Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!lll-crg!lll-lcc!pyramid!ncr-sd!greg From: greg@ncr-sd.UUCP (Greg Noel) Newsgroups: net.arch Subject: Re: Paging Message-ID: <1162@ncr-sd.UUCP> Date: Mon, 8-Sep-86 16:23:39 EDT Article-I.D.: ncr-sd.1162 Posted: Mon Sep 8 16:23:39 1986 Date-Received: Mon, 8-Sep-86 20:50:22 EDT References: <8494@duke.duke.UUCP> <147@eneevax.UUCP> <536@cubsvax.UUCP> <654@hropus.UUCP> <7092@utzoo.UUCP> Reply-To: greg@ncr-sd.UUCP (Greg Noel) Organization: NCR Corporation, San Diego Lines: 27 In article <7092@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes: >> ... Does virtual memory imply demand paging or just address translation? > >"Virtual memory", strictly speaking, refers to memory that acts like it's >always there but sometimes isn't. That is, demand paging or some analogous >scheme (paging is not the only memory-management technique possible, and >demand paging is not the only page-fetching policy possible). Address >translation, by itself, is not virtual memory. Hmmmmm..... I think I want to take issue with this. Although what Henry says is valid, his point is too narrow. Two immediate counter-examples are the SDS-940 (later the XDS-940) and the PDP-11, both of which are virtual- memory machines, and yet essentially just only do address translation. (I know, I know, later versions of the PDP-11 can restart instructions; the point remains.) 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. That is, the operating system can assign arbitrary portions of real memory to the program and this mapping can change over time. In this context, some amusing definitions: Real: You've got it, and you know you've got it. Transparent: You've got it, but you don't know you've got it. Virtual: You don't have it, but you don't know it. -- -- Greg Noel, NCR Rancho Bernardo Greg@ncr-sd.UUCP or Greg@nosc.ARPA