Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!sdcsvax!ucbvax!DBSTU1.BITNET!I7100501
From: I7100501@DBSTU1.BITNET.UUCP
Newsgroups: comp.os.vms
Subject: RE: buffered i/o count
Message-ID: <8709200626.AA15095@ucbvax.Berkeley.EDU>
Date: Sun, 20-Sep-87 02:26:36 EDT
Article-I.D.: ucbvax.8709200626.AA15095
Posted: Sun Sep 20 02:26:36 1987
Date-Received: Sun, 20-Sep-87 19:37:15 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: world
Organization: The ARPA Internet
Lines: 29

Todd Warnock asked:

>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.

Buffered I/O is a method (as opposed to Direct I/O) of transfering data from a
user process to the driver. Buffered I/O uses an intermediate system buffer in
non-paged pool which interacts with the process in process context and to which
the driver has access in fork context. If such a buffer is allocated for a user
process, its buffered i/o count is incremented. If the buffer is released, it
is decremented. Obviously, resources for buffered i/o are limited, so quotas
are used.

If the driver uses direct i/o, the process pages containing the user buffer
are locked in memory and the driver can access it directly. The direct i/o
count is incremented if pages are locked and decremented when released.

Normally all DMA devices uses direct i/o, whereas non-DMA devices may use
buffered i/o. The driver determines what transfer mode is used.

This and more information is in the "Guide to Writing a Device Driver".

-----------------------------------------------------------------------------
Eckart Meyer                            BITNET: I7100501@DBSTU1
Inst. f. Nachrichtentechnik             PHONE:  0531 / 391-2454
Technical University of Braunschweig    ADDR:   Schleinitzstr. 23
Germany                                            3300 Braunschweig
-----------------------------------------------------------------------------