Which of the following is the list that contains A through D sorted starting with the shortest effective access time of the main memory? | Cache memory | Cache memory | Cache memory | Main memory | Does the system have cache memory? (yes/no) | Access time (nanoseconds) | Hit rate (%) | Access time (nanoseconds) A | No | - | - | 15 B | No | - | - | 30 C | Yes | 20 | 60 | 70 D | Yes | 10 | 90 | 80

IPA FE Subject A sample Question 12

A question IPA Japan published in 基本情報技術者試験科目A サンプル問題問12, translated into English. ITPEC is based on the same exams.

Same question on ITPEC: Apr 2022 Q13

Which of the following is the list that contains A through D sorted starting with the shortest effective access time of the main memory?

	Cache memory			Main memory
	Does the system have cache memory? (yes/no)	Access time (nanoseconds)	Hit rate (%)	Access time (nanoseconds)
A	No	-	-	15
B	No	-	-	30
C	Yes	20	60	70
D	Yes	10	90	80

a)A, B, C, Db)A, D, B, Cc)C, D, A, Bd)D, C, A, B

Explanation

A, D, B, C — compute each system's effective access time; a cache helps only if its hit rate and speed outweigh a slow main memory.

With a cache, effective time = hit rate × cache time + miss rate × main memory time:

•A (no cache): 15 ns
•B (no cache): 30 ns
•C: 0.6 × 20 + 0.4 × 70 = 12 + 28 = 40 ns
•D: 0.9 × 10 + 0.1 × 80 = 9 + 8 = 17 ns

Shortest first: A 15, D 17, B 30, C 40.

Answer (b)

Why not others:

•(a) — ranks by main memory access time alone (15, 30, 70, 80), ignoring the caches of C and D
•(c) — puts the cached systems first, and C ahead of D, but C is the slowest at 40 ns
•(d) — assumes any cache beats no cache; D at 17 ns is slower than A at 15 ns, and C at 40 ns is slower than both

Key rule: A low hit rate in front of a slow main memory can make a cached system slower than one with fast memory and no cache; always compute.

Computer Component · AI-written, may contain errors

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This question comes from the past questions IPA Japan publishes, translated into English by ITPEC Practice. ITPEC Practice is an independent study tool and is not affiliated with IPA or ITPEC. See the original paper (PDF) or Report an issue.