1. To delete something, usually superfluous, or to abort an operation.
"Flush" was standard ITS terminology for aborting an output operation. One spoke of the text that would have been printed, but was not, as having been flushed. It is speculated that this term arose from a vivid image of flushing unwanted characters by hosing down the internal output buffer, washing the characters away before they could be printed.
2. To force temporarily buffered data to be written to more permanent memory. E.g. flushing buffered disk I/O to disk, as with C 's standard I/O library "fflush(3)" call. This sense was in use among BLISS programmers at DEC and on Honeywell and IBM machines as far back as 1965. Another example of this usage is flushing a cache on a context switch where modified data stored in the cace which belongs to one processes must be written out to main memory so that the cache can be used by another process.
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