Example on Procedure Call 

From Chapter 9 "A Programmer's View of Computer Architecture: with assembly language examples from MIPS RISC architecture," by Goodman and Miller, 1993

.data
array:  .word  0:4
message:    .asciiz "Enter integer: "

.text                                  ; Executable code follows

; start of the program
__start:




_start: .global _start                  ; "_start" is required by the linker
        .global main                    ; "main" is our main program

        b       main                    ; Start running the main program




main: 
       lda  a1, array	; load the address of array into a1
       bl fill_array	; branch to procedure fill_array



; procedure to get 4 integers from the user and put them
;   into the array.

fill_array:

         sub  sp, sp, #8
         str  lr, [sp,#4]

         mov  a4, a1			;  the array address is in a4 now
	 mov  a2, 0
         mov  a3, 3
for:     bgt a2, a, done_for

         str  a4, [sp,#0]
         lda  a1, message		; address of str string
         bl print_string		; call procedure to print string

         bl get_int   			; returns integer in a1
         ldr  a4, [sp,#0]               ; load array address
         str  a1, [a4], 4		; store int in array and increamet a4		
         add a2, a2, #1
         b for

done_for:
         ldr lr, [sp,#4]
         add  sp, sp, #8
         mov pc, lr



; Problem with the procedure as written:  it ignores the
; fact that function get_int will also use the a-register
; for its own local variables.  That might cause the values
; in registers a2, and a2 to be over rwritten.

; Solution:  Have procedure fill_array save the values of
; t-registers in its activation record.
; AND, we don't need to save a1, since it always gets reset
;      just before it is used.  AND, can set a3 inside the
;      loop just before it is used to avoid putting it in the
;      activation record.

; Better implementation:


data
array:  .word  0:4
message:    .asciiz "Enter integer: "

.text                                  ; Executable code follows

; start of the program
__start:




_start: .global _start                  ; "_start" is required by the linker
        .global main                    ; "main" is our main program

        b       main                    ; Start running the main program




main: 
       lda  a1, array	; load the address of array into a1
       bl fill_array	; branch to procedure fill_array



; procedure to get 4 integers from the user and put them
;   into the array.

fill_array:

         sub  sp, sp, #12
         str  lr, [sp,#8]

         mov a4, a1			;  the array address is in a4 now
	 mov  a2, 0
   for:  mov  a3, 3
        bgt a2, a, done_for

         str  a4, [sp,#4]
         lda  a1, message		; address of str string
         bl print_string		; call procedure to print string

         str  a2, [sp,#0]		; save a2 register
         bl get_int   			; returns integer in a1
         ldr  a2, [sp,#0]		; restore a2 register
         ldr  a4, [sp,#4]               ; load array address
         str  a1, [a4], 4		; store int in array and increamet a4		
         add a2, a2, #1
         b for

done_for:
         ldr lr, [sp,#8]
         add  sp, sp, #12
         mov pc, lr