GATE Overflow Test Series | Mock GATE | Test 3 | Question: 54

Question

Consider a five-stage pipeline with ideal CPI. Assume that $25\%$ of the instructions executed are branch instructions out of which $70\%$ are taken branches. The pipeline architecture is such that there is no penalty for a non-taken branch whereas every taken branch incurs on average a penalty of $1$ cycle. What will be the effective pipeline speedup?

• A. $4.44$
• B. $4.59$
• C. $4.25$
• D. $4.66$

Answer 1

Speedup $=\dfrac{\text{Depth}}{\text{Ideal CPI}+\text{Overhead}} = \dfrac{5}{1+0.125} = 4.44$
    
$=\dfrac{\text{Depth}}{\text{Ideal CPI}+\text{Taken Branch instruction %}\times \text{Branch penalty}} = \dfrac{5}{1+0.175 \times 1} =\dfrac{5}{1.175}= 4.255.$

Comments

Depth means no. of stages, right ?

---

Why didn’t you take numerator as just the CPI of old system, as it is pipeline its CPI should be 1.