Subroutines

 Frequently, the similar piece of code must write over again in many dissimilar parts of a program.  Instead of repeating the code every time it is required, there is an obvious advantage if the common instructions are written only once.  A set of common instructions that can be used in a program many times is known as a subroutine.  Every time that a subroutine is used in the main part of the program, a branch is implemented to the beginning of the subroutine. After the subroutine has been implemented, a branch it is made back to the main program.