lw 1 402 add 2 3 3 add 1 1 2 sw 1 202 a Find all data depenc

lw $1, 40($2)

add $2, $3, $3

add $1, $1, $2

sw $1, 20($2)

a) Find all data depencies in this instruction sequence.

b) Find all hazards in this instruction sequence for a five-stage pipeline with and then without forwarding.

c) To reduce clock cycle time, we are considering a split of the MEM stage into two stages. Repeat (b) for this six-stage pipeline.

Solution

A)

Instruction sequence                        RAW                                   WAR                                      WAW

L1: lw $1, 40($2)                            ($1) L1 to L3                         ($2) L1 to L2                           ($1)L1 to L3

L2: add $2, $3, $3                         ($2) L2 to L3, L4

L3: add $1, $1, $2                          ($1) L3 to L4

L4: sw $1, 20($2)

B)

Instruction sequence                                    without forwarding                        

L1: lw $1, 40($2)                                               ($1) L1 to L3

L2: add $2, $3, $3                                            ($2) L2 to L3, L4

L3: add $1, $1, $2                                           ($1) L3 to L4  

L4: sw $1, 20($2)            

Without forwarding

Instructions                                                        Pipeline stage                

lw $1, 40($6)                        IF   ID   EX   MEM   WB

add $2, $3, $3                            IF   ID   EX   MEM   WB

nop                                                     IF   ID   EX   MEM   WB

nop                                                            IF   ID   EX   MEM   WB

add $1, $1, $2                                                IF   ID   EX   MEM   WB

nop                                                                          IF   ID   EX   MEM   WB

nop                                                                                IF   ID   EX   MEM   WB

sw $1, 20($2                                                                       IF   ID   EX   MEM   WB

With forwarding

Instructions                                                              Pipeline stage

lw $1, 40($6)                                                IF   ID   EX   MEM   WB

add $2, $3, $3                                                     IF   ID   EX   MEM   WB

add $1, $1, $2                                                           IF   ID   EX   MEM   WB

sw $1, 20($2)                                                                     IF   ID   EX   MEM   WB

C)

Instruction sequence                                           without forwarding                             with forwarding

L1: lw $1, 40($2)                                                    ($1) L1 to L3                                              ($1) L1 to L3

L2: add $2, $3, $3                                                  ($2) L2 to L3, L4    

L3: add $1, $1, $2                                                 ($1) L3 to L4           

L4: sw $1, 20($2)