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From: jcb@lfcs.ed.ac.uk (Julian Bradfield)
Newsgroups: comp.os.misc
Subject: Re: a very naive Question???
Message-ID: <840@etive.ed.ac.uk>
Date: 12 Oct 88 18:57:48 GMT
References: <835@amethyst.ma.arizona.edu>
Sender: news@etive.ed.ac.uk
Reply-To: jcb@lfcs.ed.ac.uk (Julian Bradfield)
Organization: Laboratory for the Foundations of Computer Science, Edinburgh U
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In article <835@amethyst.ma.arizona.edu> chris@spock (Chris Ott) writes:

>heiby@mcdchg.UUCP (Ron Heiby) writes:
>> The actual definition of Virtual Memory involves the concept of allowing
>> programs to believe that they are running at a particular address, when
>> in fact they may actually be at a completely different *physical* address.

>     Wrong. The actual concept of virtual memory is the ability to allow
>programs to use more memory than the machine physically has by swapping
>the least recently used pages to secondary storage. For instance, if your

Not necessarily. You could have more real memory than total virtual
memory, and still have a virtual memory system. (One of the main
points of virtual memory is to increase the apparent size of memory,
but it's not a necessary part of the concept.) 

>     Swapping in a virtual memory system refers to the swapping of those
>"chunks" or pages to and from secondary storage. Most people actually call
>this "paging", since only pages of the process are swapped, rather than the
>whole process. We say that a process is "swapped out" when all of its pages
>have been placed on secondary storage. This usually only occurs on a heavily
>loaded system.

I call paging paging and swapping swapping! For example, MVS pages
memory; but there is also the concept of swapping out an address
space, which puts the whole (private part of the) address space on to
secondary storage, together with the page tables, with the result that
you can't do cross-memory communication with the address space, and no
task in it can be dispatched. The traditional Unix swapping concept
has much the same effect, does it not?