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From: djhr@crosfield.co.uk (dave redman)
Newsgroups: comp.sys.acorn
Subject: virtual memory
Keywords: virtual memory
Message-ID: <10396@suns502.crosfield.co.uk>
Date: 25 Jun 91 10:16:35 GMT
Article-I.D.: suns502.10396
Organization: Crosfield Electronics, Hemel Hempstead, United Kingdom.
Lines: 40

About Virtual Memory,

   I actually wrote some code ( C and assembler ) that enables page
swapping to and from memory to harddisc. I wrote to Acorn asking about how
to hook it into riscos and was told in NO uncertain terms that riscos CAN
NOT use virtual memory.

   It appears that this is because any page swap that occurs in supervisor
mode will corrupt the supervisor return link, thus the code cannot re-start
after the page fault. The only cure to this is to save the return link on
entry to every supervisor mode subroutine that MAY get swapped.

  I managed to get BASIC to read and write to virtual memory including
swapping, but I effectively reduced the amount of memory available for
swapping by marking VIDEO (obviously), RMA and sub 8000 address space as
NON mobile.

  I eventually gave up after getting the DATA-ABORT routine, and half the
PREFETCH-ABORT working because I discovered that page faults that occur on
instruction fetchs at a page boundary do funny things and needs extra code
to fix it. I couldn't be bothered to work it out and after Acorn said they
wouldn't tell me how to hook it into riscos anyway I just gave up.

  Anyway if you want to talk over my approach etc e-mail me.

  Actually, does anyone out there in netland know how I can convince the
harddisc controller that the disc is partitioned ? ( means convincing
riscos too ). I ask because the current version uses direct disc address
writes for speed ( but via adfs ) but if the disc was compacted or the file
( that I use as swap space ) is not contigous then the disc gets trashed !!!!
Having the ability to write to different partitions would solve this..

  Ideas and sugestions will be gratefully accepted.. and I know it is
possible because riscix MUST use partitions.

  Acorn - a chance at last for you to redeem yourself in my eyes !!

dave.

djhr@crosfield.co.uk