MIPS Pipeline Datapath Modifications q What do we

MIPS Pipeline Datapath Modifications q What do we need to add/modify in our MIPS datapath? State registers between each pipeline stage to isolate them l IF: IFetch ID: Dec EX: Execute MEM: Mem. Access WB: Write. Back Add Read Addr 2 Data 1 File Write Addr Write Data 16 Sign Extend Read Data 2 ALU Exec/Mem Register Read Dec/Exec Read Address Read Addr 1 IFetch/Dec PC Instruction Memory Add Data Memory Address Write Data Read Data Mem/WB Shift left 2 4 32 System Clock CSE 431 L 06 Basic MIPS Pipelining. 1 Irwin, PSU, 2005

Pipelining the MIPS ISA q What makes it easy l all instructions are the same length (32 bits) - can fetch in the 1 st stage and decode in the 2 nd stage l few instruction formats (three) with symmetry across formats - can begin reading register file in 2 nd stage l memory operations can occur only in loads and stores - can use the execute stage to calculate memory addresses q What makes it hard l l l structural hazards: what if we had only one memory? control hazards: what about branches? data hazards: what if an instruction’s input operands depend on the output of a previous instruction? CSE 431 L 06 Basic MIPS Pipelining. 2 Irwin, PSU, 2005

Graphically Representing MIPS Pipeline q Reg ALU IM DM Reg Can help with answering questions like: l l l How many cycles does it take to execute this code? What is the ALU doing during cycle 4? Is there a hazard, why does it occur, and how can it be fixed? CSE 431 L 06 Basic MIPS Pipelining. 3 Irwin, PSU, 2005

Why Pipeline? For Performance! Time (clock cycles) Inst 3 IM Reg DM IM Reg ALU Inst 2 DM ALU Inst 1 Reg ALU IM ALU O r d e r Inst 0 ALU I n s t r. Once the pipeline is full, one instruction is completed every cycle, so CPI = 1 Inst 4 Reg Reg DM Reg Time to fill the pipeline CSE 431 L 06 Basic MIPS Pipelining. 4 Irwin, PSU, 2005

Can Pipelining Get Us Into Trouble? q Yes: Pipeline Hazards l structural hazards: attempt to use the same resource by two different instructions at the same time l data hazards: attempt to use data before it is ready - An instruction’s source operand(s) are produced by a prior instruction still in the pipeline l control hazards: attempt to make a decision about program control flow before the condition has been evaluated and the new PC target address calculated - branch instructions q Can always resolve hazards by waiting l l pipeline control must detect the hazard and take action to resolve hazards CSE 431 L 06 Basic MIPS Pipelining. 5 Irwin, PSU, 2005

A Single Memory Would Be a Structural Hazard Time (clock cycles) q Mem Reg Reg Mem Inst 3 Inst 4 Reg ALU Inst 2 Reg ALU Mem Reading data from memory Mem ALU Inst 1 Reg ALU O r d e r Mem ALU I n s t r. lw Mem Reading instruction from memory Mem Reg Fix with separate instr and data memories (I$ and D$) CSE 431 L 06 Basic MIPS Pipelining. 6 Irwin, PSU, 2005

How About Register File Access? Time (clock cycles) Inst 2 DM IM Reg ALU Inst 1 Reg ALU IM ALU O r d e r add $1, ALU I n s t r. add $2, $1, clock edge that controls register writing CSE 431 L 06 Basic MIPS Pipelining. 8 Reg Fix register file access hazard by doing reads in the second half of the cycle and writes in the first half Reg DM Reg clock edge that controls loading of pipeline state registers Irwin, PSU, 2005

Register Usage Can Cause Data Hazards q Dependencies backward in time cause hazards Reg DM IM Reg ALU $8, $1, $9 IM ALU or DM ALU and $6, $1, $7 Reg ALU sub $4, $1, $5 IM ALU add $1, xor $4, $1, $5 q Reg Reg DM Reg Read before write data hazard CSE 431 L 06 Basic MIPS Pipelining. 10 Irwin, PSU, 2005

Loads Can Cause Data Hazards q Reg DM IM Reg ALU sub $4, $1, $5 IM ALU $1, 4($2) ALU O r d e r lw ALU I n s t r. Dependencies backward in time cause hazards and $6, $1, $7 or $8, $1, $9 xor $4, $1, $5 q Reg Reg DM Reg Load-use data hazard CSE 431 L 06 Basic MIPS Pipelining. 11 Irwin, PSU, 2005

One Way to “Fix” a Data Hazard Reg DM Reg IM Reg DM IM Reg ALU IM ALU O r d e r add $1, ALU I n s t r. Can fix data hazard by waiting – stall – but impacts CPI stall sub $4, $1, $5 and $6, $1, $7 CSE 431 L 06 Basic MIPS Pipelining. 12 Reg DM Reg Irwin, PSU, 2005

Another Way to “Fix” a Data Hazard or $8, $1, $9 xor $4, $1, $5 CSE 431 L 06 Basic MIPS Pipelining. 14 IM Reg DM IM Reg ALU and $6, $1, $7 DM ALU sub $4, $1, $5 Reg ALU IM ALU O r d e r add $1, ALU I n s t r. Fix data hazards by forwarding results as soon as they are available to where they are needed Reg Reg DM Reg Irwin, PSU, 2005

Forwarding with Load-use Data Hazards or $8, $1, $9 xor $4, $1, $5 q IM Reg DM IM Reg ALU and $6, $1, $7 DM ALU sub $4, $1, $5 Reg ALU $1, 4($2) IM ALU O r d e r lw ALU I n s t r. Reg Reg DM Reg Will still need one stall cycle even with forwarding CSE 431 L 06 Basic MIPS Pipelining. 16 Irwin, PSU, 2005

Branch Instructions Cause Control Hazards q Inst 4 CSE 431 L 06 Basic MIPS Pipelining. 17 IM Reg DM IM Reg ALU Inst 3 Reg ALU lw IM ALU O r d e r beq ALU I n s t r. Dependencies backward in time cause hazards DM Reg Reg DM Reg Irwin, PSU, 2005

One Way to “Fix” a Control Hazard beq O r d e r stall IM Reg ALU I n s t r. DM Fix branch hazard by waiting – stall – but affects CPI Reg stall CSE 431 L 06 Basic MIPS Pipelining. 18 Reg DM IM Reg ALU Inst 3 IM ALU lw Reg DM Irwin, PSU, 2005

Corrected Datapath to Save Reg. Write Addr q Need to preserve the destination register address in the pipeline state registers IF/ID ID/EX EX/MEM Add Shift left 2 4 PC Instruction Memory Read Address Read Addr 1 Read Addr 2 Data 1 File Write Addr 16 Sign Extend Read Data 2 MEM/WB Data Memory Register Read Write Data CSE 431 L 06 Basic MIPS Pipelining. 20 Add ALU Address Read Data Write Data 32 Irwin, PSU, 2005

MIPS Pipeline Control Path Modifications q All control signals can be determined during Decode l and held in the state registers between pipeline stages ID/EX EX/MEM IF/ID Control Add Shift left 2 4 PC Instruction Memory Read Address Read Addr 1 Read Addr 2 Data 1 File Write Data 16 CSE 431 L 06 Basic MIPS Pipelining. 21 Data Memory Register Read Write Addr Sign Extend Read Data 2 MEM/WB Add ALU Address Read Data Write Data 32 Irwin, PSU, 2005