Function 10h (16)        Close File, Using FCBs

    Close a file and update the file directory, using a File Control Block
    (FCB).

       On entry:      AH         10h
                      DS:DX      Pointer to an opened FCB

       Returns:       AL         00h       If file closed
                                 FFH       If file not found

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       Notes:         If you're closing a file on a floppy disk, DOS
                      attempts to determine if the disk in the drive has
                      been changed, and returns 0FFh in AL if so. The
                      determination is not reliable in versions of DOS
                      prior to 3.0; using this function in DOS 2 when the
                      disk has been changed may result in a trashed disk.