Question

We implemented a new 5-stage pipeline with the following features: the delay by data and control hazards are as follows: 1 cycle stall for the load by immediate use, 2 cycle stalls for branch taken.

Assume we now run 10,000 instructions on the pipeline, among them:

(1) 35% are lw instructions. 10% of lw instructions are followed by instructions that use lw result immediately in ALU input;

(2)15% are branch instructions with 40% possibility of branch taken;

(3) the remaining 50% are sw and R-type instructions that don’t cause any stalls.

How many cycles we need to execute the 10,000 instructions on the pipeline? You may ignore the pipeline overhead such as start-up cost.

Answer 1

1 cycle stall for the load by immediate use,
2 cycle stalls for branch taken.

stalls due to load by immediate use
=> 10000*(0.35 * 0.1* 1) = 350 cycles

stalls due to branch taken
=> 10000*(0.15*0.4*2) = 1200 cycles

cycle taken to execute 10000 instruction in 5-stage pipeline without any stalls.
First instruction needs 5 cycles for execution
Next 9999 instruction needs only 1 clocks
Total cycles = 5 + 9999 = 10004 cycles

Total cycles we need to execute the 10,000 instructions on the pipeline = 10004 + 1200 + 350 = 11554 cycles