ITPEC FE Morning April 2019 Question 9

20 ns — MIPS is millions of instructions per second, so the time per instruction is its reciprocal.

• •50 MIPS = 50 × 10⁶ instructions per second
• •Time per instruction: 1 ÷ (50 × 10⁶) s = 2 × 10⁻⁸ s = 20 ns

A shortcut: 1 MIPS means 1 µs per instruction, so 50 MIPS means 1 ÷ 50 µs = 0.02 µs = 20 ns.

Answer (a)

Why not others:

• •(b) 50 ns — takes the MIPS figure itself as the time; 50 ns per instruction would be 20 MIPS
• •(c) 2 µs — 100 times too long; it corresponds to 0.5 MIPS
• •(d) 5 µs — 250 times too long; it corresponds to 0.2 MIPS

Key rule: Average instruction time in µs = 1 ÷ MIPS; convert with 1 µs = 1,000 ns.

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