INT 2F - DBLSPACE.BIN - "GetDriveSpace" - GET SPACE AVAIL ON COMPRESSED DRIVE
        AX = 4A11h
        BX = 0007h
        DL = compressed drive number (0=A:)
Return: AX = status (0000h,0102h) (see also AX=4A11h/BX=0001h)
            0000h successful
                DS:SI -> free space record (see below)
Program: DBLSPACE.BIN is the resident driver for DoubleSpace, the
          disk-compression software bundled with MS-DOS 6.0
SeeAlso: AX=4A11h/BX=0000h,AX=4A11h/BX=0008h

Format of free space record:
Offset  Size    Description
 00h    DWORD   total number of sectors in drive's sector heap
 04h    DWORD   number of free sectors in drive's sector heap

.