General note: Stalls due to data hazzards prevent the ID stage from
completing.  If one instruction hasn't completed the ID stage, the
next can't complete its IF stage.

2(a) No forwarding, hardware assumes (correctly) branch not taken. 

sll  $2, $2, 2  IF ID EX MEM WB
sub  $5, $5, $5    IF ID EX  MEM WB
lw   $8, 1000($2)     IF -   -   ID EX MEM WB
add  $5, $5, $8                  IF -  -   -  ID EX MEM WB
addi $2, $2, -4                               IF ID EX  MEM WB
bne  $2, $0, loop                                IF -   -   - ID EX MEM WB
sw   $5, 500($0)                                              IF ID EX  MEM WB


2(b) No forwarding, hardware assumes branch not taken but branch
is taken.

bne  $2, $0, loop IF ID EX MEM WB   (see note 1)
sw   $5, 500($0)     IF ID EX       (see note 2)
lw   $8, 1000($2)              IF ID EX MEM WB
add  $5, $5, $8                   IF -  -   -  ID EX MEM WB
addi $2, $2, -4                                IF ID EX  MEM WB
bne  $2, $0, loop                                 IF -   -   -  ID EX MEM WB

   Note 1: A more accurate diagram has 3 stall cycles in the first
bne, waiting for the data from register $2 from the previous addi 
instruction.
   Note 2: The sw and probably several other instructions begin
execution before the branch is determined to be taken. But they
are killed before they write anything into registers or memory.


2(c) Switching the positions of the add and addi instructions would
eliminate stall cycles. The loop body of the new code would appear as
shown below and contain 2 stalls per iteration (in addition to the
cycles lost on the branch).

bne  $2, $0, loop IF ID EX MEM WB
sw   $5, 500($0)     IF ID EX
lw   $8, 1000($2)              IF  ID EX MEM WB
addi $2, $2, -4                    IF ID EX  MEM WB
add  $5, $5, $8                       IF -   -   ID EX MEM WB
bne  $2, $0, loop                                   IF ID EX MEM WB


2(d) with forwarding

sll  $2, $2, 2  IF ID EX MEM WB
sub  $5, $5, $5    IF ID EX  MEM WB
lw   $8, 1000($2)     IF ID  EX  MEM WB
add  $5, $5, $8          IF  -   ID  EX MEM WB
addi $2, $2, -4                  IF  ID EX  MEM WB
bne  $2, $0, loop                    IF ID  EX  MEM WB
sw   $5, 500($0)                        IF  ID  EX  MEM WB