pipeline Computer Architecture Lecture 12: Designing a Pipeline Processor

pipeline.2 Overview of a Multiple Cycle Implementation °The root of the single cycle processor’s problems: The cycle time has to be long enough for the slowest instruction °Solution: Break the instruction into smaller steps Execute each step (instead of the entire instruction) in one cycle -Cycle time: time it takes to execute the longest step -Keep all the steps to have similar length This is the essence of the multiple cycle processor °The advantages of the multiple cycle processor: Cycle time is much shorter Different instructions take different number of cycles to complete -Load takes five cycles -Jump only takes three cycles Allows a functional unit to be used more than once per instruction

pipeline.3 Multiple Cycle Processor °MCP: A functional unit to be used more than once per instruction Ideal Memory WrAdr Din RAdr 32 Dout MemWr 32 ALU 32 ALUOp ALU Control Instruction Reg 32 IRWr 32 Reg File Ra Rw busW Rb busA 32busB RegWr Rs Rt Mux 0 1 Rt Rd PCWr ALUSelA Mux 01 RegDst Mux PC MemtoReg Extend ExtOp Mux Imm 32 << 2 ALUSelB Mux 1 0 Target 32 Zero PCWrCondPCSrcBrWr 32 IorD

pipeline.4 Outline of Today’s Lecture °Recap and Introduction °Introduction to the Concept of Pipelined Processor °Pipelined Datapath and Pipelined Control °How to Avoid Race Condition in a Pipeline Design? °Pipeline Example: Instructions Interaction °Summary

pipeline.5 Pipelining is Natural! °Laundry Example °Sammy, Marc, Griffy, Albert each have one load of clothes to wash, dry, and fold °Washer takes 30 minutes °Dryer takes 30 minutes °“Folder” takes 30 minutes °“Stasher” takes 30 minutes to put clothes into drawers ABCD

pipeline.6 Sequential Laundry °Sequential laundry takes 8 hours for 4 loads °If they learned pipelining, how long would laundry take? 30 TaskOrderTaskOrder B C D A Time 30 6 PM AM

pipeline.7 Pipelined Laundry: Start work ASAP °Pipelined laundry takes 3.5 hours for 4 loads! TaskOrderTaskOrder 12 2 AM 6 PM Time B C D A 30

pipeline.8 Pipelining Lessons °Pipelining doesn’t help latency of single task, it helps throughput of entire workload °Multiple tasks operating simultaneously using different resources °Potential speedup = Number pipe stages °Pipeline rate limited by slowest pipeline stage °Unbalanced lengths of pipe stages reduces speedup °Time to “fill” pipeline and time to “drain” it reduces speedup °Stall for Dependences 6 PM 789 Time B C D A 30 TaskOrderTaskOrder

pipeline.9 Why Pipeline? °Suppose we execute 100 instructions °Single Cycle Machine 45 ns/cycle x 1 CPI x 100 inst = 4500 ns °Multicycle Machine 10 ns/cycle x 4.6 CPI (due to inst mix) x 100 inst = 4600 ns °Ideal pipelined machine 10 ns/cycle x (1 CPI x 100 inst + 4 cycle drain) = 1040 ns

pipeline.10 Timing Diagram of a Load Instruction Clk PC Rs, Rt, Rd, Op, Func Clk-to-Q ALUctr Instruction Memory Access Time Old ValueNew Value RegWrOld ValueNew Value Delay through Control Logic busA Register File Access Time Old ValueNew Value busB ALU Delay Old ValueNew Value Old ValueNew Value Old Value ExtOpOld ValueNew Value ALUSrcOld ValueNew Value AddressOld ValueNew Value busWOld ValueNew Delay through Extender & Mux Data Memory Access Time Instruction FetchInstr Decode / Reg. Fetch AddressReg WrData Memory Register File Write Time

pipeline.11 The Five Stages of Load °Ifetch: Instruction Fetch Fetch the instruction from the Instruction Memory °Reg/Dec: Registers Fetch and Instruction Decode °Exec: Calculate the memory address °Mem: Read the data from the Data Memory °Wr: Write the data back to the register file Cycle 1Cycle 2Cycle 3Cycle 4Cycle 5 IfetchReg/DecExecMemWrLoad

