Control as State within Program

1.  The term control exactly means to check effect of input within a program.

For illustration, in Figure, after the ATM card is inserted (as an input) the control of program is transferred to the further state (i.e. to request password state).

2.  This is traditional approach to represent control inside a program. The location of control within a program implicitly defines program state. Every state transition corresponds to an input statement. After input is read, program branches depending on an input event produce some result. Each input statement handles any input value which could be received at that time. In case of extremely nested procedural code, low-level procedures must allow inputs which may be passed to upper level procedures. After receiving the input they pass them up through many levels of procedure calls. There may be any procedure prepared to handle these lower level calls.