Function 3Eh            Close File with Handle

    Closes an open file.

        Entry   AH = 3Eh
                BX = File handle

        Return  AX = Error code, if CF is set
                     | 06h Invalid handle

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    This function flushes the file's buffers, unlocks any locked
    regions of the file, releases the handle, and updates the disk
    directory to reflect any changes in the file size, date, or time.

    Although closing a file invalidates the corresponding handle, DOS
    may reuse the handle to identify a file that is subsequently
    opened or created. Programs can use Is File or Device Remote
    (function 440ah) to determine whether a given handle is valid.