pipeline.12 Pipelining the Load Instruction °The five independent functional units in the pipeline datapath are: Instruction Memory for the Ifetch stage Register File’s Read ports (bus A and busB) for the Reg/Dec stage ALU for the Exec stage Data Memory for the Mem stage Register File’s Write port (bus W) for the Wr stage °One instruction enters the pipeline every cycle One instruction comes out of the pipeline (complete) every cycle The “Effective” Cycles per Instruction (CPI) is 1 Clock Cycle 1Cycle 2Cycle 3Cycle 4Cycle 5Cycle 6Cycle 7 IfetchReg/DecExecMemWr1st lw IfetchReg/DecExecMemWr2nd lw IfetchReg/DecExecMemWr3rd lw

pipeline.13 Conventional Pipelined Execution Representation IFetchDcdExecMemWB IFetchDcdExecMemWB IFetchDcdExecMemWB IFetchDcdExecMemWB IFetchDcdExecMemWB IFetchDcdExecMemWB Program Flow Time

pipeline.14 Single Cycle, Multiple Cycle, vs. Pipeline Clk Cycle 1 Multiple Cycle Implementation: IfetchRegExecMemWr Cycle 2Cycle 3Cycle 4Cycle 5Cycle 6Cycle 7Cycle 8Cycle 9Cycle 10 LoadIfetchRegExecMemWr IfetchRegExecMem LoadStore Pipeline Implementation: IfetchRegExecMemWrStore Clk Single Cycle Implementation: LoadStoreWaste Ifetch R-type IfetchRegExecMemWrR-type Cycle 1Cycle 2

pipeline.15 Why Pipeline? Because the resources are there! I n s t r. O r d e r Time (clock cycles) Inst 0 Inst 1 Inst 2 Inst 4 Inst 3 ALU Im Reg DmReg ALU Im Reg DmReg ALU Im Reg DmReg ALU Im Reg DmReg ALU Im Reg DmReg

pipeline.16 Can pipelining get us into trouble? °Yes: Pipeline Hazards structural hazards: attempt to use the same resource two different ways at the same time -E.g., combined washer/dryer would be a structural hazard or folder busy doing something else (watching TV) data hazards: attempt to use item before it is ready -E.g., one sock of pair in dryer and one in washer; can’t fold until get sock from washer through dryer -instruction depends on result of prior instruction still in the pipeline control hazards: attempt to make a decision before condition is evaulated -E.g., washing football uniforms and need to get proper detergent level; need to see after dryer before next load in -branch instructions °Can always resolve hazards by waiting pipeline control must detect the hazard take action (or delay action) to resolve hazards

pipeline.17 Mem Single Memory is a Structural Hazard I n s t r. O r d e r Time (clock cycles) Load Instr 1 Instr 2 Instr 3 Instr 4 ALU Mem Reg MemReg ALU Mem Reg MemReg ALU Mem Reg MemReg ALU Reg MemReg ALU Mem Reg MemReg Detection is easy in this case! (right half highlight means read, left half write)

pipeline.18 Structural Hazards limit performance °Example: if 1.3 memory accesses per instruction and only one memory access per cycle then average CPI  1.3 otherwise resource is more than 100% utilized More on Hazards later

pipeline.19 Pipelining the R-type and Load Instruction °We have a problem: Two instructions try to write to the register file at the same time! Clock Cycle 1Cycle 2Cycle 3Cycle 4Cycle 5Cycle 6Cycle 7Cycle 8Cycle 9 IfetchReg/DecExecWrR-type IfetchReg/DecExecWrR-type IfetchReg/DecExecMemWrLoad IfetchReg/DecExecWrR-type IfetchReg/DecExecWrR-type Ops! We have a problem!

