GATE CS 2011 Q21 — Machine Instructions and Addressing Modes

The given instruction is LW R1, 20(R2), which loads a $32\text{-bit}$ word from memory into register $\text{R1}$.

The effective address (EA) of the memory operand is calculated as:

Let us analyze the given addressing modes to find the best fit:

• Immediate Addressing: The operand itself is specified directly within the instruction (e.g., MOV R1, #20). No memory access is involved for the operand.
• Register Addressing: The operand is stored inside a register (e.g., ADD R1, R2). No memory access is involved.
• Register Indirect Scaled Addressing: The effective address is calculated by scaling (multiplying by $2, 4, 8,$ etc.) the contents of an index register and adding it to a base register.
• Base Indexed Addressing (or Displacement Addressing): The effective address is obtained by adding a constant displacement (offset) to the contents of a base/index register. Here, $20$ is the displacement and $\text{R2}$ is the base register.

Therefore, the addressing mode used for the memory operand is Base Indexed Addressing.