ITPEC FE Subject A April 2024 Question 8
ITPEC FE Subject A April 2024 — Question 8 of 60
CPU Instruction Execution — 32-bit vs 16-bit comparison — calculate percentage improvement in total execution time.
Given:
- Instruction mix: 32-bit = 40%, 16-bit = 40%, 8-bit = 20%
- 32-bit CPU: executes all instructions in 1 cycle
- 16-bit CPU: executes 16-bit and 8-bit in 1 cycle, 32-bit in 2 cycles
Step 1 — Average cycles per instruction:
- 32-bit CPU: 0.4×1 + 0.4×1 + 0.2×1 = 1.0
- 16-bit CPU: 0.4×2 + 0.4×1 + 0.2×1 = 1.4
Step 2 — Percentage improvement:
- Formula: (old - new) / old × 100%
- (1.4 - 1.0) / 1.4 × 100% = 28.57%
Answer: (a) 28.57
Why not others:
- (b) 40 — divides by new CPU time (0.4/1.0) instead of old
- (c) 50 — no valid calculation leads here
- (d) 71.43 — ratio 1.0/1.4, not an improvement percentage
Key rule: Improvement % is always relative to the original (slower) value: (old − new) / old × 100%.
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