20.1 Specifying Parameter-Passing Mechanisms
  When you pass data between routines that are not writ-
  ten in the same language, you have to specify how you want
  that data to be represented and interpreted. You do this by
  specifying a parameter-passing mechanism . The gen-
  eral parameter-passing mechanisms and their keywords in
  BASIC are as follows:

  .
        By reference-BY REF
  .
        By descriptor-BY DESC
  .
        By value-BY VALUE

  The following sections outline each of these parameter-
  passing mechanisms in more detail.