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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...

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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]

addi $11,$11,1 # i = i+1

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
- if $a != $b then jump to LABEL

- branch if not equal
- beq $a,$b,LABEL
- branch if equal
- if $a == $b jumpt to LABEL

- branch if equal
- slt $a,$b,$c
- set if less than
- if ($b < $c) then $a = 1 else $a = 0

- set if less than

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

addi $7,$0,100 # $7 =100

j CONTINUE # jump to continue

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

L0: addi $20,$0,20 # $20 gets 20

j BREAK

L1: addi $20,$0,10 # $20 gets 10

j BREAK

L2: addi $20,$0,11 # $20 gets 11

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
- j LABEL #jump to the instruction referenced by LABEL
- jr $a #jump to instruction referenced in register $a

Still potential for a mess!...

- Adding 100 numbers x[] is fine.
- Suppose later we want to add 1011 numbers using sequence y[]?
- Do we have to repeat the same set of instructions all over again, with small modifications?
- No way!

- Use procedures!!!

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