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Pipelined Processor II (cont’d) CPSC 321
Andreas Klappenecker
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Pipelined Datapath Pipeline separation registers, width varies
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Control Lines Instruction fetch:
control signal to read instruction memory and to write PC are always asserted - nothing special here Instruction decode/register file read: same thing happens every clock cycle, so no optional control lines to set Execution/address calculation RegDst selects the result register, ALUOp selects the ALU operation ALUSrc selects Read data 2 or sign-extd. immediate
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Pipelined Datapath with Control Signals
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Control Lines Memory access Write back Branch set by branch equal
MemRead set by load instructions MemWrite set by store instructions Write back MemtoReg send ALU result or memory value RegWrite selects register
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Pipelined Datapath with Control Signals
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Pipeline Control Pass control signals along just like the data
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Datapath with Control
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Data Hazards Assume that the compiler has to guarantee that no hazards occur Where do we insert the “nops” ? sub $2, $1, $3 and $12, $2, $5 or $13, $6, $2 add $14, $2, $2 sw $15, 100($2)
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Dependencies Data hazard: a dependency that “goes backward in time”
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Forwarding
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Forwarding
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Obstructions to Forwarding
Load word can still cause a hazard: an instruction trying to read a register following a load instruction writing to the same register. Need a hazard detection unit to “stall” pipeline
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Hazard Detection Unit Stall by letting an instruction that won’t write anything go forward
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Branch Hazards When we decide to branch, other instructions are in the pipeline! We are predicting “branch not taken” need to add hardware for flushing instructions if we are wrong
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Flushing Instructions
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Dynamic Scheduling The hardware performs the “scheduling”
hardware tries to find instructions to execute out of order execution is possible speculative execution and dynamic branch prediction All modern processors are very complicated DEC Alpha 21264: 9 stage pipeline, 6 instruction issue PowerPC and Pentium: branch history table Compiler technology important This class has given you the background you need to learn more - read Chapter 6! More material will be posted!
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