INT 2fH 4B05H: Identify Instance Data

                                                         Compatibility: 5.0+ 
 Expects: AX    4B04H  (INT 2fH)
          ES:BX 0000:0000
          CX:DX address of switcher service function handler
 Returns: ES:BX 0000:0000 if no instance data chain
                else, address of a SwStartupInfoRec
    Info: This provides a way for application programs (such as TSRs) to
          tell the switcher about any "instance data" (memory areas that
          are unique for each task).  For example, Doskey preserves a
          command-line history for each DOS session, even though the
          program code need be in memory once.

          This fn creates a linked list of Identify Instance Data handlers.
          Application programs which need to preserve instance data must
          intercept INT 2fH and process fn AX=4b05H as follows:

          If you don't need to process Instance Data requests, just JMP to
          the previous INT 2fH handler.

          If you do need instance data, your INT 2fH handler should:

          ■ First, chain to any previously-installed handlers by pushing
            the FLAGS register and performing a FAR CALL to the previous

          ■ On return from that CALL, create a SwStartupInfoRec from local
            memory and fill it in (including the address of your list of
            SwInstanceItemRec structures.   Copy the returned value of
            ES:BX into the pfNextDev field of the structure.

          ■ Place the address of your own SwCallbackInfoRec into ES:BX and
            execute an IRET.

          These steps result in a chain of INT 2fH handlers, each of which
          can process subsequently-installed handlers.

See Also: INT 2fH 4BxxH: Task Switcher Functions
          Switcher Services and Notifications
          INT 2fH: Multiplex Interrupt
          DOS Functions