Instruction Sets and Addressing Modes

At A-Level you need to understand what an instruction set is, the format of a machine-code instruction, and the different addressing modes used to specify operands. You should also be able to trace through short sequences of assembly-level instructions.

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What Is an Instruction Set?

An instruction set (or instruction set architecture — ISA) is the complete collection of all the machine-code instructions that a particular processor can recognise and execute. Each instruction is a binary pattern stored in memory.

The instruction set defines:

• The operations the processor can perform (e.g. ADD, SUB, LOAD, STORE, BRANCH).
• The addressing modes supported.
• The format of each instruction (how many bits for the opcode, how many for the operand).
• The registers available to the programmer.

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Instruction Format

A typical machine-code instruction is divided into two parts:

┌──────────────┬──────────────────────┐
│   Opcode     │      Operand         │
│  (operation) │  (data or address)   │
└──────────────┴──────────────────────┘