Switcher API Overview

  Starting with DOS 5.0, Microsoft published information on coexisting with
  the DOS task-switcher.  For most applications, actions of a task switcher
  are transparent, but a few programs need to avoid the potential pitfalls
  possible when they are turned off and on by an external program.

█▌Who Needs the Switcher API?▐█
  Some programs need to take special precautions when running under a task
  switcher.  For instance, modem programs will be in big trouble if they get
  switched-out while online with a remote computer.  These programs can
  process SwNtfyFn 0001H and indicate when switching-out would be bad news.

  Some TSR programs installed before the switcher can provide different
  context for each DOS session.  For instance, the Doskey TSR maintains a
  separate "command history" and "macro set" for each DOS session.  All
  sessions share the same code space, but a separate data area is maintained
  for each session.

  Some programs need to take precautions about the use of global resources
  that may be destroyed without proper handling.  For instance, a word
  processor may create temporary files.  Each session of the program should
  maintain separate files to avoid problems.

    Note: Much of the Switcher API is concerned with a situation in which
          two or more switchers are active simultaneously and having them
          coexist peacefully.  Unless you are writing a task switcher, you
          may ignore most of the API.

█▌Using the Switcher API▐█
  If you need to be switcher-aware, follow these steps:

   1) Use INT 2fH 4B02H to see if a task-switcher is active.  This also
      returns the service entry address.

   2) Use INT 2fH 4B05H to identify regions of your memory which must be
      maintained for each session.  The switcher will automatically save
      and restore these areas for each session when tasks are switched.

   3) If you need to be notified of a pending task-switch (in order to
      clean up before the switch or to block the switch altogether), call
      the service entry address with AX=0004H to "hook" the notification
      chain (see SwSrvcFn 0004H).  Thereafter, the switcher will call a
      designated procedure in your code before switching you out.

      And remember to use SwSrvcFn 0005H later, to unhook yourself before
      you terminate.

See Also: Switcher API
          INT 2fH
          TSR Functions
          DOS Functions