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Path: utzoo!mnetor!seismo!columbia!topaz!husc6!panda!genrad!decvax!tektronix!orca!tekecs!kendalla@blast.gwd.tek.com (Kendall Auel)
From: kendalla@blast.gwd.tek.com (Kendall Auel)
Newsgroups: net.arch
Subject: Re: Paging
Message-ID: <7621@tekecs.UUCP>
Date: Tue, 9-Sep-86 15:25:36 EDT
Article-I.D.: tekecs.7621
Posted: Tue Sep  9 15:25:36 1986
Date-Received: Wed, 10-Sep-86 20:14:25 EDT
References: <8494@duke.duke.UUCP> <147@eneevax.UUCP>
Sender: news@tekecs.UUCP
Reply-To: kendalla@blast.UUCP (Kendall Auel)
Organization: Tektronix, Wilsonville OR
Lines: 34

In article <1162@ncr-sd.UUCP> greg@ncr-sd.UUCP (Greg Noel) writes:
>In article <7092@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes:
>>> ... Does virtual memory imply demand paging or just address translation?
>> ... Address translation, by itself, is not virtual memory.
> ... Perhaps more to the point would be to say that the memory image seen by
>     the program is independent of the real memory provided by the machine.

I had always thought that virtual memory meant address translation only, and
that a memory hierarchy (cache, RAM, disk, tape, ...) did not necessarily
exist in a virtual memory system. When I looked it up, however, my textbooks
speak of virtual memory as a way to map backing store into the address space
of the processor:

	"Virtual memory is a technique which 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."
	- Peterson & Silberschatz; _Operating_Systems_Concepts_

	"Virtual memory provides the user with a memory that can be larger
	than physical Mp. The virtual memory system handles overlays in a
	user-transparent manner by deferring the binding between user and
	physical addresses until instruction execution time."
	- Siewiorek, Bell, Newell; _Computer_Structures:_Principles_and_
	Examples_

I got two points from this reading. 1) Virtual memory *technically* implies
only address translation, and 2) The main advantage to using virtual memory
is to extend physical memory via a backing store (disk, drum, ...). Thus, the
concept of virtual memory for all intents and purposes implies swapping memory
to and from a disk, as well as address translation.

Kendall Auel
Tektronix, Inc.