INT 2F U - DR-DOS 6 - VIDEO MEMORY SPACE CONTROL
        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
SeeAlso: AX=D201h/BX=4849h

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