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MIPS Instruction Set

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

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  1. MIPS Instruction Set Conditional Expressions and Branching

  2. Outline • Motivation for conditional expressions and loops • How to add a sequence of numbers • Conditional expressions (branching) • Jumping • Switching

  3. 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!

  4. 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

  5. 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

  6. Conditional Expressions • bne $a,$b,LABEL • branch if not equal • if $a != $b then jump to LABEL • 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

  7. 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...

  8. 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; }

  9. 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

  10. 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

  11. 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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