6. Insert an appropriate number of NOPs in the following code to eliminate as ma

Question

6. Insert an appropriate number of NOPs in the following code to eliminate as many hazards as possible. The MIPS pipeline for this question does not support forwarding: lw $s0, 4($t0) add $s1, $s0, $t0 addi $s2, $s1, 5 or $s3, $zero, $s1 sw $s2, 8($t0) 7. Reorder instructions or insert an appropriate number of NOPs in the following code to eliminate as many hazards as possible. The MIPS pipeline for this question does not support forwarding. Only insert if it is not possible to reorder instructions: NOPs sub $t5, $t4, $t3 lw $s0, 4($t0) add $s3, $t1, $t2 add $s1, $s0, $t0 lw $t3, 0($s1) subi $s2, $s1, 5 or $s3, $s2, $sl sw $s2, 80$t0) 8. Why do branch delay slots occur in the MIPS pipeline? Can the following code be modified to avoid the use of NOPs? If not, how many NOPs are needed? add $s0, $s1, $s2 sub $s3, $s2, $s1 j label

Explanation / Answer

6.

Consider instruction as I1, I2.... I5. Here we are having data dependency between I1 and I2 due to s0 and between I2 AND I3 due to s1

Space time diagram for given code (N=NOP)

Hence 6 nop is required.

7.

I1: sub $t5, $t4, $t3

I2:lw $s0,4($t0)

I3:add $s3, $t1,$t2

I4:add $s1, $s0, $t0

I5:lw $t3, 0($s1)

I6:subi $s2,$s1,5

I7:or $s3, $s2, $s1

I8:sw $s2, 8($t0)

New order of execution.

I2:lw $s0,4($t0)

I1: sub $t5, $t4, $t3

I3:add $s3, $t1,$t2

NOP

I4:add $s1, $s0, $t0

I6:subi $s2,$s1,5

NOP

NOP

NOP

NOP

I5:lw $t3, 0($s1)

I6:subi $s2,$s1,5

NOP

NOP

NOP

NOP

I7:or $s3, $s2, $s1

I8:sw $s2, 8($t0)

8

MIPS uses 5 stage pipeline.These stages are IF,ID,EX,MA,WB. Stage 2 is Instruction decode and operand fetch.

When we are executing branch instruction branch target is known at 2nd stage(ID: At this stage only we update PC with target address) until then next instruction in the pipeline will be fetched which make out of order execution .To prevent this we insert delay slots after the branch instruction.

We can prevent delay slot in this program by rearranging the code.

add $s0, $s1, $s2

j label

sub $s3 $s2,$s1

1 2 3 4 5 6 7 8 9 10 12 13 14 15 WB I1 N N N I2 N N N I3 I4 MA I1 N N N I2 N N N I3 I4 I5 EX I1 N N N I2 N N N I3 I4 I5 ID I1 N N N I2 N N N I3 I4 I5 IF I1 N N N I2 N N N I3 I4 I5

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