What are Addressing Modes?

An addressing mode specifies how the operand part of a machine code instruction identifies the actual data to be used. Different addressing modes allow the operand to refer to data in different ways — as a literal value, a memory address, or a register.

OCR H446 requires knowledge of four main addressing modes: immediate, direct, indirect, and indexed.

1. Immediate Addressing

The operand is the actual data value to be used. The value is embedded directly in the instruction itself — no memory lookup is needed.

• How it works: the instruction contains the literal value to use (e.g. the number 5).
• Example: MOV R1, #5 — the value 5 is placed directly into R1. The # symbol indicates immediate addressing in many assembly languages.
• Advantage: fast — no memory access required to fetch the data.
• Disadvantage: the value is fixed — it cannot change during program execution. Limited to small constants.
• Use case: loading constants or small literal values (e.g. setting a counter to 0, comparing to a fixed threshold).

2. Direct Addressing

The operand contains a memory address. The CPU accesses that address in memory to retrieve the actual data.

• How it works: the operand is a memory address; the CPU goes to that address to get the data value.
• Example: LDR R1, 200 — load the value stored at memory address 200 into R1.
• Advantage: can access any location in memory; the value at that address can change during program execution.
• Disadvantage: requires one memory access to fetch the data (slower than immediate); the address is fixed in the instruction.
• Use case: accessing a specific named variable stored at a known memory location.

3. Indirect Addressing

The operand contains a memory address, but that address holds another address — the actual data is at the second address. The CPU must make two memory accesses.

• How it works: operand → memory address A → value at A is another address B → actual data is at B.
• Example: LDR R1, (200) — go to address 200, read what is there (say, address 350), then go to address 350 to get the actual data.
• Advantage: the pointer (address stored at the operand address) can be changed at runtime, so indirect addressing supports dynamic data access — e.g. pointers in C, implementing linked lists, passing arrays by reference.
• Disadvantage: requires two memory accesses — slower than direct addressing.
• Use case: pointer-based data access, implementing dynamic data structures.

4. Indexed Addressing

The operand contains a base address. An index register (e.g. IX) holds an offset value. The effective address is calculated as: base address + index register value.

• How it works: effective address = base address (in instruction) + value of index register (IX).
• Example: LDR R1, 200, IX where IX = 3 → effective address = 200 + 3 = 203 → load value from address 203 into R1.
• Advantage: by incrementing the index register, the same instruction can access successive memory locations — ideal for iterating through arrays.
• Disadvantage: requires an additional register (the index register) and a calculation to determine the effective address.
• Use case: iterating through arrays and sequential data structures — the index register acts as the array index.

Summary Comparison Table

Memory Map Example

Suppose memory contains:

• Immediate: MOV R1, #200 → R1 = 200 (the number 200 itself).
• Direct: LDR R1, 200 → R1 = 350 (value stored at address 200).
• Indirect: LDR R1, (200) → CPU reads address 200, gets 350, then reads address 350, gets 17 → R1 = 17.
• Indexed (IX=2): LDR R1, 200, IX → effective address = 200 + 2 = 202 → R1 = 99.