INT 2fH 4B01H: Build Notification Chain

                                                         Compatibility: 5.0+ 
 Expects: AX    4B01H  (INT 2fH)
          CX:DX address of switcher service function handler
          ES:BX 0000:0000 (on initial call; see below)
                ...or...
                address of a SwCallbackInfoRec needing processing (when
                intercepting this INT 2fH fn; see below)
          ──────────────────────────────────────────────────────────────────
 Returns: ES:BX 0000:0000 (on initial call)
                ...or...
                address of a local SwCallBackInfoRec (when intercepting)
          ──────────────────────────────────────────────────────────────────
    Info: This fn creates a linked list of notification function handlers.
          Applications which need to be notified of task-switcher
          activities may intercept this fn and process subsequent calls
          (made by other programs) as follows:

          If you don't need to process switcher notifications, just JMP to
          the previous INT 2fH handler.

          If you do need notifications, 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
            handler.

          ■ On return from that CALL, create a SwCallbackInfoRec from local
            memory and fill it in (including the address of your code that
            will handle switcher notifications).   Copy the returned value
            of ES:BX into the pfNextHandler 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
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