GATE CS 2018 Q50 — Instruction Pipelining

Since there are no data or control hazards, instructions flow through the pipeline sequentially. The stages before and after the Perform Operation ($PO$) stage are $IF$, $ID$, $OF$ (each taking $1$ cycle) and $WB$ (taking $1$ cycle).

Because the $PO$ stage takes at least $1$ clock cycle for every instruction, it acts as the bottleneck. Once the first instruction reaches the $PO$ stage, the $PO$ stage will run continuously without any idle cycles until all $100$ instructions complete their $PO$ stage.

We can calculate the total clock cycles as follows:

• Cycles before $PO$ for the first instruction: $3$ cycles ($IF$, $ID$, and $OF$)
• Total cycles spent in the $PO$ stage for all $100$ instructions:
  $$\text{Total } PO \text{ cycles} = (40 \times 3) + (35 \times 2) + (25 \times 1) = 120 + 70 + 25 = 215 \text{ cycles}$$
• Cycles after $PO$ for the last instruction: $1$ cycle ($WB$)

$$\text{Total Clock Cycles} = 3 + 215 + 1 = 219 \text{ cycles}$$