Forwarding and Hazards MemberRole William ElliottTeam Leader Jessica Tyler ShulerWiki Specialist Tyler KimseyLead Engineer Cameron CarrollEngineer Danielle TurnerPowerPoint Leader* Chris SmithPresentation Leader* Saeed AlshahraniPowerPoint Team* Matt WoodPowerPoint Team* *Roles change with Phase

Overview What are Hazards?  Hazards are problems that occur within the instruction pipeline of the central processing unit (CPU) microarchitecture that can potentially result in an incorrect computation.  There are several types of hazards  Structural Hazards  Data Hazards  Control Hazards

Block Diagram

Structural Hazards  When the current instruction cannot execute in the proper clock cycle because the hardware does not support the combination of instructions that are set to execute  These are not a problem in the MIPS architecture, as there is sufficient hardware to implement everything we need  The only solution, would they occur, would be to add more hardware

Data Hazards  When the current instruction cannot execute in the proper clock cycle because data that is needed to execute instruction is not available.  Load-use data hazard: a specific form of data hazard in which the data being loaded by a load instruction has not yet become available when it is needed by another instruction

Read-after-Read (RAR)  This is a false dependency  Reading won’t alter the contents of the register file, and the registers are read at different times MIPSPseudo-code add $t0, $t1, $t2 sub $t4, $t1, $t5 c = a + b d = a + e

 This is not a hazard to be concerned with  The output of the second operation in no way relies on the output of the first  Output is stored again in the next pipeline stage, overwriting the value from the first instruction MIPSPseudo-code add $t1, $t2, $t3 add $t1, $t4, $t5 c = a + b c = d + e Write-after-Write (WAW)

 This is also not a hazard  The “sub” instruction reads in the value of $t3 five stages before it’s written into the register file MIPSPseudo-code sub $t1, $t2, $t3 add $t3, $t4, $t5 c = a + b b = d + e Write-after-Read (WAR)

 This refers to a situation where a previous instruction has not finished writing a value to memory before it needs to be used  This can occur because of the pipelined nature of our processor. Data is written back to the register file two stages after the subsequent instruction needs this information.  This is known as a true-data dependency MIPSPseudo-code add $t1, $t2, $t3 add $t4, $t1, $t5 c = a + b d = c + e Read-after-Write (RAW)

Data Hazards True-Data Dependencies  Stalling – inserts a bubble, or a nop (no operation) into the pipeline  Forwarding – Copies data from inside the pipeline before it is written back to the register.

Forwarding Example add $t1, $t2, $t3 add $t4, $t1, $t5

Forwarding Logic

Selector MUXs Mux ControlSourceExplanation ForwardA = 00ID/EXThe first ALU operand comes from the register file ForwardA = 10EX/MEMThe first ALU operand is forwarded from the prior ALU result ForwardA = 01MEM/WBThe first ALU operand is forwarded from data memory or an earlier ALU result ForwardB = 00ID/EXThe second ALU operand comes from the register file ForwardB = 10EX/MEMThe second ALU operand is forwarded from the prior ALU result ForwardB = 01MEM/WBThe second ALU operand is forwarded from data memory or an earlier ALU result

Data Hazards Load-Use Hazard  There is a case where forwarding will not solve our problems, as the load will not resolve until the last clock cycle in the pipeline, while the subsequent instruction needs the information in the previous cycle.  Therefore, we must stall the pipeline in order to let the load instruction to “catch up”

Load-Use Example

Hazard Detection Unit

Bubble Control Flow Chart

Handling the Bubble

Inserting the Bubble

Control Hazards  Branch (beq)  Two paths that can be taken, uses a statement that checks which branch will be taken.  (Think of an “if” statement in Java, or C.)  Jump (j)  Always “jumps” to a different location in the code.

Control Hazards Flush Logic  Because there are two options in a branch, we assume the branch will not be taken. If the branch is taken we must remove the instructions that have entered the pipeline.  We do this with a “flush” where we turn all values in the pipeline into nop.

Flush Logic

Questions?