Question

Consider a 5-stage instruction pipeline which can execute 3 types of instructions:

1. Load & Store type
2. ALU type
3. Other type

Each load & store type instructions take 8 cycles to execute in Pipeline. Each ALU type instruction take 6 cycles to execute to pipeline and each other type instruction take 5 cycles to execute in pipeline. There are 20 load & store type, 30 ALU type and 10 other type instructions to execute in the pipeline minimum. How many cycles are needed to execute all instructions if all instructions can be overlapped?

Answer 1

option (c)

Assuming only one stage takes extra cycles for each of the instructions.
Total number of instructions(n) = 20 +30 + 10 = 60
Number of stages (k) = 5
without extra cycles. number of cycles needed to execute 60 instructions
= (k + n - 1) = 5 + 60 - 1 = 64

without extra cycles each type of instructions take k(5) cycles to execute.

$Extracycles\Rightarrow (8-5)=3\Rightarrow 3\times 20=60$

$\Rightarrow (6-5)=1\Rightarrow 1\times 30=30$

= 60 + 30 = 90

Total cycles = 64 + 90 = 154