines what actions the instruction requires, and carries out those actions. This cycle is repeated continuously by the central processing unit(CPU), from boot up to when the computer is shut down.

FETCH INSTRUCTION

DECODE INSTRUCTION

EXECUTE INSTRUCTION

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THE FETCH-DECODE-EXECUTE CYCLE

Von Neumann architecture (e.g. Pentium is using an von Neumann architecture for the external memory), which – – – Allows instructions and data to be mixed and stored in the same memory module More flexible and easier to implement Suitable for most of the general purpose processors

The fetch-decode-execute cycle represents the steps that a computer follows to run a program. The CPU fetches an instruction (transfers it from main memory to the instruction register), decodes it (determines the op-code and fetches any data necessary to carry out the instruction), and executes it (performs the operation(s) indicated by the instruction). A large part of this cycle is spent copying data from one location to another. When a program is initially loaded, the