Compatibility: 3.3+
Expects: AH    68H
BX    file handle of file to flush
                                                                  
Returns: AX    error code if CF is set to CY
                                                                  
Info: This function forces DOS to flush its RAM buffers of file data
for the selected file handle.

DOS normally avoids disk I/O by writing data to RAM buffers until
a sector is full or the file is closed.  This provides a way to
force DOS to write the buffered data to disk.

Versions:   Prior to DOS 3.3, you can force a file flush by closing, then
reopening the file.  But by using fn 45H to duplicate the
handle and then close the duplicate.

  Starting with DOS 4.0, you can force a flush on each write by
opening the file via fn 6cH (extended file open/create).

Notes:   This does NOT flush the Smartdrv cache buffers (the data may
sit in memory for a few seconds).  You can force Smartdrv to
write all data by using fn 0dH (reset disk).

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DOS Fn 68H: Commit File