Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!gatech!akgua!usl!elg From: elg@usl.UUCP (Eric Lee Green) Newsgroups: net.arch Subject: Re: Paging Message-ID: <913@usl.UUCP> Date: Sat, 13-Sep-86 13:33:03 EDT Article-I.D.: usl.913 Posted: Sat Sep 13 13:33:03 1986 Date-Received: Mon, 15-Sep-86 18:44:26 EDT References: <8494@duke.duke.UUCP> <147@eneevax.UUCP> Reply-To: elg@usl.UUCP (Eric Lee Green) Organization: USL, Lafayette, La. Lines: 38 Summary: virtual machines, virtual memory Re: What is virtual memory?: One of the big programmer benefits of modern operating systems is that they give each user his own "virtual machine" to run his programs on, that is, to some extent, each program thinks it has the machine all to itself, and doesn't have to worry about what other processes are doing. Virtual memory is basicaly the "memory" of this virtual machine. For example, in the PDP-11 example -- each process thought it was running on a 64K machine. While on this Vax, each program thinks it is running on a 4 gigabyte machine. While on the 80286, the machine's virtual memory consists of the fact that it makes it look to each process as if it can access 64,000 64k segments. Note that paging is not the only way to do things -- the 80286 has no paging and yet its memory management scheme certainly qualifies as "virtual memory", while the PDP-11 also swapped out its 64K segments when something else needed to be run. But, basically, the point is: a) Each process is given its own virtual machine, which has a constant amount of memory, so that b) programs can be designed for that virtual amount of memory, and do not have to contain a bunch of special clauses for when the actual physical amount of memory changes sometime in the future. The grossest abuse of course is VM, where not only do they make it look like the process is running on its own processor, but has its own directly addressible i/o devices also. Talk about taking things to extremes! Eric -- Eric Green {akgua,ut-sally}!usl!elg (Snail Mail P.O. Box 92191, Lafayette, LA 70509) -- Tengo lo mismo que doy y solo sirve al presente.