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  The next three chapters describe the DOS functions accessed through
  interrupt 21H. DOS version 1 had 42 interrupt 21H functions. This variety
  of functions was strongly rooted in the 8-bit microcomputer tradition
  typified by the CP/M operating system, whose services many of the DOS
  functions resembled.

  DOS version 1 was adequate for diskette-based microcomputers with
  keyboards and video displays, but the advent of high-capacity fixed disks
  and a wider variety of diskette formats called for a new set of
  sophisticated disk file-management functions. These were supplied in DOS
  version 2, and roughly patterned after the disk file-management services
  used in the UNIX operating system. In version 3, DOS continued to evolve,
  but offered only a few new functions, primarily in support of new hardware
  such as the PC/AT, networks, and the PS/2s.

  Although some interrupt 21H functions introduced in later versions of DOS
  provide services similar to those in earlier versions, all version 1
  functions continue to be supported in later versions. When you have a
  choice between two similar functions, you should, in general, use the
  higher-numbered, more recent function. We'll point out why as we go along.