Int 2F Fn 4B01  C - DOS 5+ Task Switcher - Build Callout Chain             [T]

   AX = 4B01h
   CX:DX -> task switcher entry point (see AX=4B02h)
   ES:BX = 0000h:0000h

Return: ES:BX -> callback info structure (see below) or 0000h:0000h

Notes: called by the task switcher
   this function is hooked by clients which require notification of task
     switcher activities; the call must first be passed on to the prior
     handler with registers unchanged using a simulated interrupt.  On
     return, the client must build a callback info structure and store
     the returned ES:BX in the "next" field, then return the address of
     its own callback info structure.
   a client program must add itself to the notification chain if it
     provides services to other programs; before terminating, it must
     remove itself from the chain by calling the task switcher's entry
     point with AX=0005h (see AX=4B02h)
   the task switcher entry point should not be saved, as it is subject to
     change and will be provided on any notification call
   the Windows 3.1 Standard Mode supports this API

See Also: AX=160Bh,AX=4B02h

Format of callback info structure:
Offset Size    Description
 00h   DWORD   pointer to next callback info structure
 04h   DWORD   pointer to notification function (see below)
 08h   DWORD   reserved
 0Ch   DWORD   address of zero-terminated list of API info structures
       (see AX=4B02h)

Notification function is called with:
   AX = function
       0000h switcher initialization

       Return: AX = 0000h if OK to load
              = nonzero to abort task switcher
       0001h query suspend
       BX = session ID

       Return: AX = 0000h if OK to switch session
              = 0001h if not
       0002h suspend session
       BX = session ID
       interrupts disabled

       Return: AX = 0000h if OK to switch session
              = 0001h if not
       0003h activate session
       BX = session ID
       CX = session status flags
           bit 0: set if first activation of session
           bits 1-15: reserved (0)
       interrupts disabled

       Return: AX = 0000h
       0004h session active
       BX = session ID
       CX = session status flags
           bit 0: set if first activation of session
           bits 1-15: reserved (0)

       Return: AX = 0000h
       0005h create session
       BX = session ID

       Return: AX = 0000h if OK to create session
              = 0001h if not
       0006h destroy session
       BX = session ID

       Return: AX = 0000h
       0007h switcher termination
       BX = flags
           bit 0: set if calling switcher is only switcher loaded
           bits 1-15: reserved (0)

       Return: AX = 0000h
   ES:DI -> task switcher entry point (see AX=4B02h)

Notes: function 0000h is generally called by the program which controls or
     invokes the task switcher, rather than by the task switcher itself;
     the entry point supplied to this function is not necessarily the
     entry point to the task switcher itself, and may be 0000h:0000h.  If
     any client indicates that loading is not possible, all clients will
     be called with function 0007h; thus it is possible for a client to
     receive a termination notice without a corresponding initialization
     notice.
   except for functions 0002h and 0003h, the notification handler is
     called with interrupts enabled and may make any INT 21h function
     call; interrupts must not be enabled in functions 0002h and 0003h
   function 0007h may be called with ES:DI = 0000h:0000h if the entry
     point is no longer valid