Instruction Sets I

Lecturer: Graham Bragg

Topics: Instructions, Assembly Code, Instruction Decoding, RISC vs CISC, Memory Alignment

What is an Instruction?

An instruction tells the CPU what operation to perform. It's the fundamental unit of computation.

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Key Concept: Instructions control the logic of the CPU by encoding operations, addresses, and data in binary format that feeds into the underlying hardware logic.

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CPU Architecture Overview

A CPU consists of:

• Memory Interface - connects to memory
• Instruction Fetcher - retrieves instructions from memory
• Instruction Decoder - converts binary instructions into hardware control signals
• Registers - fast temporary storage
• Arithmetic Logic Unit (ALU) - performs computations

Instruction Set Architecture (ISA)

Definition: The structure of a computer that defines the list of commands/operations that a CPU can execute.

Key Points about ISAs:

• Defined in the 1960s (IBM System/360)
• Lists all operations the CPU supports (addition, subtraction, multiplication, etc.)