Pipelining

Definition

A technique that improves CPU efficiency by overlapping the execution of multiple instructions. While one instruction is being executed, the next is being decoded, and another is being fetched.

Without Pipelining:

Cycle	1	2	3	4	5	6	7	8	9
Inst 1	F	D	E
Inst 2				F	D	E
Inst 3							F	D	E

With Pipelining:

Cycle	1	2	3	4	5
Inst 1	F	D	E
Inst 2		F	D	E
Inst 3			F	D	E

3 instructions complete in 5 cycles instead of 9!

Advantage	Disadvantage
Increases instruction throughput	Pipeline stalls if one stage takes longer
Better utilisation of CPU components	Branch instructions cause pipeline flushes
No increase in clock speed needed	Adds complexity to CPU design

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Question

Pipelining

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Answer

One instruction is fetched while the previous one is decoded and the one before that is executed, so stages of the cycle overlap. This increases throughput: more instructions are completed in a given time and no part of the processor sits idle.

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Browse all 1 flashcards on Pipelining

Pipelining
One instruction is fetched while the previous one is decoded and the one before that is executed, so stages of the cycle overlap. This increases throughput: more instructions are completed in a given time and no part of the processor sits idle.

Exam questions on Pipelining

2 OCR past paper questions (6 marks) with mark schemes

Question 1H446/01 · June 20243 marks
Three ways of improving the performance of a CPU are increasing the clock speed, adding more cores and using pipelining. Explain how pipelining improves the performance of a CPU.
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1 mark per bullet: • Allows the next instruction to be fetched whilst the previous one is being decoded/executed // allows the overlapping of different parts of the FDE • It increases throughput // increases the number of instructions processed in a set period of time • It prevents the CPU having to wait // prevents idle components Allow a diagram demonstrating pipelining for MP1. Allow 'it will take less time to do the same amount of instructions' for MP2.
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Question 2H446/01 · June 20213 marks
The CPU uses pipelining to improve efficiency. Explain what is meant by the term 'pipelining'.
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Concurrent processing of multiple instructions One instruction can be fetched while previous is being decoded And the one before is being executed In case of a branch pipeline is flushed Increases speed of execution (3 marks - max 3 from list)
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Exam questions on Pipelining

Past paper questions