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  In this chapter we'll discuss the interrupt 21H functions introduced in
  DOS versions 2.0 and later. These functions provide a wide range of
  operating system services within a more sophisticated and flexible
  framework than the original 42 functions we described in Chapter 16.

  Almost every DOS upgrade has increased the number of services provided to
  programmers. DOS 2.0 initiated the most dramatic changes: It added 33 new
  functions to the existing 42; it changed the way you access file
  information as a result of these new functions; and it made it possible to
  adapt DOS to work with almost any hardware device through the use of
  programs called installable device drivers. Before discussing the newer
  DOS functions in detail, we'll briefly cover how some of these
  enhancements can affect your programming practices.