pipeline.20 The Four Stages of R-type °Ifetch: Instruction Fetch Fetch the instruction from the Instruction Memory °Reg/Dec: Registers Fetch and Instruction Decode °Exec: ALU operates on the two register operands °Wr: Write the ALU output back to the register file Cycle 1Cycle 2Cycle 3Cycle 4 IfetchReg/DecExecWrR-type

pipeline.21 Important Observation °Each functional unit can only be used once per instruction °Each functional unit must be used at the same stage for all instructions: Load uses Register File’s Write Port during its 5th stage R-type uses Register File’s Write Port during its 4th stage IfetchReg/DecExecMemWrLoad IfetchReg/DecExecWrR-type 1234

pipeline.22 Solution 1: Insert “Bubble” into the Pipeline °Insert a “bubble” into the pipeline to prevent 2 writes at the same cycle The control logic can be complex °No instruction is completed during Cycle 5: The “Effective” CPI for load is >1 Clock Cycle 1Cycle 2Cycle 3Cycle 4Cycle 5Cycle 6Cycle 7Cycle 8Cycle 9 IfetchReg/DecExecWrR-type IfetchReg/DecExec IfetchReg/DecExecMemWrLoad IfetchReg/DecExecWr R-type IfetchReg/DecExecWr R-type Pipeline Bubble IfetchReg/DecExecWr

pipeline.23 Solution 2: Delay R-type’s Write by One Cycle °Delay R-type’s register write by one cycle: Now R-type instructions also use Reg File’s write port at Stage 5 Mem stage is a NOOP stage: nothing is being done Clock Cycle 1Cycle 2Cycle 3Cycle 4Cycle 5Cycle 6Cycle 7Cycle 8Cycle 9 IfetchReg/DecExecWrR-type IfetchReg/DecExecWrR-type IfetchReg/DecExecMemWrLoad IfetchReg/DecExecWrR-type IfetchReg/DecExecWrR-type IfetchReg/DecExecWrR-typeMem 12345

pipeline.24 The Four Stages of Store °Ifetch: Instruction Fetch Fetch the instruction from the Instruction Memory °Reg/Dec: Registers Fetch and Instruction Decode °Exec: Calculate the memory address °Mem: Write the data into the Data Memory Cycle 1Cycle 2Cycle 3Cycle 4 IfetchReg/DecExecMemStoreWr

pipeline.25 The Four Stages of Beq °Ifetch: Instruction Fetch Fetch the instruction from the Instruction Memory °Reg/Dec: Registers Fetch and Instruction Decode °Exec: ALU compares the two register operands Adder calculates the branch target address °Mem: If the registers we compared in the Exec stage are the same, Write the branch target address into the PC Cycle 1Cycle 2Cycle 3Cycle 4 IfetchReg/DecExecMemBeqWr

pipeline.26 A Pipelined Datapath IF/ID Register ID/Ex Register Ex/Mem Register Mem/Wr Register PC Data Mem WA Di RADo IUnit A I RFile Di Ra Rb Rw MemWr RegWr ExtOp Exec Unit busA busB Imm16 ALUOp ALUSrc Mux 1 0 MemtoReg 1 0 RegDst Rt Rd Imm16 PC+4 Rs Rt PC+4 Zero Branch 1 0 Clk IfetchReg/DecExecMemWr

pipeline.27 The Instruction Fetch Stage IF/ID: lw $1, 100 ($2) ID/Ex Register Ex/Mem Register Mem/Wr Register PC = 14 Data Mem WA Di RADo IUnit A I RFile Di Ra Rb Rw MemWr RegWr ExtOp Exec Unit busA busB Imm16 ALUOp ALUSrc Mux 1 0 MemtoReg 1 0 RegDst Rt Rd Imm16 PC+4 Rs Rt PC+4 Zero Branch 1 0 Clk IfetchReg/DecExecMem You are here! °Location 10: lw $1, 0x100($2) $1 <- Mem[($2) + 0x100]

pipeline.28 A Detail View of the Instruction Unit °Location 10: lw $1, 0x100($2) IF/ID: lw $1, 100 ($2) PC = Adder Instruction Memory “4” Instruction Address Clk Ifetch You are here! Reg/Dec

