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  The services introduced with DOS versions 2 and 3 have three important new
  features that directly affect the way you use the services:

  .  Most of the functions return a set of consistent error codes in the AX
     register.

  .  All functions that use string input require a special string format
     known as the ASCIIZ format--a string of ASCII characters terminated by
     a single zero byte.

  .  The newer DOS functions use a 16-bit number called a handle, instead of
     an FCB, to identify the files and I/O devices that a program
     communicates with.

  We'll discuss each of these enhancements on the next few pages.