Compatibility: 6.0+
Expects: AX    4a11H (mux number)
BX    0007H (DoubleSpace fn code)
DL    compressed drive to query
Returns: AX    DoubleSpace API Error Code (0000H = success)
DS:SI address of DsSpaceInfoRec (see below)
Info: This obtains the true total space and free space available in the
specified volume.

Note: the newer (6.2) DS IOCTL 'S' function is the preferred
method of obtaining this information.

DS:SI on return, DS:SI points to the following data structure:

Offset Size Contents
+0      4  lTotSects   total data sectors in the disk
+4      4  lFreeSects  available sectors in the disk
8              size of a DsSpaceInfoRec (undocumented)

Notes:   This changes DS, pointing it into DBLSPACE.BIN local data area.
So take notice (and change it back to point to your local data
area after this call).

  The return values in lTotSects and lFreeSects are expressed in
512-byte sectors.

  The maximum physical size of a DoubleSpace compressed volume
is 512 MB.  Thus, the largest value you can expect returned in
lTotSects is 1,048,576 (0010 0000H) sectors.

  Use DOS fn 36H to obtain the estimated storage for the drive.

  The documented API does not provide an estimated compression
ratio, but you can derive it by comparing lTotSects to the
value obtained from DOS fn 36H.

DS IOCTL 'S' (Get DriveSpace Pointers)
- -

INT 2fH 4a11H BX=0007H: Get Dblspace Disk Size / Free Space