Tutorial 3: ARM assembly function call
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.. include:: shared/notformark.rst

	     
Question 1: Understanding the stack 
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Report the final contents of the stack and the registers when the following ARM assembly instructions are executed. Assume that the initial stack is empty and :code:`R0=-1 R1=22 R2=3 R4=0`.After that, rewrite the instructions using :code:`Pop/Push` only.
 


.. code-block:: arm

  STR R4, [SP, #-4]!
  STR R4, [SP, #-4]!
  STR R0, [SP, #-4]!
  LDR R1, [SP], #4
  STR R1, [SP, #-4]!
  STR R2, [SP, #-4]!
  LDR R0, [SP], #4



Question 2: Function call with no argument passed or value returned
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Identify the caller, the callee and the instruction that the link register will have its address when the following program finishes execution. 


.. code-block:: arm

 .global _start

 _start: 

        MOV R0, #0
        MOV R1 #2
        ADD R1, R0, R1
        BL function_1
        LSR R3, #2
        AND R0, R1, R3
        .end
        
 function_1:
        BX LR
       

Question 3: Function call with argument passed by value and a value returned
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Convert the following C program into ARM assembly.

.. note::
 To follow  ARM calling convention:  use :code:`R0–R3` for passing arguments, and use :code:`R0` for the return value. You can  use the stack to pass the arguments if you want. Also Please make sure to follow the Callee-save convention (callee is responsible for leaving the registers as it found them)

.. code-block:: c

 int MAC (i1, i2, i3, i4);

 void main(){

         int weight0 = 2;
         int    ifm0 = 18;
         int weight1 = 3;
         int    ifm1 = 8;
         int     ofm = MAC(weight0, ifm0, weight1, ifm1);
 }

 int MAC (i1, i2, i3, i4){
        return i1*i2 + i3*i4;
  }

Question 4: Passing arguments by reference vs passing by value
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Given the following C program, what's the result of each `printf` statement?

.. code-block:: c

 #include <stdio.h>

 int ASR_Val (int i) {

        i = i >> 1;
        return i ;
 }
  
 void ASR_Ref (int *i) {

        *i = (*i) >> 1;
 }

 void main () {

        int w = 14;
        int w_returned ;

        w_returned = ASR_Val(w) ;
        printf("%d\n",w);
        printf("%d\n", w_returned);

        ASR_Ref(&w) ;
        printf("%d\n",w);
    
 }


Question 5: Structs (Always passed by value)
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Show, in ARM assembly, how to pass the struct in the following C program as an argument using the stack. Also, report the result of each `printf` statement.

.. code-block:: c

 #include <stdio.h>

 struct Vec { //Vector struct that has two integers x and y.

        int x;
        int y; 
 };

 struct Vec add2(struct Vec v);

 void main(){

        struct Vec vector_to_pass;
        struct Vec vector_returned;

        vector_to_pass.x = 12;
        vector_to_pass.y = 43;

        vector_returned = add2(vector_to_pass);
        
        printf("%d\n",vector_to_pass.x);
        printf("%d\n",vector_to_pass.y);
        printf("%d\n",vector_returned.x);
        printf("%d\n",vector_returned.y);
 
 }

 struct Vec add2(struct Vec v){ 
 //Function `add2`, that takes a struct of type `vec` and adds 2 to its integers and returns a struct of type Vec
       
        v.x = v.x + 2;
        v.y = v.y + 2;
        return v;
 }
 
Question 6: Frame Pointer
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Modify the previous program (Q5) to use the Frame Pointer :code:`FP (R11)` with relative addressing to access arguments.

.. note::
 Frame Pointer contains the base address of the function frame on the stack (Stack Pointer value right after entering a function). It is used to simplify addressing when passing arguments through the stack.

The following instructions will help you save and intialize FP properly.

.. code-block:: arm

 start: 
        ....
        BL add2
 
 add2:
        PUSH {FP}
        ADD FP, SP, #4
        ....
        POP {FP}