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Pipelined Control (Simplified)

Pipelined Control (Simplified). Control Refresher. Here are the control signals that were previously identified:. Control Signals Grouped by Stages. pass to EX stage. pass to MEM stage. pass to WB stage. Control Signal Forwarding. Control signals derived from instruction opcode, as before:.

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Pipelined Control (Simplified)

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  1. Pipelined Control (Simplified)

  2. Control Refresher • Here are the control signals that were previously identified:

  3. Control Signals Grouped by Stages pass to EX stage pass to MEM stage pass to WB stage

  4. Control Signal Forwarding • Control signals derived from instruction opcode, as before:

  5. Pipelined Control Overview

  6. 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, but how do we detect when to forward?

  7. Dependencies & Forwarding

  8. Fwd fromEX/MEMpipeline reg Fwd fromMEM/WBpipeline reg 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 when 1a. EX/MEM.RegisterRd = ID/EX.RegisterRs 1b. EX/MEM.RegisterRd = ID/EX.RegisterRt 2a. MEM/WB.RegisterRd = ID/EX.RegisterRs 2b. MEM/WB.RegisterRd = ID/EX.RegisterRt

  9. 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

  10. ALU Operand Selection Value from register fetch in ID stage Value from WB stage Value from ALU execution

  11. Forwarding Paths Select source for $rs Select source for $rt register #s from current instruction $rd # from MEM stage Must select correct operand sources $rd # from EX stage

  12. Conditions for EX Hazard • If ( EX/MEM.RegWrite and • EX/MEM.RegisterRd ≠ 0 and • EX/MEM.RegisterRd = ID/EX.RegisterRs )then • ForwardA = 10 • If ( EX/MEM.RegWrite and • EX/MEM.RegisterRd ≠ 0 and EX/MEM.RegisterRd = ID/EX.RegisterRt ) • thenForwardB = 10 QTP: could BOTH occur with respect to the same instruction?

  13. Conditions for MEM Hazard • If ( MEM/WB.RegWrite and • MEM/WB.RegisterRd ≠ 0 and MEM/WB.RegisterRd = ID/EX.RegisterRs ) • thenForwardA = 01 • If ( MEM/WB.RegWrite and • MEM/WB.RegisterRd ≠ 0 and MEM/WB.RegisterRd = ID/EX.RegisterRt ) • thenForwardB = 01 QTP: could BOTH an EX hazard and a MEM hazard occur with respect to the same instruction?

  14. Double Data Hazard • Consider the sequence: add $1,$1,$2add $1,$1,$3add $1,$1,$4 • Both hazards occur… which value do we want to forward? • Revise MEM hazard condition: • Only forward if EX hazard condition is not true

  15. Revised Conditions for 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 ) • thenForwardA = 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 ) • thenForwardB = 01

  16. Datapath with Forwarding

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