struct Vector { float x; float y; }
Vecrot foo(Vector a, Vector b) {
 Vecrot c;
 c.x = a.x +  b.x;
 c.y = a.y + b.y;
 return c;
}
    

#include <iostream.h>
extern Vector Add(Vector a,Vector b);

void main()
{
        Vector Vector1,Vector2,Vector3;
        Vector3=AddVector(Vector1,Vector2);
        cout<<Vector3.x<<" "<<Vector3.y;
}

    

Fasm ..... all stuff blah blah....

AddVector:
          push ebp
          mov ebp,esp
          finit
          ;------
          fld dword [ebp+8]   Vector1.x
          fadd dword [ebp+16]  x + x
          fld  dword [ebp+12]  Vector1.y
          fadd  dword [ebp+20] y+y
          
           OK st0 = y+y  ,st1= x+x       
          What do I need to do here to return both values above to vector3 in the Caller

          pop ebp
          retn    

// My source.
struct Vector
{
        float x;
        float y;
        float z;
        float w;
};

Vector foo(Vector a, Vector b)
{
        Vector c;

        c.x = a.x +  b.x;
        c.y = a.y + b.y;
        c.z = a.z + b.z;
        c.w = a.w + b.w;

        return c;
}

// Out put from g++.

  .file   "vec.cpp"
 .text
       .align 2
.globl __Z3foo6VectorS_
 .def    __Z3foo6VectorS_;       .scl    2;      .type   32;     .endef
__Z3foo6VectorS_:
     pushl   %ebp
        movl    %esp, %ebp
  movl    8(%ebp), %eax
       flds    12(%ebp)
    fadds   28(%ebp)
    fstps   (%eax)
      flds    16(%ebp)
    fadds   32(%ebp)
    fstps   4(%eax)
     flds    20(%ebp)
    fadds   36(%ebp)
    fstps   8(%eax)
     flds    24(%ebp)
    fadds   40(%ebp)
    fstps   12(%eax)
    popl    %ebp
        ret     $4
    

Now, I recall Vid giving
you a short quiz that you seem to have passed.
But perhaps an additional question should have
been asked for you to explain the difference
between "call by value" and "call by reference".

After studying the GAS assembly for sometime
(more than I wanted to; it's weird), I can see
that eax is being used as a pointer to somewhere.
And that "somewhere" is the return structure of the
function call. That I will agree with you on,
provided you agree that my example was not the GAS
generated code.