ITPEC FE Subject A April 2025 Question 10

Cache Hit Ratio — find common hit ratio when two systems have equal effective access time.

Formula: T = h × cache_time + (1 - h) × main_time

Given both systems share the same h and T:

• •System A: cache = 14 ns, main = 80 ns
• •System B: cache = 12 ns, main = 90 ns

Set equal:
14h + 80(1 - h) = 12h + 90(1 - h)
-66h + 80 = -78h + 90
12h = 10
h = 10/12 ≈ 0.83

Why not others:
- (a) 0.17 — complement of the correct answer (1 − 0.83)
- (b) 0.20 — does not satisfy the equation
- (d) 1.00 — would mean cache-only access; 14 ≠ 12

Key rule: When two systems share hit ratio and effective time, equate h × cache + (1−h) × main for both and solve for h.

AI-generated — may contain errors