Tutorial 3: ARM assembly function call
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.. note::

   This is for practice, not for marks. Treat this as additional study material to help you develop your knowledge and skills on these topics. During the tutorial session, the TA will explain how to solve these problems.
   

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:: asm 

  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:: asm

 .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. Aslo 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;
 }