Another usage of the stack

Recall we had trouble performing the following computation: 

   result = square(3) + square(4);

This code will not work all the time: 

   /* -------------------------------------------------
      Call square(3)
      ------------------------------------------------- */
   mov     r0, #3
   bl      square

   mov	r10, r0           // Save 3^2 in r10 *** FAIL !!!

   /* -------------------------------------------------
      Call square(4)
      ------------------------------------------------- */
   mov     r0, #4
   bl      square     <--- r10 overwritten by this call 

   add	r0, r0, r10       // FAIL to compute 32 + 42

DEMO:   /home/cs255001/demo/asm/8-sub/reg-problem.s

Recall the important facts to notice in the last example
 

  • Before main( ) make a function call, the program stack was empty

  • After main( ) has clean up the parameters in the program stack, the program stack is empty again !!!
 

In general:   the stack right before and right fater a function call are equal in content 

   ....
   instr1          <------+
   push parameters        | The content of the
   bl   f                 | runtime stack are
   pop  parameters	  | equal at these instructions
   instr2          <------+

Using the stack to perserve values in registers
 

There is a very simple technique to perserve values in registers when you need to make a function call: 

   ....
   // How to perserve the value in register r0       
   // we need to make a function call                

   push  {r0}    // Save r0 on stack
                   <------+
   push parameters        | The content of the
   bl   f                 | runtime stack are
   pop  parameters	  | equal at these instructions  
                   <------+
   push  {r0}    // Restore the old r0 value

Using the stack to perserve values in registers

Example:   result = square(3) + square(4); 

  mov     r0, #3
  bl      square

  push    {r0}           // Save 32 in stack *** !!!
  mov     r0, #4
  bl      square
  pop     {r1}           // Retrieve 32 from stack *** !!!

  add	r0, r0, r1       // Computes 32+ 42

  /* -------------------------------------------------
     Store in result
     ------------------------------------------------- */      
  movw    r1, #:lower16:result
  movt    r1, #:upper16:result

  str     r0, [r1]

DEMO:   /home/cs255001/demo/asm/reg-problem.s
Note: this technique will also work when parameters + local vars are in the stack