ITPEC FE Morning October 2022 Question 12

50 MIPS — MIPS is the number of instructions per second in millions, the reciprocal of the average instruction time.

• •Instructions per second: 1 ÷ (20 × 10⁻⁹) = 5 × 10⁷
• •In millions: 5 × 10⁷ ÷ 10⁶ = 50

Equivalently, 20 ns = 0.02 µs, and 1 ÷ 0.02 = 50 instructions per microsecond, which is the same as millions per second.

Answer (d)

Why not others:

• •(c) 20 — takes the instruction time in nanoseconds as the MIPS value instead of taking its reciprocal
• •(a) 5 — a factor-of-ten slip; it would need an average of 200 ns per instruction
• •(b) 10 — would need an average of 100 ns per instruction

Key rule: MIPS = 1 ÷ (average instruction time in µs); a faster instruction gives a larger MIPS value.

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