pipeline.29 The Decode / Register Fetch Stage IF/ID: ID/Ex: Reg. 2 & 0x100 Ex/Mem Register Mem/Wr Register PC Data Mem WA Di RADo IUnit A I RFile Di Ra Rb Rw MemWr RegWr ExtOp Exec Unit busA busB Imm16 ALUOp ALUSrc Mux 1 0 MemtoReg 1 0 RegDst Rt Rd Imm16 PC+4 Rs Rt PC+4 Zero Branch 1 0 Clk IfetchReg/DecExecMem You are here! °Location 10: lw $1, 0x100($2) $1 <- Mem[($2) + 0x100]

pipeline.30 Load’s Address Calculation Stage IF/ID: ID/Ex Register Ex/Mem: Load’s Address Mem/Wr Register PC Data Mem WA Di RADo IUnit A I RFile Di Ra Rb Rw MemWr RegWr ExtOp=1 Exec Unit busA busB Imm16 ALUOp=Add ALUSrc=1 Mux 1 0 MemtoReg 1 0 RegDst=0 Rt Rd Imm16 PC+4 Rs Rt PC+4 Zero Branch 1 0 Clk IfetchReg/DecExecMem You are here! °Location 10: lw $1, 0x100($2) $1 <- Mem[($2) + 0x100]

pipeline.31 A Detail View of the Execution Unit ID/Ex Register Ex/Mem: Load’s Memory Address ALU Control ALUctr 32 busA 32 busB Extender Mux 16 imm16 ALUSrc=1 ExtOp=1 3 ALU Zero ALUout 32 Adder 3 ALUOp=Add << 2 32 PC+4 Target 32 Clk Exec You are here! Mem

pipeline.32 Load’s Memory Access Stage IF/ID: ID/Ex Register Ex/Mem Register Mem/Wr: Load’s Data PC Data Mem WA Di RADo IUnit A I RFile Di Ra Rb Rw MemWr=0 RegWr ExtOp Exec Unit busA busB Imm16 ALUOp ALUSrc Mux 1 0 MemtoReg 1 0 RegDst Rt Rd Imm16 PC+4 Rs Rt PC+4 Zero Branch=0 1 0 Clk IfetchReg/DecExecMem You are here! °Location 10: lw $1, 0x100($2) $1 <- Mem[($2) + 0x100]

pipeline.33 Load’s Write Back Stage IF/ID: ID/Ex Register Ex/Mem Register Mem/Wr Register PC Data Mem WA Di RADo IUnit A I RFile Di Ra Rb Rw MemWr RegWr=1 ExtOp Exec Unit busA busB Imm16 ALUOp ALUSrc Mux 1 0 MemtoReg=1 1 0 RegDst Rt Rd Imm16 PC+4 Rs Rt PC+4 Zero Branch 1 0 Clk IfetchReg/DecExecMem You are somewhere out there! °Location 10: lw $1, 0x100($2) $1 <- Mem[($2) + 0x100] Wr

pipeline.34 How About Control Signals? IF/ID: ID/Ex Register Ex/Mem: Load’s Address Mem/Wr Register PC Data Mem WA Di RADo IUnit A I RFile Di Ra Rb Rw MemWr RegWr ExtOp=1 Exec Unit busA busB Imm16 ALUOp=Add ALUSrc=1 Mux 1 0 MemtoReg 1 0 RegDst=0 Rt Rd Imm16 PC+4 Rs Rt PC+4 Zero Branch 1 0 IfetchReg/DecExecMem °Key Observation: Control Signals at Stage N = Func (Instr. at Stage N) N = Exec, Mem, or Wr °Example: Controls Signals at Exec Stage = Func(Load’s Exec) Wr

pipeline.35 Pipeline Control °The Main Control generates the control signals during Reg/Dec Control signals for Exec (ExtOp, ALUSrc,...) are used 1 cycle later Control signals for Mem (MemWr Branch) are used 2 cycles later Control signals for Wr (MemtoReg MemWr) are used 3 cycles later IF/ID Register ID/Ex Register Ex/Mem Register Mem/Wr Register Reg/DecExecMem ExtOp ALUOp RegDst ALUSrc Branch MemWr MemtoReg RegWr Main Control ExtOp ALUOp RegDst ALUSrc MemtoReg RegWr MemtoReg RegWr MemtoReg RegWr Branch MemWr Branch MemWr Wr

