---

General discussion on passing parameters to functions, passing return values to caller and storing local variables

---

• High level programming view of a subroutine/function call and return
  • Consider the following example program:

    void main( ) // I omitted the 'public static' for simplicity { int x = 4, y = 7, z; z = F(x, y); // Pass parameters x, y and receive return value } int F(int a, int b) // I omitted the 'public static' for simplicity { int s, t; // Local variables .... return(77); }

    The subroutine call statement:

    z = F(x, y); // Subroutine call statement

    packs a lot of meaning in a high level programming language

    We are here to spell out what exactly happens in a subroutine invocation (call) statement.

    ---

    ---

  • You were told in CS170 that:

    The subroutine invocation F(x, y) will passes the variables x and y (to the function F( )) Then the statements (= instructions) in the subroutine F( ) are executed using a = x (= 4 in example) and b = y (= 7 in example)

  ---

  ---

  ---

  ---

• The common element in (1) passing parameters (2) passing return values and (3) storing (creating) local variables
  • The one thing that

    Passing parameters to a function Passing the return value to a caller function Creating local variables for a function

    is:

    They all require the use of memory cells: You need memory cells to store the parameter (variables) You need memory cells to store the return value You need memory cells to store the local variables

  ---

  ---

  ---

  ---

• Memory elements in the computer system
  • Memory elements (= devices that can be used to store values) can be found in:

    The computer memory (of course... it has the word memory in it !!!) The registers in the CPU !!!

    So passing parameters, passing return values and allocating memory for local variables can be done using:

    Registers in the ALU              Computer memory - specifically: memory space on the system stack !!!

  ---

  ---

  ---

  ---

• What happens exactly during a subroutine/function call and return
  • Consider the following example program:

    void main( ) // I omitted the 'public static' for simplicity { int x = 4, y = 7, z; z = F(x, y); // Pass parameters x, y and receive return value } int F(int a, int b) // I omitted the 'public static' for simplicity { int s, t; // Local variables .... return s; }

    The subroutine call statement:

    z = F(x, y); // Subroutine call statement

    packs a lot of meaning in a high level programming language

    We are here to spell out what exactly happens in a subroutine invocation (call) statement.

    ---

    ---

  • You were told in CS170 that:

    The subroutine invocation F(x, y) will passes the variables x and y (to the function F( )) Then the statements (= instructions) in the subroutine F( ) are executed

  ---

  ---

  ---

  ---

• Using registers
  • When main( ) calls F(x, y):

    z = F( x, y );

    main( ) passes its variable x and y to F( )

    We can pass these parameters by copying the values in these variables in some registers

    For example:

    Copy the variable x in register R0 Copy the variable y in register R1

    ---

  • In this case:

    The function F( ) must use register R0 as its parameter variable a !!! The function F( ) must use register R1 as its parameter variable b !!!

    Schematically explained:

  Consequently:

  You can use registers to pass parameters to a function You can use registers to return a value to the caller You can use registers as local variables However: Using registers to pass parameters and storing local variable can only be done in a leaf subroutine because When a non-leaf subroutine calls another subroutine, the values in the registers will be lost !!! In other words: Never use registers to pass parameters nor as local variable in a non-leaf subroutine !!!

  ---

  ---

  ---

  ---

• Teaching example 2 on function calls
  • Consider the 2nd example program to illustrate passing paremeters, passing return values and creating local variables:

    void main( ) // I omitted the 'public static' for simplicity { int x, y, z; z = squaresum(x, y); // Pass parameters x, y and receive return value } /* -------------------------------------- Returns: a^2 + (a+1)^2 + ... + b^2 -------------------------------------- */ int squaresum(int a, int b) // I omitted the 'public static' for simplicity { int i, s; // Local variables s = 0; for ( i = a; i <= b; i++ ) s = s + square(i); return s; } int square(int x) { return(x*x); }