Subroutines

1
Subroutines
2
Subroutines

• Repeat same code with different parameters
• Provide for separate compilation
• Provide for reuse

3
Skeleton

• Subroutine name
• save sp,
  frame_size, sp
• lt body of codegt
• ret
• restore

4

• Save
• To allocate a new set of registers
• Changes the register mapping
• Restore
• Restore old register windows
• Changes back to previous mapping

5
registers

• i0-i5 -- incoming parameters
• i6 -- frame pointer
• i7 -- address of call instruction
• l0-l7 -- all yours
• o0 -- all yours, but also used to pass a
  parameter to and/or return a value from a
  subroutine
• o1-o5 -- all yours, but also used to pass
  parameters to a subroutine
• o6 -- stack pointer
• o7 -- address of call instruction
• g0 -- always zero
• g1-g7 -- all yours

6
Pass parameters to subroutinesreturn value

• before save after save
• i7 i0
• l7 l0
• o7 call ? ? i7 call
• o6 old sp ? ? i6 new fp
• o5 ? ? i5
• .
• .
• .
• o0 ? ? i0
• l7 l0
• o7 o0

7
Example

• int sum( int a, int b )
• return ( a b )
• c sum( 1, 2 )

• mov 1,o0
• mov 2,o1
• call a
• nop
• -------------------------------------
• a
• save sp,-96,sp
• mov i0,l0 o6
• mov i1,l1
• add l0,l1,l2
• mov l2,i0
• ret
• restore

8
Stack

• Space on Stack
• Callers registers
• Local registers for the called subroutine

9
Call by reference

• void prt_sum( int x, int n )
• register int i, sum 0
• for( i 0 i lt n i )
• sum sum xi
• printf("d\n",sum)

10
Arrays

• .global main
• main
• save sp, -96, sp
• clr l0
• clr l1
• mov i0, l2
• loop
• cmp l0, i1
• bge done
• nop
• ld l2, o0
• add l1, o0, l1
• add l2, 4, l2
• inc l0
• ba loop
• nop
• done
• set fmt, o0
• mov l1, o0

11
Leaf Subroutine

• Not call other subroutines
• Not execute save or restore
• Uses calling subroutines register sets
• But only output and global calling registers
• Return address is stored in o78 not i78
• Correct return instruction is retl

12

• .align 4
• void main() .global main
• void swap() .type main,function
• .proc 020
• main
• !PROLOGUE 0
• int a,b save sp, -120, sp
• !PROLOGUE 1
• a 5 mov 5, o0
• st o0, fp-20
• b 44 mov 44, o0
• st o0, fp-24
• swap(a,b) add fp, -20, o0
• call swap, 0
• add fp, -24, o1 ! scheduled in delay slot
• ret
• restore
• .align 4
• void swap(x,y) .global swap