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From: leichter@VENUS.YCC.YALE.EDU.UUCP
Newsgroups: comp.os.vms
Subject: re: buffered i/o count
Message-ID: <8709191208.AA03446@ucbvax.Berkeley.EDU>
Date: Fri, 18-Sep-87 10:06:00 EDT
Article-I.D.: ucbvax.8709191208.AA03446
Posted: Fri Sep 18 10:06:00 1987
Date-Received: Sun, 20-Sep-87 13:44:09 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: "Jerry Leichter" <leichter@venus.ycc.yale.edu>
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Organization: The ARPA Internet
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	Where does BUFFERED I/O count come from ?  Does DMA affect it ?  What 
	determines when it gets incremented ?  Pointers to manuals would be
	appreciated.  I need to know for accounting purposes.

VMS I/O can be done using either of two mechanims:  Direct or buffered.

The direct I/O mechanism lets the I/O device read or write memory within your
process directly:  The pages are locked down into physical memory, the device
is told where they are and then told to "go".

The buffered I/O mechanism allocates a buffer in system space (charging
against the process's BYTLM quota); the I/O device is given access to the
buffer, and the data transferred is copied to/from the process's address
space.

The choice of which mechanism to use is entirely up to the writer of the
device driver.  In theory, a driver could even use different mechanisms for
different kinds of I/O requests - e.g., direct output and buffered input.  I
know of no driver that does this; I doubt any has ever been written (though
technically non-I/O QIO's like SETMODE are "direct" even for otherwise
buffered drivers).

There is no completely general principle that determines which drivers use
the direct mechanism, and which the buffered.  Disks and tapes use direct I/O;
in general, you'd expect "high-speed" devices to do so, though there are
exceptions, like the Ethernet drivers (which need to deal with link-level
encapsulation).  Terminals, line printers, and similar devices are buffered.

Writing a driver that uses direct I/O is somewhat more involved than writing
one that uses buffered I/O.

Direct I/O devices are almost certain to use DMA, but plenty of buffered
devices use it too - even many terminal interfaces.
							-- Jerry
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