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From: as@prg.ox.ac.uk (Andrew Stevens)
Newsgroups: comp.sys.acorn
Subject: MEMC and video DMA question
Message-ID: <1277@culhua.prg.ox.ac.uk>
Date: 14 Feb 91 13:03:49 GMT
Sender: news@prg.ox.ac.uk
Reply-To: as@prg.ox.ac.uk (Andrew Stevens)
Organization: Oxford University Computing Laboratory, UK
Lines: 36


A question to the knowledgeable:

The recent comments on speeding up Arch's when using big modes
reminds me of something that has puzzled me for some time about
the Arch architecture:

In a basic Arch you get contention between the video-subsystem and the
CPU - the MEMC multiplexes the two onto the same memory sub-system.  It
even generates the video-DMA addresses as I understand it. So, what
happends in a machine with 2 or more MEMC's?

Can you fix things up so that RAM controlled by one MEMC supports video
DMA (and runs slow), whilst a second bank with its own MEMC runs at full
tilt.  In an A540, for example, does the machine run faster when more
than one RAM bank is added?

Also, does MEMC support general-purpose DMA as well as video?
E.g. for winchesters etc, or is all that sort of stuff (as I
heard rumoured) done by the main CPU?

The latter would at least partially explain the reputedly poor
through-put of Rxy0 UNIX systems when paging.  A 32K page size certainly
doesn't help, but it shouldn't impact through-put all that much. If the
system isn't thrashing other runnable processes should happily soak up
CPU time while a page-fault is fixed.  However, if the CPU was *busy*
during page-faults sluggish performance during paging would be no
suprise at all.



Andrew
        Andrew Stevens                  
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