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  To be candid and blunt, let us admit right away that we can't give you
  everything you need here. Working with BASIC and interfacing to BASIC are
  very, very complicated subjects--complex enough to fill several books by
  themselves. Frankly, interfacing with interpreted BASIC is a particularly
  messy area, made even messier by the number of BASIC versions used with
  the different models of the extended PC family. The specific techniques we
  describe here apply to the most popular interpreted BASICs: the BASIC
  distributed by IBM with every PC and PS/2; and Microsoft's GW-BASIC.

  In this section we describe the interface between assembly-language
  subroutines and interpreted BASIC programs. We discuss only those
  subroutines accessed through BASIC's CALL statement.

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  NOTE:
    Interpreted BASIC supports a second subroutine-call mechanism through
    the USR statement, but in our opinion, USR functions involve annoying
    and unnecessary complications. We recommend that you stick to the CALL
    interface instead.
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