Analyze the program and try to evaluate the value of the res if the execution of the instruction that causes WAW stall was proceeded without waiting for the execution of the previous instruction to complete.

Calculated “wrong” res value:

Explain why the execution of the instruction indicated in the answer to the previous question had to be suspended.

W-A-W hazard

This type of hazard may occur when two adjacent instructions (i) and (j) save the result of their execution to the same resource (register, memory cell) and the time of the execution of the earlier instruction (i) is longer than the time of the execution instruction (j). It may cause an erroneous result of executing this sequence of instructions to be written into the resource. The typical way to remove such a dependency is to delay the completion of the execution instruction (j) until the execution of the instruction (i) is completed. 

;WinMIPS64 WAW hazard example

    s.d f3,res(r0)

Very important note! : Before you execute this program, be sure that the data forwarding mechanism is disabled.  If  data forwarding is enabled the MIPS hardware cannot detect, signal and prevent WAW hazard. 

Execute the example program. Analyze the clock cycle diagram and find the instruction that causes  WAW stall.

Mark the instruction that caused the WAW hazard.

What is the value stored in res variable at the end of the program execution?

Data Forwarding

A data forwarding (or operand forwarding) mechanism is one of the method to reduce the impact of raw hazards on the pipeline instruction execution. It relies on the existence of additional data paths that allow the results of the instruction execution to be used earlier than after the execution is fully completed. To see how it works, we will use again the example program used for R-A-W hazard investigation.

Before you execute the program in WinMIPS64 simulator, enable the data forwarding mechanism (in menu “Configure” set the option “Enable forwarding”)

Analysez le diagramme du cycle d'horloge et identifiez les instructions provoquant   un blocage RAW (RAW). 

Exécutez le programme d'exemple RAW dans le simulateur WinMIPS.

Important ! Avant d'exécuter le programme dans le simulateur WinMIPS64, désactivez le mécanisme de transfert de données (dans le menu « Configurer », désactivez l'option « Activer le transfert »).

Combien de cycles d'horloge ont été nécessaires pour l'exécution du programme pipeliné de l'exemple RAW ?

Quelle est la valeur CPI pour l'exécution séquentielle d'un programme ( CPIseq ) ?

Quelle est la valeur CPI ( CPIpip ) pour l'exécution en pipeline du programme d'exemple RAW ?

(Ce chiffre peut être calculé manuellement ou obtenu à partir de la fenêtre de statistiques du simulateur WinMIPS64)

Combien d'instructions ont été exécutées ?