GATE CS 2025 Set 2 Q.61: ALU, Datapath and Control Unit

An application executes $6.4 \times 10^8$ number of instructions in 6.3 seconds. There are four types of instructions, the details of which are given in the table. The duration of a clock cycle in nanoseconds is _________. (rounded off to one decimal place)

Instruction type	Clock cycles required per instruction (CPI)	Number of instructions executed
Branch	2	$2.25 \times 10^8$
Load	5	$1.20 \times 10^8$
Store	4	$1.65 \times 10^8$
Arithmetic	3	$1.30 \times 10^8$

Your answer

Official answer and solution

Official answer: 3
From: Official GATE 2025 answer key

Solution

Compute the total clock cycles $C = \sum (\text{CPI}_i \times I_i)$ and divide the total execution time ( $6.3\text{ s}$ ) by $C$ .

Total clock cycles executed ( $C$ ): $C = (2 \times 2.25 + 5 \times 1.20 + 4 \times 1.65 + 3 \times 1.30) \times 10^8$ $C = (4.50 + 6.00 + 6.60 + 3.90) \times 10^8 = 21.0 \times 10^8 = 2.1 \times 10^9\text{ cycles}$
Clock cycle duration ( $T_{\text{clk}}$ ): $T_{\text{clk}} = \frac{\text{Execution time}}{C} = \frac{6.3\text{ s}}{2.1 \times 10^9\text{ cycles}} = 3.0 \times 10^{-9}\text{ s} = 3.0\text{ ns}$

Final answer

3.0

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 2 (CS2)
Question: 61 of 65
Section: Computer Science and Information Technology

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