Path: utzoo!mnetor!uunet!husc6!uwvax!oddjob!ncar!gatech!purdue!i.cc.purdue.edu!j.cc.purdue.edu!pur-ee!uiucdcs!uiucdcsm!grunwald From: grunwald@uiucdcsm.cs.uiuc.edu Newsgroups: comp.arch Subject: Re: Today's dumb question... Message-ID: <3300026@uiucdcsm> Date: 10 May 88 14:32:00 GMT References: <674@cernvax.UUCP> Lines: 24 Nf-ID: #R:cernvax.UUCP:674:uiucdcsm:3300026:000:976 Nf-From: uiucdcsm.cs.uiuc.edu!grunwald May 10 09:32:00 1988 VM offers more than protection from errant programs. You can use it to: + do cheap heap based allocation. See the DEC-SRC report by Li and Appel (and someone else) on a heap based allocation scheme which uses page level protection to make heap allocation almost as efficient as stack based allocation. + Process migration. See Zayas (sp?) in the last SOSP. Using demand paging for process migration *even given crappy hardware* was a big win. + Cheap memory copies, less memory fragmentation, etc. + You *need* paging hardware for the software solutions to shared memory hardware. Also, I want a single-user, multi-programming machine. That means I *still* need protection -- from myself. As for caches being expensive. Well, if you presume that SRAM costs drop faster than cache controller chips, maybe. Even if you put 32Mb in a system, it's not going to help if you can only afford 32Mb of 150ns DRAM but you need 25ns SRAM to pump your system.