Path: utzoo!yunexus!geac!syntron!jtsv16!uunet!husc6!mailrus!uflorida!gatech!hubcap!mark
From: mark@hubcap.UUCP (Mark Smotherman)
Newsgroups: comp.os.misc
Subject: Re: a very naive Question???
Summary: definitions
Message-ID: <3211@hubcap.UUCP>
Date: 11 Oct 88 19:43:00 GMT
Article-I.D.: hubcap.3211
References: <835@amethyst.ma.arizona.edu>
Organization: Clemson University, Clemson, SC
Lines: 80

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.


It's a shame that we can't agree on terminology.  I believe that we have
been too heavily influenced by marketing types.

The older academic textbooks agree with Ron.  E.g., A.M. Lister (1st ed.
1975, - 3rd ed. 1984) states "... the programmer 'sees', and programs for,
a _virtual memory_ whose characteristics differ from those of the real
memory."  Lister then goes on to a section on "Implementation of Virtual
Memory" that _begins with_ base and limit registers (i.e. GE-635, Honeywell
6000, Univac 1100, CDC 6600, etc.).  Obviously these machine did not
support demand paging.

However, Chris may just be out of school and may have just taken an OS
course using a textbook such as Silberschatz and Peterson (alt. ed. 1988).
These authors state "Virtual memory is a technique that allows the execution
of processes that may not be completely in memory.  The main visible
advantage of this scheme is that user programs can be larger than physical
memory."

I have fun in class with those of the second persuasion when I bring up
the DECsystem 10 model KI10, which has a 24-bit _physical address_ and
an 18-bit _virtual address_!  DEC used a paging scheme and a large
physical memory to balance these systems and extend the life of the 10
series - thus, as the CPU got faster it needed more memory and more ready
processes, yet for compatibility purposes the older 18-bit software still
had to run.  According to your definition, Chris, is this _paging_ system
a virtual memory system???


> 
> > The definition the author of the article (and the UW editor) are mistakenly
> > using involves the concept of *demand paging*.
> > ... It's just that the sharing of memory by more programs than would
> > physically fit all at once was done by a process called "swapping".
> 
>      Close, but not quite. Demand paging involves only the code portion of
                                                    ^^^^^^^^^^^^^^^^^^^^^
> the program.


Page only the code portion?  News to me!


> 
> >                    Anyway, even the "swapping" style of memory sharing
> > uses Virtual Memory.
> 
>      Wrong. The definition swapping depends on what type of memory you
> have (virtual or non-virtual).
>      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.


The base and limit scheme mentioned above uses swapping.


> 
> > Ron Heiby, heiby@mcdchg.UUCP    Moderator: comp.newprod & comp.unix
> >
>  Chris Ott
>  Computational Fluid Mechanics Lab            Just say "Whoa!!" and
>  University of Arizona                          vote for Randee!!

-- 
Mark Smotherman, Comp. Sci. Dept., Clemson University, Clemson, SC 29634
INTERNET: mark@hubcap.clemson.edu    UUCP: gatech!hubcap!mark