INT 2F - DOS Protected-Mode Interface - INSTALLATION CHECK
        AX = 1687h
Return: AX = 0000h if installed
            BX = flags
                bit 0: 32-bit programs supported
            CL = processor type (02h=80286, 03h=80386, 04h=80486)
            DH = DPMI major version
            DL = two-digit DPMI minor version
            SI = number of paragraphs of DOS extender private data
            ES:DI -> DPMI mode-switch entry point
        AX nonzero if not installed
SeeAlso: INT 38/AH=10h

Call mode switch entry point with:
        AX = flags
            bit 0: set if 32-bit program
        ES = real mode segment of buffer for DPMI private data (ignored if
                SI was zero)
Return: CF set on error
            program still in real mode
            AX = error code (DPMI 1.0+)
               8011h unable to allocate all necessary descriptors
               8021h 32-bit program specified, but 16-bit DPMI host
        CF clear if successful
            CS = 16-bit selector corresponding to real-mode CS
            SS = selector corresponding to real-mode SS (64K limit)
            DS = selector corresponding to real-mode DS (64K limit)
            ES = selector to program's PSP (100h byte limit)
            FS = GS = 0
            high word of ESP = 0 if 32-bit program
            program now in protected mode
Note:   this entry point is only called for the initial switch to protected
          mode