---

Implementing the "Pass-by-value" and "Pass-by-reference" techniques in assembler

---

• Implementing the "Pass-by-value" mechanism
  • Recall the pass-by-value construct written in a high level programming language:

    #include <iostream> using namespace std; /* ==================================================== Function with a parameter that is passed by value ==================================================== */ void f(int a) // Var a is passed by value { a = a + 1; } int main(int argc, char *argv[]) { int k = 7; f(k); }

    ---

  • For simplicity, I will use a permanent variable k to show you how the pass-by-value mechinism is implemented in assembler.

    The main( ) function will pass the value (= content) of the variable k to the function f( ).

    This is how it is done in ARM assembler code:

    main: /* ========================================= Pass k to f( ) by value ========================================= */ movw r0, #:lower16:k movt r0, #:upper16:k // r0 = address of k ldr r0, [r0] // r0 = value of k push {r0} // Pass k by value (pass k's value) bl f // Call function f add sp, sp, #4 // Clean up parameters on the stack /* ----------------------------------------------------------- */ .data k: .4byte 7 // This is the variable k

    ---

  • A passed-by-value parameter is access and updated as follows:

    /* ================================================ Function f that gets a parameter by value Stack frame of f( ): FP--> +-----------+ | old FP | FP + 0 +-----------+ | ret addr | FP + 4 +-----------+ | param a | FP + 8 +-----------+ ================================================ */ f: /* **************************************** Prelude: build stack frame **************************************** */ push {lr} // Save return address in LR push {fp} // Save Frame Pointer in FP mov fp, sp // Initialize my own FP sub sp, sp, #0 // I create 0 local variable in stack /* ======================================= a = a + 1 ======================================= */ ldr r0, [fp, #8] // r0 = a add r0, r0, #1 // r0 = a + 1 str r0, [fp, #8] // a = a + 1 /* ************************************************ Postlude: break down stack frame ************************************************ */ mov sp, fp // De-allocate the local variables pop {fp} // Restore old FP pop {pc} // Return

    ---

    ---

  • Summary:

    In pass-by-value: The value of the variable is passed (= copied) to the function The function will access/update the copy

  ---

  ---

  ---

  ---

• Implementing the "Pass-by-reference" mechanism
  • Recall the pass-by-reference construct written in a high level programming language:

    #include <iostream> using namespace std; /* ==================================================== Function with a parameter that is passed by value ==================================================== */ void f(int & a) // Var a is passed by reference { a = a + 1; } int main(int argc, char *argv[]) { int k = 7; f(k); }

    ---

  • For simplicity, I will again use a permanent variable k to show you how the pass-by-value mechinism is implemented in assembler.

    The main( ) function will pass the address of the variable k to the function f( ).

    This is how it is done in ARM assembler code:

    main: /* ========================================= Pass k to f( ) by value ========================================= */ movw r0, #:lower16:k movt r0, #:upper16:k // r0 = address of k push {r0} // Pass k by reference (pass k's address) bl f // Call function f add sp, sp, #4 // Clean up parameters on the stack /* ----------------------------------------------------------- */ .data k: .4byte 7 // This is the variable k

    ---

  • A passed-by-value parameter is access and updated as follows:

    /* ================================================ Function f that gets a parameter by value Stack frame of f( ): FP--> +-----------+ | old FP | FP + 0 +-----------+ | ret addr | FP + 4 +-----------+ | param a | FP + 8 +-----------+ ================================================ */ f: /* **************************************** Prelude: build stack frame **************************************** */ push {lr} // Save return address in LR push {fp} // Save Frame Pointer in FP mov fp, sp // Initialize my own FP sub sp, sp, #0 // I create 0 local variable in stack /* ======================================= a = a + 1 ======================================= */ ldr r0, [fp, #8] // r0 = address of (original variable) a ldr r1, [r0] // r1 = value in variable a add r1, r1, #1 // r0 = a + 1 str r1, [r0] // orignal a = a + 1 !!! /* ************************************************ Postlude: break down stack frame ************************************************ */ mov sp, fp // De-allocate the local variables pop {fp} // Restore old FP pop {pc} // Return

    Note:

    Because the function f( ) is given (= passed) the address of the variable a in main( ), it can update the original value !!!        You can see in the assembler code how this is achieved

    ---

    ---

  • Summary:

    In pass-by-reference: The address of the (original) variable is passed (= copied) to the function The function will use the address to access/update the value in the original variable