Parameters are placeholders for originalparameterprocedurereferenceroutinevaluevariables needed by routines and supplied at call time (when the originalparameterprocedurereferenceroutinevaluevariables or funtion is called).

A call by originalparameterprocedurereferenceroutinevaluevariables parameter is a pointer referring to the address in main memory of the originalparameterprocedurereferenceroutinevaluevariables variable. Changes made to this parameter within the body of the routine are made to the original variable. So in effect the new value is passed back to the program where the originalparameterprocedurereferenceroutinevaluevariables was called;

'Call by originalparameterprocedurereferenceroutinevaluevariables' parameter: any changes made to this originalparameterprocedurereferenceroutinevaluevariables will not affect the original originalparameterprocedurereferenceroutinevaluevariables where the routine is called from;