What is the approximate performance of a CPU in MIPS, when the instruction mix of the CPU is shown in the table below? Here, the CPU does not use a pipeline architecture. Instruction type | Instruction execution time in µs | Appearance ratio Register to register operation | 0.3 | 30% Register to memory operation | 0.6 | 50% Conditional branch operation | 0.1 | 20%

ITPEC FE Subject A October 2024 Question 8

What is the approximate performance of a CPU in MIPS, when the instruction mix of the CPU is shown in the table below? Here, the CPU does not use a pipeline architecture.

Instruction type	Instruction execution time in µs	Appearance ratio
Register to register operation	0.3	30%
Register to memory operation	0.6	50%
Conditional branch operation	0.1	20%

a)0.03b)0.41c)2.44d)35.00

Explanation

2.44 MIPS — the CPU completes one instruction every 0.41 µs on average, and MIPS is the reciprocal of that time in microseconds.

Average instruction time, weighting each type by its appearance ratio:

0.3 × 0.30 + 0.6 × 0.50 + 0.1 × 0.20 = 0.09 + 0.30 + 0.02 = 0.41 µs

Instructions per second: 1 ÷ (0.41 × 10⁻⁶) ≈ 2.44 × 10⁶, that is 2.44 million instructions per second.

Answer (c)

Why not others:

•(b) 0.41 — the average execution time per instruction in µs, not its reciprocal
•(a) 0.03 — would mean about 33 µs per instruction, yet even the slowest instruction takes 0.6 µs, so MIPS cannot fall below 1 ÷ 0.6 ≈ 1.67
•(d) 35.00 — would mean about 0.029 µs per instruction, faster than the fastest instruction (0.1 µs), so MIPS cannot exceed 1 ÷ 0.1 = 10

Key rule: Without a pipeline, MIPS is 1 ÷ (average instruction time in µs); the average must lie between the fastest and slowest instruction times, which bounds the answer.

Computer Component · AI-written, may contain errors

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