Int 2F Fn DE01  - Desqview V2.26+ Xdi - Custom Subfunction, Ralf Brown's   [U]

   AX = DE01h
   BX = 5242h ("RB")
   CX:DX = program identifier
       656F7000h ("eop",0) for DVeop

Return: AX = 5242h ("RB") if installed
       ES:BX -> data or entry point
       CX = version number (CH = major, CL = minor)

Call DVeop entry point with:
   ES:DI -> callback address or 0000h:0000h to remove callback

Return: AX = status
       0000h failed (callback table full or attempted to remove non-
           existent callback)
       0001h successful
       ES:DI -> chaining address
   BX,CX,DX destroyed

Notes: the callback function is called with a simulated interrupt when the
     DESQview window containing it is closed; it should perform all
     necessary cleanup and then perform a FAR jump to the chaining address
     or an IRET if the chaining address is 0000h:0000h
   if the program wishes to remove itself before the window is closed, it
     should call the DVeop entry point with the previously returned
     chaining address and ignore the returned chaining address.