Register-to-Register (RR) Instruction Class Which description best characterizes RR instructions in a typical instruction set architecture?

Introduction / Context:
Instruction formats are often categorized by where operands reside: register-to-register (RR), register-to-memory (RM), and memory-to-memory (MM). Recognizing RR instructions is core to understanding CPU datapaths and pipeline throughput.

Given Data / Assumptions:

• General-purpose registers are present.
• Opcodes specify operand sources and destination.

Concept / Approach:
RR instructions fetch operands exclusively from registers and write the result back to a register. This minimizes memory traffic and is favored in RISC designs, enabling single-cycle ALU operations when operands are ready in the register file.

Step-by-Step Solution:
Identify operand locations: both sources are registers.Execution path: register file → ALU → register file.Result placement: destination is a register (e.g., ADD R1, R2 → R1).No direct memory access happens during the ALU operation itself.

Verification / Alternative check:
In contrast, RM instructions use one register and one memory operand, while MM instructions use two memory operands (rare in modern CPUs due to pipeline and bandwidth costs).

Why Other Options Are Wrong:
Option A: Describes memory-to-memory instructions.Option B: Describes register-to-memory (or memory-to-register) class.Option D: Cannot be correct because A and B contradict the RR definition.Option E: Not applicable because Option C is correct.

Common Pitfalls:

• Confusing addressing mode with instruction class—RR defines operand location, not how addresses are computed.
• Assuming RR always takes one cycle; pipeline hazards can introduce stalls.

Final Answer:
Operate on two registers and leave the result in a register.