DOS Fn 36H: Get Disk Free Space

                                                         Compatibility: 2.0+ 
 Expects: AH    36H
          DL    drive number (0=default, 1=A, etc.)
 Returns: AX    ffffH   if DL was an invalid drive number
                else    sectors per cluster if no error
          BX    available clusters (allocation units)
          CX    bytes per sector (usually 512)
          DX    total clusters on the disk
    Info: Returns information useful for calculating total and available
          disk space.

          If AX returns ffffH, you sent a bad drive number in DL.

          Otherwise,  free space in bytes = (AX * CX * BX)
                     total space in bytes = (AX * CX * DX)

          When sectors are 512 bytes, the calculation for Ks is simpler:
                      free space in K-bytes = (AX * BX) / 2
                     total space in K-bytes = (AX * DX) / 2

Notes:    ■ A common error in the use of this fn is to disregard arithmetic
            overflow on the multiplications.  The product of three 16-bit
            values can be very large indeed.

Versions: DOS 6.0
          ■ This call is intercepted by the resident portion of Undelete
            (when Delete Sentry is active).  The free-space value is
            increased to reflect the disk space that will be made available
            if Undelete needs to physically delete files in the \SENTRY

            Once per day, this call may take several seconds before
            returning -- when it triggers Undelete's daily purge.

          ■ In DoubleSpace compressed volumes, the available and free space
            values are estimates, based on the average compression ratio
            obtained for data currently on the drive.

See Also: Fn 32H (get disk info)
          Fn 1bH (get FAT info)
          DOS Functions