INT 10 - TopView - GET SHADOW BUFFER
        AH = FEh
        ES:DI -> assumed video buffer
                B800h:0000h color text/CGA graphics, B000h:0000h mono text,
                  or A000h:0000h EGA/VGA graphics (RSIS environments only)
Return: ES:DI -> actual video buffer for calling process
Notes:  if no multitasker or RSIS-compliant environment is installed, ES:DI is
          returned unchanged; RSIS is the Relocated Screen Interface
          Specification
        for display pages other than 0, use AH=05h and AH=0Fh to determine
          whether a particular page exists
        TopView requires a call to AH=FFh to notify it that the screen has
          changed; DESQview will check for changes itself until the first call
          to AH=FFh
SeeAlso: AX=5201h,AH=FFh,INT 15/AX=1024h,INT 21/AH=2Bh"DESQview"