Pipelined Control (Simplified)

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Presentation transcript:

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:

Pipelined Control Overview

Data Hazards in ALU Instructions Consider this sequence: sub $2, $1,$3 and $12,$2,$5 or $13,$6,$2 add $14,$2,$2 sw $15,100($2) We can resolve hazards with forwarding, but how do we detect when to forward?

Dependencies & Forwarding

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 Fwd from EX/MEM pipeline reg Fwd from MEM/WB pipeline reg

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

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

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

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 ) then ForwardB = 10 QTP: could BOTH occur with respect to the same instruction?

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

Double Data Hazard Consider the sequence: add $1,$1,$2 add $1,$1,$3 add $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

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 ) then ForwardA = 01 EX/MEM.RegisterRd ≠ 0 and EX/MEM.RegisterRd = ID/EX.RegisterRt ) and MEM/WB.RegisterRd = ID/EX.RegisterRt ) then ForwardB = 01

Datapath with Forwarding