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  DOS has evolved since the release of version 1.0 in 1981. Even though each
  new release has contained both improvements and bug-fixes, the driving
  force behind each release has been a hardware change, and a hardware
  change has usually involved a disk-drive change. (See Figure 14-1.)

  In all but versions 2.1 and 3.1, changes to DOS involved significant
  modifications to disk support (including new disk-storage formats). The
  main change to 2.1 was relatively minor, but was disk-related: The
  diskette control head-settle time was adjusted to allow for differences in
  the performance of the half-height drives used in the PCjr and Portable
  PC. Version 2.1 also corrected a few of the known bugs in 2.0. Version 3.1
  incorporated networking functions that were designed for version 3.0, but
  not ready when 3.0 was released. The following list summarizes the main
  differences between these versions:

  Version        Release Date   Hardware Change
  --------------------------------------------------------------------------
  1.0            August 1981    Original IBM PC (single-sided diskette
                                drive)
  1.1            May 1982       Double-sided diskette drive
  2.0            March 1983     PC/XT
  2.1            October 1983   PCjr and Portable PC
  3.0            August 1984    PC/AT
  3.1            March 1985     PC Network
  3.2            January 1986   Support for 3-1/2-inch diskette drives
  3.3            April 1987     PS/2s
  --------------------------------------------------------------------------

  Figure 14-1.  DOS releases and associated changes to hardware.

  Version 1.0 supported the single-sided, 8-sector diskette format. All
  basic DOS services were included in this release.

  Version 1.1 added support for double-sided diskettes. The DOS services
  remained the same.

  Version 2.0 added support for 9-sector diskettes (both single- and
  double-sided) and for the PC/XT fixed disk. The DOS services were enhanced
  extensively in this version. (See Chapter 17.)

  Version 2.1 added neither new disk formats nor new DOS services; it did,
  however, adjust its disk operation timing to benefit the PCjr and the
  Portable PC.

  Version 3.0 added support for the PC/AT's 1.2 MB diskette drive and
  additional fixed-disk formats. It also laid the groundwork for network
  disks.

  Version 3.1 added network disks, which include a file-sharing capability.

  Version 3.2 introduced support for 3-1/2-inch diskette drives.

  Version 3.3 was announced concurrently with IBM's introduction of the
  PS/2s. Several new commands and functions were included specifically to
  support the PS/2 hardware.

  --------------------------------------------------------------------------
  NOTE:
    Each version of DOS is compatible with prior versions, except in some
    very detailed respects (these sorts of details always seem to be
    unavoidable).
  --------------------------------------------------------------------------

  With each release of DOS, there has been a question among software
  developers about which version of DOS to target.

  In particular, DOS versions 2.0 and later supported a much wider variety
  of disk hardware and provided significantly more programming services than
  did versions 1.0 and 1.1, so programs that used the more advanced features
  of the later DOS versions wouldn't run at all on versions 1.0 and 1.1.
  Fortunately, the number of people still using version 1.0 or 1.1 is very
  small, so most software developers target their applications toward
  versions 2.0 and later. The differences between these later DOS versions
  are relatively minor and can usually be accommodated in software that
  verifies which version of DOS is running.

  Far-sighted software developers must also tackle the question of
  compatibility with future versions of DOS. Both IBM and Microsoft are
  looking toward OS/2 as the logical successor to DOS. In this view, DOS is
  considered a "mature" product; that is, enhancements to future versions
  aren't likely to affect existing DOS programs.

  Microsoft has published guidelines to help DOS software developers write
  programs that can later be converted for use under OS/2. In our
  discussions of DOS services in the next few chapters, we'll point out
  several techniques that can help ensure the future compatibility of your
  DOS programs.

  In any case, a program can detect which version of DOS it is running under
  by using DOS function 30H (decimal 48). Unless you can be sure of your
  audience, you should include this safeguard in your programs and always
  check to be certain that the correct DOS version is installed.