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From: farren@hoptoad.UUCP
Newsgroups: comp.arch
Subject: Re: Using a DMA chip in strange ways
Message-ID: <1839@hoptoad.uucp>
Date: Wed, 18-Feb-87 03:23:57 EST
Article-I.D.: hoptoad.1839
Posted: Wed Feb 18 03:23:57 1987
Date-Received: Thu, 19-Feb-87 06:52:47 EST
References: <4343@columbia.UUCP>
Reply-To: farren@hoptoad.UUCP (Mike Farren)
Distribution: world
Organization: Nebula Consultants in San Francisco
Lines: 26

In article <4343@columbia.UUCP> dupuy@amsterdam.columbia.edu (Alexander Dupuy) writes:
>
>While reading Tanenbaum's new OS book (the Minix book) a sort of half baked
>idea came to me.

 No, your idea seems fully baked :-)

>    Since the DMA chip on your favorite disk/tape controller works by stealing
>bus cycles when the CPU is busy with other things (like arithmetic), would
>there be any advantage in having a DMA chip which would simply be used for
>memory to memory copies (from user to kernel space, or from one user space to
>another)?

  Many DMA circuits that are not designed for a single purpose (i.e., disk
controller to memory transfer) can be used like this, and it IS a good idea,
as long as the system is not DMA intensive.  In particular, a number of micros
(specifically, the Amiga) have this capability, and use it.  Particularly good
if you are copying large blocks of memory to other memory spaces, a task often
associated with graphics, but otherwise very useful.  DMA can usually do the
move from two to four times faster than even a tightly-coded loop.

-- 
----------------
                 "... if the church put in half the time on covetousness
Mike Farren      that it does on lust, this would be a better world ..."
hoptoad!farren       Garrison Keillor, "Lake Wobegon Days"