Procedures%202a%20MIPS%20code%20examples

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Procedures 2aMIPS code examples

• Making Use of a Stack

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Outline

• Assumptions for example
• Implementation sprod/prod/add2
• Example factorial

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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

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• Strategy for example
• First examine code for simple procedures
• Prod
• Add2
• Then examine code for more complex procedure
• sprod

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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

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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

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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

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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

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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 n1, in which case
  n!1)
• Function to implement factorial
• fact(n)
• if n1 then return 1
• else return nfact(n-1)

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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.

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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 n1 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 84
• jr 31 return

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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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Next Time

• How to handle arguments to procedures
• How to write programs for the MIPS simulator SPIM
• Answering your questions about the Coursework.