In which addressing mode is the effective address of the operand generated by…

Concept: An addressing mode is the rule an instruction set uses to compute the effective address (EA) — the actual memory location from which the operand is taken. Modes are told apart by two things: what the address field inside the instruction means, and whether the current content of a register is combined arithmetically with that field to build the EA. Displacement-style modes are exactly those that ADD an instruction-supplied constant to a register's content.

Application: The rule described here is a sum of two quantities — one constant that comes from the instruction and one value that is read out of a register at run time. Written as a formula this is the displacement form EA = X + [Ri], built as follows:

1. The instruction supplies a fixed constant X in its address field.

2. The processor reads the current content of the named register, written [Ri].

3. It adds the two to form the effective address: EA = X + [Ri].

4. The operand is then fetched from memory location EA.

A mode whose effective address is formed by that constant-plus-register sum, with the register acting as the varying part, is index addressing.

Cross-check: Setting the effective-address rule of each offered mode side by side makes the distinction explicit:

Only the sum rule EA = X + [Ri] matches “a constant value added to the contents of a register”, so the mode being described is index addressing. This is also why index mode is the natural fit for arrays: the constant X names the array’s base while stepping the index register moves to the next element.