INT 2fH 4a11H BX=0001H: Get Dblspace Drive Mapping

                                                         Compatibility: 6.0+ 
 Expects: AX    4a11H (mux number)
          BX    0001H (DoubleSpace fn code)
          DL    drive number to check (0=A, 1=B, etc)
 Returns: AX    DoubleSpace API Error Code (0000H=success)
          BH    compressed drive sequence number (0-255)
          BL    bits 0-6: host drive number (if drive is compressed)
                   bit 7: compressed drive flag (1=compressed)
    Info: You may use this fn as part of a system to determine whether a
          particular drive ID represents a CVF or a host drive.
          Interpreting the return results can be a little bit tricky:

       AX on return, if AX is non-zero, then DoubleSpace has not been
          loaded and DL is a standard (uncompressed, unswapped) drive ID.

       BH on return, BH is the sequence number; that is, the number of the
          file extension of the CVF.  For instance, if BH returns with 01H,
          (and if BL indicates that this is a compressed drive) then the
          CVF is named DBLSPACE.001 (in the root of its host).

       BL on return, if the high bit is set, then drive DL is a compressed
          drive and the low 7 bits of BL identify the drive ID of the host.

          For instance, a return value of BL=83H means that DL was a
          compressed volume whose CVF is now on drive D (0=A, 1=B, etc.).
          All DOS-level fns will be able to access that host as drive D,
          but its drive ID may have been swapped, so BIOS-level fns to
          access the physical drive may use a different drive number.

          To learn the original drive number for the drive on which the CVF
          is stored, you must make a second call to this fn, this time
          setting DL to the value returned in the low 7 bits of BL.  If
          BL=DL upon return from that second call, then that drive is host
          to itself (the drive IDs have been swapped); therefore BIOS
          access is via the BL value returned in the first call.

   Notes: This seems far too complicated; I hope I made it clearer than the
          Microsoft dox!

See Also: DoubleSpace Overview
          DoubleSpace API
          INT 2fH: Multiplex Interrupt
          DOS Functions