Path: utzoo!mnetor!tmsoft!torsqnt!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!mcsun!ukc!edcastle!dcl-cs!cam-cl!news
From: nbvs@cl.cam.ac.uk (Nicko van Someren)
Newsgroups: comp.sys.acorn
Subject: Re: MEMC and video DMA question
Message-ID: <1991Feb14.181211.3078@cl.cam.ac.uk>
Date: 14 Feb 91 18:12:11 GMT
References: <1277@culhua.prg.ox.ac.uk>
Reply-To: nbvs@cl.cam.ac.uk (Nicko van Someren)
Organization: U of Cambridge Comp Lab, UK
Lines: 34

In article <1277@culhua.prg.ox.ac.uk> as@prg.ox.ac.uk (Andrew Stevens) writes:
>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?

One MEMC is the master MEMC and all others are slave MEMCs.  A MEMC knows
which it is by the state of the byte/word line when the last reset took place.

>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?

No.  The CPU memory accesses and the video DMA go along the same data bus
so one must hold up the other even if there is more than one bank or RAM.

>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 rumour you heard was right.  MEMC provides DMA for video and sound but
has no general DMA.

>Andrew

Nicko



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| Nicko van Someren, nbvs@cl.cam.ac.uk, (44) 223 358707 or (44) 860 498903    |
|       "Go and buy an Aleph One ARM3 card and stop whining!!!"               |
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