Types of pass value with Subroutines & How To Use Subroutines In SAP ABAP Program.

Introduction :   
A subroutine is a reusable section of code.
It is a modularization unit within the program where a function is encapsulated in the form of source code.
Subroutines are used to modularize the program.
Generally Subroutines are called by using PERFORM keyword and defined within FORM and ENDFORM block.
here are three ways to pass the values with Subroutines.

1. pass by value:
In this type, the actual and formal parameters will be referring to separate memory.
The formal parameter is changed, the actual parameter will not be changed.
The parameters which are passed in the perform call are known as actual parameters or actual arguments.
The parameters which are received by the called Form & endform  are known as formal parameters or formal arguments.
Example is shown below:

example1 : pass by value : using    - using value().

output

example2 : pass by value : changing - using value().

                                                             
                                                                 output

2. pass by reference:
In this type, the actual and formal parameters are referred to the same memory.
If the formal parameter is changed, the actual parameter is also changed.

example1 : pass by reference : using - using.

output

example2 : pass by reference : using - changing.

output

3. pass by value and return:
In this type, the actual and formal parameters will be referring to separate memories.
If the formal parameter is changed, the actual parameter is also changed after executing the FORM...ENDFORM.

example1 : pass by value and return : using    - changing value().

output

example2 : pass by value and return : changing - changing value().

output

you can use subroutines according to your requirements.
this is a basic approach .