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From: ted@cgl.ucsf.edu@blia.UUCP
Newsgroups: mod.computers.vax
Subject: Re: BIO's vs. DIO's
Message-ID: <8701301850.AA05273@blia.BLI>
Date: Fri, 30-Jan-87 13:50:49 EST
Article-I.D.: blia.8701301850.AA05273
Posted: Fri Jan 30 13:50:49 1987
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In article <8701291247.AA11373@ucbvax.Berkeley.EDU>, RMALOUF@SBCCMAIL.BITNET writes:
> [question on direct i/o's vs buffered i/o's]

Each device driver specifies whether it does direct or buffered i/o's. Direct
i/o's occur when the driver sets up a device DMA directly into or out of the
user's buffer. This produces a low-CPU-overhead transfer but the I/O must
complete quickly because the user's pages are locked in physical memory and
Unibus mapping registers are allocated (on Unibus devices). Disks and tapes
generally do direct i/o.

Buffered i/o's have the executive copy the user's buffer to or from non-
paged pool and the driver does i/o to or from this buffer. There is the
increased overhead of copying the buffer but this allows the driver to
modify the data (for example, put protocol headers on for ethernet devices),
do i/o to slow devices without locking the user's buffer, or even do non-DMA
i/o. Terminals, line-printers, and network devices are good examples of
devices using buffered i/o.

For normal users, disk and tape i/o is generally direct and terminal i/o is
generally buffered.

Hope this helps.

===============================================================================
            Ted Marshall
            Britton Lee, Inc.
p-mail:     14600 Winchester Blvd, Los Gatos, Ca 95030
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disclaimer: These opinions are my own and may not reflect those of my employer;
            I leave them alone and they leave me alone.
fortune for today:
Green light in A.M. for new projects.  Red light in P.M. for traffic
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