Obtain Real-to-Protected Mode Switch Entry Point

This function can be called in real mode only to test for
the presence of a DPMI host, and to obtain an address of a
mode switch routine that can be called to begin execution in
protected mode.

Call With:

   AX     = 1687h

Returns:

   if function successful
   AX     = 0
   BX     = flags
           Bit    Significance
           0      0 = 32-bit programs are not supported
                  1 = 32-bit programs are supported
           1-15   not used
   CL     = processor type
           02H = 80286
           03H = 80386
           04H = 80486
           05H-FFH  Reserved for future Intel processors
   DH     = DPMI major version as a decimal number
           (represented in binary)
   DL     = DPMI minor version as a decimal number
           (represented in binary)
   SI     = number of paragraphs required for DPMI host
           private data (may be 0)
   ES:DI  = segment:offset of procedure to call to enter
           protected mode

   if function unsuccessful (no DPMI host present)
   AX     = nonzero

Notes:

o  The entry point returned by Int 2FH Fn 1687H is
   only called for the first switch to protected mode by a
   DPMI client.  For further details on the protocol for
   switching to protected mode and the environment after
   switching to protected mode, see page 22.

o  Under DPMI hosts, the major version number is returned in
   DH and the minor version number is returned in DL. There
   are two decimal digits for the minor version number with
   the least-significant digit representing the revision
   number of the minor version number. Under DPMI version
   0.9 hosts, DH is returned as 0, and DL is returned as
   decimal 90 (5AH). In hypothetical DPMI version 2.3, DH
   would be returned as 2 and DL would be returned as 30
   (1EH).