Addressing Modes

Computer Organization & Embedded System — Control and Central Processing Units, NEC licence examination syllabus (Nepal Engineering Council).

Addressing Modes: all the ways to say "this data, over there"

The single highest-yield topic in this entire chapter.

An instruction rarely contains the actual data it operates on — instead, it contains a hint about where to find that data. That hint is the addressing mode, and mastering all of them (plus computing the effective address for each) is pure exam gold.

The single question every addressing-mode problem is really asking: "given this instruction's address field and these register/memory values, what is the Effective Address (EA), and what value gets used?"

📍 All Ten Modes, Simplest to Most Powerful

ImpliedNo address needed at all — e.g. CLC clears carry, target is implicit.

ImmediateThe operand itself sits right in the instruction. EA doesn't exist; the value IS the data.

RegisterOperand is in a named register.

Register-indirectThe register holds an address, and THAT address holds the actual data.

DirectEA = address field, directly.

IndirectEA = memory[address field] — one extra hop.

RelativeEA = PC + address field. Used heavily for branch instructions.

IndexedEA = index register + address field. Great for looping through arrays.

Base-registerEA = base register + address field. Used for relocating whole programs.

Autoincrement/decrementLike register-indirect, but the register automatically bumps up/down after use — perfect for stepping through an array without a separate increment instruction.

💡 Practice a single table: given AR=825, R1=400, and memory contents at a few addresses, compute EA for direct, indirect, relative, indexed, and register-indirect modes side by side. This exact style of numerical appears almost every year.

Syllabus points

Implied, immediate, register, register-indirect
Direct, indirect, relative, indexed, base-register
Autoincrement / autodecrement
Effective Address computation (numericals)

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