3.2 Formalized

Let us formalize subroutines now. The definition of a subroutine, at least in this module, consists of the following:

• A unique name to distinguish a subroutine from all the other ones.
• Code to execute when the subroutine is invoked.

When a subroutine is invoked using a invoke statement, the following steps take place in this order:

• At least one column is created for the subroutine to store the line number to return to.
• The ``return line number'' is the line immediately following the invoke statement.
• The program continues execution on the first executable line in the definition of the subroutine.

When a subroutine reaches it end define sub line, the following happens in this sequence:

• Remember the ``return line number''.
• All columns created just for the subroutine disappear
• Execution continues on the line as remembered in step 1.

The most crucial part is that the column labeled ``return line #'' does not exist until the invoke statement. This column also disappears immediately after the end define sub statement. In other words, as a subroutine is called, at least one column is created. As a subroutine returns (when it reaches end define sub), all the column(s) created for the subroutine disappear.

In this module, each subroutine only has one dedicated column. However, when we get into parameters and variables, we'll see that many columns can be created when a subroutine is invoked.