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# MIPS Instruction Set PowerPoint PPT Presentation

MIPS Instruction Set. Conditional Expressions and Branching. Outline. Motivation for conditional expressions and loops How to add a sequence of numbers Conditional expressions (branching) Jumping Switching. Last time ...adding 3 numbers. What a mess this is...

MIPS Instruction Set

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## MIPS Instruction Set

Conditional Expressions and Branching

### Outline

• Motivation for conditional expressions and loops

• How to add a sequence of numbers

• Conditional expressions (branching)

• Jumping

• Switching

### Last time ...adding 3 numbers...

• What a mess this is...

• lw \$10,A(\$0) # \$10 = a0 (\$0 always contains 0)

• li \$4, 4# \$4 = 4

• lw \$3,A(\$4)# \$3 = a1

• add \$10,\$10,\$3# \$10 = a0 + a1

• li \$4, 8# \$4 = 8

• lw \$3,A(\$4)# \$3 = a2

• add \$10,\$10,\$3# \$10 = a0 + a1+ a2

• li \$4, 12# \$4 = 12

• sw \$10,A(\$4)# a3 gets \$10

• Suppose 100 numbers to be added?

• Doesn’t generalise!

### Adding a sequence of n numbers

• x[0], x[1], ..., x[n-1] sequence of n numbers.(n=100, say).

• Consider (abstract representation)

sum = 0 # initialise sum

n = 100 # initialise boundary condition

i = 0 # initialise counter

LOOP: # label

sum = sum + x[i] #overwrite sum

i = i+1 #increment counter

if (i != n) then go to LOOP

### Adding the Sequence in MIPS

move \$5,\$0# puts 0 into \$5 (sum)

addi \$4,\$0,100 # puts 100 into \$4 (n)

move \$11,\$0 # puts 0 into \$11 (i)

li \$12,4# puts 4 into \$12

LOOP:mult \$14,\$11,\$12# \$14 = i*4 (why?!)

lw \$6,Xstart(\$14)# retrieves x[i]

add \$5,\$5,\$6# sum = sum + x[i]

bne \$11,\$4, LOOP# if i < n go to LOOP

sw \$5,Sum(\$0)#result put in memory

### Conditional Expressions

• bne \$a,\$b,LABEL

• branch if not equal

• beq \$a,\$b,LABEL

• branch if equal

• if \$a == \$b jumpt to LABEL

• slt \$a,\$b,\$c

• set if less than

• if (\$b < \$c) then \$a = 1 else \$a = 0

### Example (introduces jump)

• Consider the following abstract code

if (a < b) then c = 100

else c = 500

• In MIPS (suppose a,b,c are \$5,\$6,\$7)

slt \$10,\$5,\$6# \$10 = 1 if \$5<\$6

beq \$10,\$0,ELSE# if \$10==0 goto else

ELSE : addi \$7,\$0,500# \$7 = 500

CONTINUE: whatever here...

### Choosing from several alternatives

• Abstract code

if k=0 then a=20

else if k=1 then a=10

else if k=2 then a=11;

• C/C++ representation (uses ‘switch’)

switch(k){

0 : a = 20; break;

1 : a = 10; break;

2 : a = 11;

}

### Using jump register (jr \$a)

• jumps to instruction referenced in \$a

• suppose Label is memory location containing addresses L0, L1, L2. (\$4=k*4)

lw \$10,Label(\$4) # \$10 = Label[k]

jr \$10# jump \$10

j BREAK

j BREAK

BREAK:continue whatever here

### Summary

• bne \$a,\$b,LABEL #branch if not equal

• beq \$a,\$b,LABEL #branch if equal

• slt \$a,\$b,\$c# select if less than