Pipelined Datapath and Control

Lecture for CPSC 5155Edward Bosworth, Ph.D.

Computer Science DepartmentColumbus State University

Chapter 4 — The Processor — 2

MIPS Pipelined Datapath§4.6 P

ipelined Datapath and C

ontrol

WB

MEM

Right-to-left flow leads to hazards

Chapter 4 — The Processor — 3

Pipeline registers Need registers between stages

To hold information produced in previous cycle

The Pipeline Registers• IF/ID This provides an execution context for the ID (Instruction

Decode and Register Fetch) stage of execution.• ID/EX This provides an execution context for the EX (Execute) phase

of instruction execution. In particular, the discrete control signals generated by the control unit as a result of instruction decoding are stored here.

• EX/MEM This provides an execution context for the MEM (Memory Access or R-Type Instruction Completion) phase of instruction execution. In addition , this register stores copies of the control signals required to complete both the MEM and WB phase of execution for this instruction.

• MEM/WB This provides an execution context for the WB (Write Back) phase of instruction execution.

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Pipeline Operation Cycle-by-cycle flow of instructions through

the pipelined datapath “Single-clock-cycle” pipeline diagram

Shows pipeline usage in a single cycle Highlight resources used

c.f. “multi-clock-cycle” diagram Graph of operation over time

We’ll look at “single-clock-cycle” diagrams for load & store

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IF for Load, Store, …

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ID for Load, Store, …

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EX for Load

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MEM for Load

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WB for Load

Wrongregisternumber

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Corrected Datapath for Load

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EX for Store

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MEM for Store

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WB for Store

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Multi-Cycle Pipeline Diagram Form showing resource usage

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Multi-Cycle Pipeline Diagram Traditional form

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Single-Cycle Pipeline Diagram State of pipeline in a given cycle

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Pipelined Control (Simplified)

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Pipelined Control Control signals derived from instruction

As in single-cycle implementation

The Control Signals by Phase• Instruction Fetch There are no control signals specific to this stage. • Instruction Decode There are no instruction–specific control

signals in this step.• Execute• There are three control signals associated with this step.• RegDst This selects which field, IR[20:16] or IR[15:11] will be used

as the register destination number for the Write Register in WB. The five bit value selected is written into EX/MEM and copied to MEM/WB.

• ALUOp This is the two–bit selector of the ALU operation.• ALUSrc This discrete control signal selects the B input to the ALU.

Control Signals by Phase• Memory Access• There are three control signals associated with this step.• Branch This indicates that a branch instruction is in this stage.• MemRead The ALU output is used as a memory address that is read.

This is set by the LW instruction.• MemWriteThe ALU output is used as a memory address, to which

the contents of the specified register are written.This is set by the SW instruction.

• Write Back• There are two control signals associated with this step.• MemToReg This selects either the ALU output or memory output to

be written back to the register file• RegWrite This causes the selected value to be written to the

specified register.

Chapter 4 — The Processor — 22

Pipelined Control

Size of the Pipeline Registers

  IF/ID ID/EX EX/MEM MEM/WBProgram Counter 32 32 32 32Machine Language Instruction 32      Register 1 Read Data   32    Register 2 Read Data   32 32  Sign Extended Address Offset   32    Discrete Control Signals   9 5 2ALU Function Code   6    Shift Amount   5    ALU Result     32 32ALU Discrete Output: Zero     1  Data read from memory       32Destination Register Number   10 5 5TOTAL BITS 64 158 107 103

Chapter 4 — The Processor — 24

Data Hazards in ALU Instructions Consider this sequence:

sub $2, $1,$3and $12,$2,$5or $13,$6,$2add $14,$2,$2sw $15,100($2)

We can resolve hazards with forwarding How do we detect when to forward?

§4.7 Data H

azards: Forwarding vs. S

talling

Chapter 4 — The Processor — 25

Dependencies & Forwarding

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Detecting the Need to Forward Pass register numbers along pipeline

e.g., ID/EX.RegisterRs = register number for Rs sitting in ID/EX pipeline register

ALU operand register numbers in EX stage are given by ID/EX.RegisterRs, ID/EX.RegisterRt

Data hazards when1a. EX/MEM.RegisterRd = ID/EX.RegisterRs1b. EX/MEM.RegisterRd = ID/EX.RegisterRt2a. MEM/WB.RegisterRd = ID/EX.RegisterRs2b. MEM/WB.RegisterRd = ID/EX.RegisterRt

Fwd fromEX/MEMpipeline reg

Fwd fromMEM/WBpipeline reg

Chapter 4 — The Processor — 27

Detecting the Need to Forward But only if forwarding instruction will write

to a register! EX/MEM.RegWrite, MEM/WB.RegWrite

And only if Rd for that instruction is not $zero EX/MEM.RegisterRd ≠ 0,

MEM/WB.RegisterRd ≠ 0

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Forwarding Paths

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Forwarding Conditions EX hazard

if (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRs)) ForwardA = 10 #Two bit control signal to MUX

if (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRt)) ForwardB = 10

MEM hazard if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0)

and (MEM/WB.RegisterRd = ID/EX.RegisterRs)) ForwardA = 01

if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0) and (MEM/WB.RegisterRd = ID/EX.RegisterRt)) ForwardB = 01

Chapter 4 — The Processor — 30

Double Data Hazard Consider the sequence:

add $1,$1,$2add $1,$1,$3add $1,$1,$4

Both hazards occur Want to use the most recent

Revise MEM hazard condition Only fwd if EX hazard condition isn’t true

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Revised Forwarding Condition MEM hazard

if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0) and not (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRs)) and (MEM/WB.RegisterRd = ID/EX.RegisterRs)) ForwardA = 01

if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0) and not (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRt)) and (MEM/WB.RegisterRd = ID/EX.RegisterRt)) ForwardB = 01

Chapter 4 — The Processor — 32

Datapath with Forwarding

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Load-Use Data Hazard

Need to stall for one cycle

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Load-Use Hazard Detection Check when using instruction is decoded

in ID stage ALU operand register numbers in ID stage

are given by IF/ID.RegisterRs, IF/ID.RegisterRt

Load-use hazard when ID/EX.MemRead and

((ID/EX.RegisterRt = IF/ID.RegisterRs) or (ID/EX.RegisterRt = IF/ID.RegisterRt))

If detected, stall and insert bubble

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How to Stall the Pipeline Force control values in ID/EX register

to 0 EX, MEM and WB do nop (no-operation)

Prevent update of PC and IF/ID register Using instruction is decoded again Following instruction is fetched again 1-cycle stall allows MEM to read data for lw

Can subsequently forward to EX stage

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Stall/Bubble in the Pipeline

Stall inserted here

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Stalls and Performance

Stalls reduce performance But are required to get correct results

Compiler can arrange code to avoid hazards and stalls Requires knowledge of the pipeline structure

The BIG Picture