Subroutines

1
Subroutines

• A programming language must provide
• A way to jump to subroutines
• return to the instruction after the calling point

int greater(int a, int b) int temp if (a gt
b) temp a else temp b return
temp
void main() int a,b,c,e,f,c,max e e
1 c greater(e,f) max max c ... b
greater(a,b)
2
Subroutines

• MIPs statements that support procedures
• Jumpj address
• jumps to address
• provides no mechanism for returning
• Jump and Linkjal subroutine_address
• jumps to subroutine address
• saves the address of the instruction after the
  jal in R31
• Jump Registerjr 31

3
Subroutines

• Example

4
Subroutines

• Still have to deal with
• nested subroutine calls
• passing parameters
• reserving storage for local variables
• A stack is often used to help implement the above

5
Subroutines

• Stack
• Last in first out queue
• stack normally grows towards the bottom of memory
  
• On the MIPs R29 is normally used as the system
  stack pointer
• other stacks can be maintained using other
  address registers
• PUSH operation performed decrementing
  R29i.e. sub 29, 29,0x4 sw 31, (29)
• POP operation performed by incrementing
  R29i.e. lw 31, (29) add 29, 29,0x4

6
Subroutines

• Consider a subroutine A that calls a subroutine B
  that calls subroutine C. C calls no subroutines

A ... jal B ... B sub 29, 29,0x4 sw 31,
(29) ... jal C ...
lw 31, (29) add 29, 29,0x4 jr
31 C ... jr 31