Procedures 2a MIPS code examples Making Use of a Stack

Outline Assumptions for example Implementation: sprod/prod/add2 Example: factorial

Assumptions for Example We suppose the following: –x1,y1,x2,y2 stored in memory starting at location X –Procedures: »add2 - adds together $4, $5 puts result in $6 »prod - multiplies $4, $5 puts result in $6 »sprod – (x1 * y1) + (x2 * y2), puts result in $8 –$29 is the stack pointer (address of ‘top’ of stack) –The stack in MIPS grows downwards »so that subtracting 4 from $29 makes room for a push »and adding 4 to $29 adjusts for a pop

Strategy for example: –First examine code for simple procedures: »Prod »Add2 –Then examine code for more complex procedure: »sprod

Prod prod :addi $29,$29,-4#step 1 of push sw $31,0($29)#step 2 of push $31 mult $6,$4,$5 lw $31,0($29)#step 1 of pop: # restore return address addi $29,$29,4 #step 2 of pop: # adjust top of stack jr $31 #return

add2 add2 :addi $29,$29,-4#step 1 of push sw $31,0($29)#step 2 of push $31 add $6,$4,$5 lw $31,0($29)#step 1 of pop: # restore return address addi $29,$29,4 #adjust top of stack jr $31 #return

Implementation of sprod sprod :lw $4,X($0)#x1 into $4 li $5, 4 lw $5,X($5)#y1 into $5 addi $29,$29,-4#step 1 for push sw $31,0($29)#step 2 for push $31 #return address now saved, #to be popped at end sprod jal prod#call prod, result in $6 add $20,$0,$6#copy result into $20 li $4, 8 lw $4,X($4)#x2 into $4 li $5, 12 lw $5,X($5)#y2 into $5 jal prod#call prod, result in $6 # continued …

sprod continued add $4,$0,$20#first result into $4 add $5,$0,$6#second result into $5 jal add2#puts result in $6 lw $31,0($29)#step 1 of pop: # restores return address addi $29,$29,4#step 2 of pop: # adjust ‘top’ of stack jr $31#return to caller of sprod

Example Factorial n! = n  (n-1) ...  2  1 4! = 4  3  2  1 = 24 4! = 4  (3  2  1) = 4  3! n! = n  (n-1)! (unless n=1, in which case n!=1) Function to implement factorial: fact(n) { if n==1 then return 1 elsereturn n*fact(n-1); }

Factorial in MIPS (Assumptions) The argument (n) is in $4 $8 is already initialised to 1 –$8 will hold the result We will use the pretend instructions – push – pop – print »for sake of simplicity.

Nearly a MIPS representation MAIN :li $4,4#the argument n = 4 li $11,1# set $11 to have the value 1 li $8,1# initialize $8 to have the value 1 jal fact#call “fact” procedure print $8 #no such instruction! exit #no such instruction - halt altogether fact:beq $4,$11,RETURN#if n==1 goto RETURN push($31) #save on stack push($4) #save argument addi $4,$4,-1#n becomes n-1 jal fact#recursive call to fact RETURN: $4 = pop() #no such instruction! $31 = pop() mult $8,$8,$4#$8 = $8*$4 jr $31#return

Summary jal –jump and link »saves return address in $31 jr $31 –jumps to return address in register $31 Stack pointer used to store nested return addresses ($29)

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