08 Pipeline Design

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Pipelined CPUs

(Second Edition: Sections 6.1-6.3Fourth Edition: Sections 4.5-4.6)from Dr. Andrea Di Blas’ notes



CMPE 110 – Spring 2011 – J. Ferguson

Outline• Pipeline Principles

• Pipelined datapath

• Pipelined control

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Single Cycle Restaurant

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Everyone gets one minute to be served,whether they need it or not.



CMPE 110 – Spring 2011 – J. Ferguson 8 - 4

Multicycle Restaurant

Everyone spends 15 seconds at each station thatshe orders from.




Pipelined Restaurant

Everyone spends 15 seconds at each station thatshe orders from, but each station can serve adifferent customer.




Pipelined Datapath• Just like with multicycle implementation there are

stages,

• but each stage executes concurrently.• A new instruction begins execution every clock

cycle.

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Pipeline concept

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• Break up instruction into tasks

• Balance the amount of work (time) betweenstages

• Allow each segment to complete and startnext instruction




Properties of Pipelines• Latency: the time it takes for a single instruction

to execute. Pipelining makes latency slightly worse.

• Throughput: number of instructions executed perunit time. Pipelining improves throughput.

• Five stages in classical pipeline: IF, ID, EX, MEM,WB. Just like our multicycle pipeline.

• Clock is constrained by slowest stage of pipeline.

• Pipelining isn’t free: complexities in design andadditional resources (more later).

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Multicycle! Pipelined

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multicycle




A MIPS pipeline

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The “pink” registers hold data between stages




Pipeline Performance• If all instructions took N Cycles, and

• each of N pipeline stages did 1/Nth of the work,

and• the clock speed didn’t change,

• the pipeline implementation’s “throughput” would beN times faster than the multicycle implementation.

•

Because an instruction would be finished every clockcycle.

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Pipeline performance… but…

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Instr.Class

Instr.Fetch

Reg.Read

ALUOp.

DataAccess

Reg.Write

TotalTime

LoadWord

200ps 100ps 200ps 200ps 100ps 800ps

StoreWord

200ps 100ps 200ps 200ps 700ps

R-Format

200ps 100ps 200ps 100ps 600ps

Branch 200ps 100ps 200ps 500ps

If each instruction type was 25% of the executed

instructions, then average execution time would be650 ps. What would the performance increase be?

650/200 = 3.25




Multicycle vs. Pipeline

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MIPS ISA and Pipelining• All instructions are the same length

• Few instruction formats (and similar as possible)

• Memory operands only in load and store and simpleaddressing mode

• Operands are aligned in memory

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Pipelining instructions example

• Concept: relatively simple

lw $10, 20($1)

sub $11, $2, $3

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Clock Cycle 1

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Clock Cycle 2

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Clock Cycle 3

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Clock Cycle 4

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Clock Cycle 5

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Clock Cycle 6

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Pipelined Control

• Generated as before, then pipelined too

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08 Pipeline Design

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