Int 2F Fn 12FF  U - Dr Dos 6, Novell Dos 7 - Emm386.exe - Video Memory Sp  [m]

   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

Notes: this functionality is provided by EMM386, and partially supported by
     HIDOS.SYS
   BL specifies which program handles the call, BH is the function number
BUG:   4DOS 5.51(a) often hangs the system (reported to be reproducable), if
     MEMMAX +V is issued from the 4DOS prompt.  To avoid this, one should
     temporary load COMMAND.COM followed by MEMMAX +v , starting the
     application, MEMMAX -v and EXIT.  4DOS 5.5c does not show this
     phenomena on the same systems where 5.51a hangs.

See Also: AX=D201h/BX=4849h