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)   │
└──────────────┴──────────────────────┘

Field	Purpose	Example
Opcode	Specifies the operation to perform	ADD, SUB, LDA (load), STA (store), BRA (branch)
Operand	Specifies the data to use, or the address where the data is located	A memory address, a literal value, or a register identifier

• The opcode width determines how many different instructions the processor can support. An 8-bit opcode allows up to 256 different instructions.
• The operand width determines the range of addresses or values that can be specified directly.

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Types of Instruction

Category	Examples	Description
Data transfer	LDA, STA, MOV	Move data between registers and memory
Arithmetic	ADD, SUB, MUL, DIV	Perform mathematical calculations
Logical	AND, OR, NOT, XOR	Perform bitwise logic operations
Comparison	CMP	Compare two values and set status flags
Branch / Jump	BRA, BEQ, BNE, BGT, BLT	Change the flow of execution (conditional or unconditional)
Shift	LSL, LSR, ASR	Shift bits left or right
I/O	IN, OUT	Read from or write to I/O ports
Halt	HLT / STOP	Stop execution

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Addressing Modes

An addressing mode specifies how the operand field of an instruction should be interpreted to find the actual data (the effective address or value). A-Level requires you to know the following modes:

1. Immediate Addressing

The operand is the data itself — not an address.

LDA #5      ; Load the value 5 into the accumulator
ADD #3      ; Add the literal value 3 to the accumulator

• Advantage: Fast — no memory access needed to get the data.
• Disadvantage: The range of values is limited by the number of bits available for the operand.

2. Direct (Absolute) Addressing

The operand gives the memory address that holds the data.