GATE CS 2025 Set 1 Q.27: ALU, Datapath and Control Unit

A partial data path of a processor is given in the figure, where RA, RB, and RZ are 32-bit registers. Which option(s) is/are CORRECT related to arithmetic operations using the data path as shown?

Processor datapath showing 32-bit registers RA and RB feeding into multiplexers Mux_A and Mux_B respectively. Both Mux_A and Mux_B also receive a 32-bit immediate value input selected by control signals Select RA/immediate and Select RB/immediate. The outputs of Mux_A and Mux_B feed the two inputs of an ALU controlled by ALU control lines, and the ALU output feeds 32-bit register RZ.

The data path can implement arithmetic operations involving two registers.

The data path can implement arithmetic operations involving one register and one immediate value.

The data path can implement arithmetic operations involving two immediate values.

The data path can only implement arithmetic operations involving one register and one immediate value.

Official answer and solution

Official answer: A, B and C
From: Official GATE 2025 answer key

Solution

Inspect the inputs to Mux_A and Mux_B feeding the two operand ports of the ALU.

In the given datapath:

Mux_A selects the first ALU input from either register RA or a 32-bit immediate value.
Mux_B independently selects the second ALU input from either register RB or a 32-bit immediate value.

Depending on the two select lines (Select RA/immediate and Select RB/immediate), the ALU can perform arithmetic operations on:

Two registers (RA and RB) $\implies$ (A) is CORRECT.
One register and one immediate value (RA + immediate, or immediate + RB) $\implies$ (B) is CORRECT.
Two immediate values (immediate + immediate) $\implies$ (C) is CORRECT.

Statement (D) is false because of the word 'only'.

Final answer

Options (A), (B), (C)

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2025 Computer Science and Information Technology - Session 1 (CS1)
Question: 27 of 65
Section: Computer Science and Information Technology

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