Functions

Functions in assembly is reasonably simple if all you need to do is modularise your code
There is no need to worry about parameter passing and return values since the code accesses the memory and registers from anywhere.
I'm sure that there are more complex implementations of functions but I won't be considering them.

The code to demonstrate the use of functions is below

    

      // funcitons_4.s

      .global main
      .extern printf

      .data
      output_msg:
	      .asciz "Result is %d.\n"
      num:
	      .quad 0
      four:
	      .quad 4
      five:
	      .quad 5
      two:
	      .quad 2

      .text
      main:
	      // prolog
	      stp x29, x30, [sp, -16]!
	      mov x29, sp

	      // main code
	      bl load_reg
	      bl add_five
	      bl mult_four
	      bl div_two
	      bl print_result

      end:
	      // cleanup
	      mov x0, #0
	      ldp x29, x30, [sp], 16
	      RET

	      // functions
      load_reg:
	      ldr x19, =num
	      ldr x19, [x19]
	      RET

      add_five:
	      ldr x0, =five
	      ldr x0, [x0]
	      add x19, x19, x0
	      RET

      mult_four:
	      ldr x0, =four
	      ldr x0, [x0]
	      mul x19, x19, x0
	      RET

      div_two:
	      ldr x0, =two
	      ldr x0, [x0]
	      sdiv x19, x19, x0
	      RET

      print_result:
	      ldr x0, =output_msg
	      mov x1, x19
	      stp x29, x30, [sp, -16]!
	      bl printf
	      ldp x29, x30, [sp], 16
	      RET
    
  

It is worth pointing out the code in the print_result function.
Before the code runs it stores the frame pointer and link

    
      andrew@master:~/assm2 $ ./functions_4
      Result is 10.
      ^C
      andrew@master:~/assm2 $ nano functions_4.s
      andrew@master:~/assm2 $ gcc -g functions_4.s -o functions_4
      andrew@master:~/assm2 $ ./functions_4
      Result is 10.
      andrew@master:~/assm2 $