Function 36h            Get Disk Free Space

    Returns the number of available clusters, and other information
    about the disk in the specified drive.

        Entry   AH = 36h
                DL = Drive number (0=default, 1=A, 2=B, etc.)

        Return  AX = FFFFh if function failed
                otherwise
                AX = Number of sectors per cluster
                BX = Number of available clusters on the disk
                CX = Number of bytes per sector
                DX = Total number of clusters on the disk

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    The number of free bytes on the disk can be calculated by
    multiplying the available clusters by the sectors per cluster by
    the bytes per sector (BX * AX * CX).

    DOS reports sectors allocated in the file allocation table (FAT)
    but not belonging to a file (lost clusters) as used clusters, just
    as if they were allocated to a file.