Function 36h (54)        Get Disk Free Space

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

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

       Returns:       AX         Sectors per cluster
                                 FFFFh if invalid drive specified
                      BX         Number of available clusters
                      CX         Bytes per sector
                      DX         Total number of clusters per drive

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    This function returns information similar to that provided by
    Functions 1Bh and 1Ch, with three exceptions. This function:

         ■   returns the number of available clusters;
         ■   does not return a pointer to the media descriptor byte;
         ■   returns an error code if the specified drive is invalid.