pipeline.36 Beginning of the Wr’s Stage: A Real World Problem °At the beginning of the Wr stage, we have a problem if: RegAdr’s (Rd or Rt) Clk-to-Q > RegWr’s Clk-to-Q °Similarly, at the beginning of the Mem stage, we have a problem if: WrAdr’s Clk-to-Q > MemWr’s Clk-to-Q °We have a race condition between Address and Write Enable! Ex/Mem Mem/Wr RegAdr RegWrMemWr Data WrAdr Data Reg File Data Memory Clk RegAdr RegWr RegWr’s Clk-to-Q RegAdr’s Clk-to-Q Clk WrAdr MemWr MemWr’s Clk-to-Q WrAdr’s Clk-to-Q

pipeline.37 The Pipeline Problem °Multiple Cycle design prevents race condition between Addr and WrEn: Make sure Address is stable by the end of Cycle N Asserts WrEn during Cycle N + 1 °This approach can NOT be used in the pipeline design because: Must be able to write the register file every cycle Must be able write the data memory every cycle Clock IfetchReg/DecExecMemWrStore IfetchReg/DecExecMemWrStore IfetchReg/DecExecMemWrR-type IfetchReg/DecExecMemWrR-type

pipeline.38 Synchronize Register File & Synchronize Memory °Solution: And the Write Enable signal with the Clock This is the ONLY place where gating the clock is used MUST consult circuit expert to ensure no timing violation: -Example: Clock High Time > Write Access Delay WrEn I_Addr I_Data Reg File or Memory Clk I_Addr I_WrEn Address Data I_WrEn C_WrEn Clk Address Data WrEn Reg File or Memory Synchronize Memory and Register File Address, Data, and WrEn must be stable at least 1 set-up time before the Clk edge Write occurs at the cycle following the clock edge that captures the signals

pipeline.39 A More Extensive Pipelining Example Clock Cycle 1Cycle 2Cycle 3Cycle 4Cycle 5Cycle 6Cycle 7Cycle 8 IfetchReg/DecExecMemWr0: Load IfetchReg/DecExecMemWr4: R-type IfetchReg/DecExecMemWr8: Store IfetchReg/DecExecMemWr12: Beq (target is 1000) End of Cycle 4 End of Cycle 5 End of Cycle 6 End of Cycle 7 °End of Cycle 4: Load’s Mem, R-type’s Exec, Store’s Reg, Beq’s Ifetch °End of Cycle 5: Load’s Wr, R-type’s Mem, Store’s Exec, Beq’s Reg °End of Cycle 6: R-type’s Wr, Store’s Mem, Beq’s Exec °End of Cycle 7: Store’s Wr, Beq’s Mem

pipeline.40 Pipelining Example: End of Cycle 4 °0: Load’s Mem 4: R-type’s Exec 8: Store’s Reg 12: Beq’s Ifetch IF/ID: Beq Instruction ID/Ex: Store’s busA & B Ex/Mem: R-type’s Result Mem/Wr: Load’s Dout PC = 16 Data Mem WA Di RADo IUnit A I RFile Di Ra Rb Rw RegWr=0 ExtOp=x Exec Unit busA busB Imm16 ALUOp=R-type ALUSrc=0 Mux 1 0 MemtoReg=x 1 0 RegDst=1 Rt Rd Imm16 PC+4 Rs Rt PC+4 Zero Branch= : Beq’s Ifet 8: Store’s Reg4: R-type’s Exec0: Load’s Mem Clk MemWr=0 Clk

pipeline.41 Pipelining Example: End of Cycle 5 °0: Lw’s Wr 4: R’s Mem 8: Store’s Exec 12: Beq’s Reg 16: R’s Ifetch IF/ID: 16 ID/Ex: Beq’s busA & B Ex/Mem: Store’s Address Mem/Wr: R-type’s Result PC = 20 Data Mem WA Di RADo IUnit A I RFile Di Ra Rb Rw RegWr=1 ExtOp=1 Exec Unit busA busB Imm16 ALUOp=Add ALUSrc=1 Mux 1 0 MemtoReg=1 1 0 RegDst=x Rt Rd Imm16 PC+4 Rs Rt PC+4 Zero Branch= : R’s Ifet 12: Beq’s Reg8: Store’s Exec4: R-type’s Mem 0: Load’s Wr Clk MemWr=0 Clk

