SwSrvc Fn 0004H: Hook Notification Chain

 Expects: AX    0004H
          ES:DI address of an SwCallbackInfoRec structure
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 Returns: AX    0000H and CF is clear (NC) if successful
                CF is set (CY) if the switcher does not support this fn.
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    Info: Applications use this fn when they wish to be notified of task-
          switcher operations such as switching session out.

          To access this fn, fill-in an SwCallbackInfoRec structure and use
          a FAR call to the switcher entry address (see INT 2fH 4b02H).
          Your program code pointed to by the pfHandler in that structure
          will then receive notifications in the form of FAR calls with AX
          specifying a notification fn number.

    Note: Before terminating your program (for instance, a TSR which
          receives notifications) you must use SwSrvcFn 0005H (unhook).

See Also: Switcher Services and Notifications
          INT 2fH 4bxxH (switcher functions)
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