CSSE232 : Computer Architecture I

Name: Box: Date:

HW15

(10 points) Draw the pipeline diagram for the following code running on a pipelined MIPS processor. Identify all of the data dependencies (draw forwards and stalls).
```
add $3, $4, $2

sub $5, $3, $1

lw  $6, 200($3)

add $7, $3, $6
```
Which of the above dependencies are data hazards that will be resolved via forwarding? Which dependencies are data hazards that will cause a stall?
(5 points) Modify the following code to make use of the delayed branch slot on a pipelined MIPS processor.
```
loop: lw   $2, 100($3)
sub  $4, $4, $5
beq  $3, $4, loop
```

(10 points) Consider the following code to be run on a pipelined MIPS processor:
```
lw   $4, 4($5)
lw   $3, 0($5)
add  $7, $7, $3
addi $5, $5, 4
sw   $6, 0($5)
add  $8, $8, $4
beq  $7, $8, loop
nop #this delay slot is currently empty
```
a. Reorder the instructions to maximize performance. Performance may already by maximized.

b. Reorder the instructions to minimize performance. Performance may already be minimized.
(10 points) We wish to add a variant of the lw (load word) instruction, which increments the index register after loading the word from memory. This instruction (l_inc) corresponds to the following two instructions:
```
lw   $rt, L($rs)
addi $rs, $rs, 4
```
Describe the changes you would need to make to the datapath. You may need to make major changes to the pipeline! Do your changes effect other instructions?