pipeline.42 Pipelining Example: End of Cycle 6 °4: R’s Wr 8: Store’s Mem 12: Beq’s Exec 16: R’s Reg 20: R’s Ifet IF/ID: 20 ID/Ex:R-type’s busA & B Ex/Mem: Beq’s Results Mem/Wr: Nothing for St PC = 24 Data Mem WA Di RADo IUnit A I RFile Di Ra Rb Rw RegWr=1 ExtOp=1 Exec Unit busA busB Imm16 ALUOp=Sub ALUSrc=0 Mux 1 0 MemtoReg=0 1 0 RegDst=x Rt Rd Imm16 PC+4 Rs Rt PC+4 Zero Branch= : R-type’s Ifet 16: R-type’s Reg12: Beq’s Exec8: Store’s Mem 4: R-type’s Wr Clk MemWr=1 Clk

pipeline.43 Pipelining Example: End of Cycle 7 °8: Store’s Wr 12: Beq’s Mem 16: R’s Exec 20: R’s Reg 24: R’s Ifet IF/ID: 24 ID/Ex:R-type’s busA & B Ex/Mem: Rtype’s Results Mem/Wr:Nothing for Beq PC = 1000 Data Mem WA Di RADo IUnit A I RFile Di Ra Rb Rw RegWr=0 ExtOp=x Exec Unit busA busB Imm16 ALUOp=R-type ALUSrc=0 Mux 1 0 MemtoReg=x 1 0 RegDst=1 Rt Rd Imm16 PC+4 Rs Rt PC+4 Zero Branch= : R-type’s Ifet 20: R-type’s Reg16: R-type’s Exec12: Beq’s Mem 8: Store’s Wr Clk MemWr=0 Clk

pipeline.44 The Delay Branch Phenomenon °Although Beq is fetched during Cycle 4: Target address is NOT written into the PC until the end of Cycle 7 Branch’s target is NOT fetched until Cycle 8 3-instruction delay before the branch take effect °This is referred to as Branch Hazard: Clever design techniques can reduce the delay to ONE instruction Cycle 4Cycle 5Cycle 6Cycle 7Cycle 8Cycle 9Cycle 10Cycle 11 IfetchReg/DecExecMemWr IfetchReg/DecExecMemWr 16: R-type IfetchReg/DecExecMemWr IfetchReg/DecExecMemWr24: R-type 12: Beq (target is 1000) 20: R-type Clk IfetchReg/DecExecMemWr1000: Target of Br

pipeline.45 The Delay Load Phenomenon °Although Load is fetched during Cycle 1: The data is NOT written into the Reg File until the end of Cycle 5 We cannot read this value from the Reg File until Cycle 6 3-instruction delay before the load take effect °This is referred to as Data Hazard: Clever design techniques can reduce the delay to ONE instruction Clock Cycle 1Cycle 2Cycle 3Cycle 4Cycle 5Cycle 6Cycle 7Cycle 8 IfetchReg/DecExecMemWrI0: Load IfetchReg/DecExecMemWrPlus 1 IfetchReg/DecExecMemWrPlus 2 IfetchReg/DecExecMemWrPlus 3 IfetchReg/DecExecMemWrPlus 4

pipeline.46 Summary °Disadvantages of the Single Cycle Processor Long cycle time Cycle time is too long for all instructions except the Load °Multiple Clock Cycle Processor: Divide the instructions into smaller steps Execute each step (instead of the entire instruction) in one cycle °Pipeline Processor: Natural enhancement of the multiple clock cycle processor Each functional unit can only be used once per instruction If a instruction is going to use a functional unit: -it must use it at the same stage as all other instructions Pipeline Control: -Each stage’s control signal depends ONLY on the instruction that is currently in that stage