Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!mcsun!ukc!cam-cl!news
From: nbvs@cl.cam.ac.uk (Nicko van Someren)
Newsgroups: comp.sys.acorn
Subject: Virtual Memory (Was: SCSI interfaces - background capability)
Message-ID: <1991Jun30.103408.24281@cl.cam.ac.uk>
Date: 30 Jun 91 10:34:08 GMT
References: <9106251106.AA05878@ucbvax.Berkeley.EDU> <1991Jun27.212244.5025@rata.vuw.ac.nz> <KERS.91Jun28093522@cdollin.hpl.hp.com>
Reply-To: nbvs@cl.cam.ac.uk (Nicko van Someren)
Organization: U of Cambridge Comp Lab, UK
Lines: 39

Back when RISC OS first came out there was very little need for VM on the
Archimedes simply because there was very little software around to run.  Now
that users have a large software base at their disposal they are tempted to use
it and use up memory in the process.  As far as I can see there are two main
problems involved in giving RISC OS virtual memory.  Firstly the kernel and 
modules of the OS do not cope well with data aborts and secondly not every one
has a fast 'swap device' to use as the backing store.

The first problem is in fact not as bad is it might be.  In RISC OS tasks only
ever get switched during a WIMP poll.  If we made the provisions that a) the
memory in the RMA, screen (naturally) and system areas never got swapped out
and b) No task could have a WIMP slot bigger that the physical memory size then
we could forego true VM for paged applications.  A task could ask for a slot
of any size as long as it was capable of fitting in memory in one go and then
all or part of the WIMP slot could be stored to disk when the task was not
the active one.  While this has many drawbacks, for instance the system does
not know when a page has been touched so it must save any page it wants to 
swap out, it does give a workable VM system as far as the user is concerned and
further more the only changes to code that need to be made are rewrites of the
WIMP paging code and the WIMP call that fetches memory from another tasks work
space.

The second problem, that not all users have a fat paging device, is in my
opinion not a reason not to do the job.  RISC OS file switch is fast enough
that the WIMP could be made to page to a file, the size of which could be set
by the user.  Since any application that is paged in is always all in memory
when it is running, and since the filer application is in the RMA, you could
even set up to page to an 800K file on a floppy on an A3000 and file switch
and ADFS would prompt you to change discs at the right moment.  Users of
bigger machines could use there hard discs and if you have eco/ether net then
the paging could happen there.

Acorn have, in the recent past, declared that they are a 'Market driven 
company'.  We, the users of usenet, are your market Acorn.  SO GET ON WITH IT!

Nicko
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| Nicko van Someren, nbvs@cl.cam.ac.uk, (44) 223 358707 or (44) 860 498903    |
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