A computer designer needs to design the pipeline of a new processor. She has a b

Question

A computer designer needs to design the pipeline of a new processor. She has a benchmark program with 10^6 instructions. Each instruction takes 10 ns to complete on a single-cycle processor. How long does it take to execute this program on a nonpipelined processor? the current state-of-the-art processor has about 20 pipeline stages. Assuming that it is perfectly pipelined, how much speedup will it achieve compared to the nonpipelined processor? Assume the clock speed stays the same as part (a). Provide a detailed answer as to how you arrived at the speedup number.

Explanation / Answer

At first you need to find a basic difference between a pipelined and non-pipelined processor. Here is a following example. Consider yourself as a Manager of a 10 floor construction site. You have 10 labors working under you. The labors get paid on daily basis and so your task is to complete the site at the earliest since you need not pay them for a longer time. Now let’s consider a following scenario. The labors has to move bricks from the ground floor to the top 10th floor. Each person can carry only 10 bricks at a time and it takes 10 minutes to reach the top floor and another 10 minutes to come down to collect the next set of bricks (assume 1 minute for each floor). So it takes 20 minutes to move 10x10=100 bricks by 10 labors and for one hour it comes only 300 bricks. Now you have a better idea to move the bricks in a faster way. You make one labor to stay in each floor. The labor who stays in the ground floor passes 10 bricks to the labor at the 1st floor (1min) and returns to the ground floor (1 min). At the same time the labor who was at the first floor moves the bricks to the second floor and returns back to 1st floor. Now in another one minute the labor of the ground floor again takes new set of bricks and passes to the person who returned to back to 1st floor. After the 1st 10 minutes, it takes only one minute to move the 10 bricks and this way you can move about 500 bricks in the first hour and 600 bricks from the next hour. That’s the logic behind pipelined processors. You process more set of instructions at the same machine cycle in similar fashion.

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