Question $4$

A pipelined processor is executing the following instruction sequence:

$1.$ LOAD R$2,$ O$($R$1)$

$2.$ LOAD R$4,1($R$3)$

$3.$ SUB R$5,$ R$4,$ R$6$

$4.$ ADD R$8,$ R$5,$ R$7$

The processor has two configurations:

$-$ With forwarding: Data is forwarded as soon as it's available

$-$ Without forwarding: Instructions dependent on results on results from previous instructions must stall until the result is written back

Assume:

$-$ Each instruction completes in $5$ cycles without stalls.

$-$ Data hazards only occur for read$-$after$-$write $($RAW$)$ dependencies.

a$)$ With forwarding enabled, fill in the blanks below:

$-$ Instruction $1$ completes in cycle

$-$ Instruction $2$ completes in cycle

$-$ Instruction $3$ completes in cycle

$-$ Instruction $4$ completes in cycle

b$)$ Without forwarding, calculate the total number of stall cycles needed to handle data hazards

$-$ Instruction $1$ requires stall cycles

$-$ Instruction $2$ requires stall cycles

$-$ Instruction $3$ requires stall cycles

$-$ Instruction $4$ requires stall cycles

$-$ Total stall cycles $=$

c$)$ Total cycles: calculate the total number of cycles for the entire sequence with and without forwarding

$-$ With forwarding : cycles

$-$ Without forwarding:

cycles