Int 2F Fn 12FF  U - Dr-dos 6 - Video Memory Space Control                  [O]

   AX = 12FFh
   BX = 0006h
   DX = 0000h
   CX = function
       0000h get status of video memory space (MEMMAX /V)
       0001h map memory into video memory space (MEMMAX +V)
       0002h unmap memory from video memory space (MEMMAX -V)

Return: CF clear if successful
       AX = 0000h (successful)
       BX = segment of reserved video RAM
       CX = segment of used video RAM
       DX = segment of first upper MCB

Note:  this functionality is provided by EMM386, and partially supported by
     HIDOS.SYS

See Also: AX=D201h/BX=4849h