Path: utzoo!attcan!uunet!aplcen!uakari.primate.wisc.edu!uflorida!mephisto!mcnc!ecsvax.uncecs.edu!dukeac!wolves!ggw From: ggw@wolves.uucp (Gregory G. Woodbury) Newsgroups: comp.arch Subject: Re: Physical >= Virtual Summary: split spaces make that 2GB Message-ID: <1989Nov26.061739.6074@wolves.uucp> Date: 26 Nov 89 06:17:39 GMT References: <6808@pt.cs.cmu.edu> <593@gp.govt.nz> Reply-To: ggw@wolves.UUCP (Gregory G. Woodbury) Followup-To: comp.arch Organization: Wolves Den UNIX BBS Lines: 26 In article <593@gp.govt.nz> don@gp.govt.nz (Don Stokes, GPO) writes: >In article <6808@pt.cs.cmu.edu>, lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay) writes: >> My trusty VAX Architecture Handbook states that a process may have at >> most 2 GB of virtual memory. > >Of course that is 2GB *virtual*. > >But my VAX/VMS internals manual gives the page table entry's page frame >number size as 21 bits = 2^21(pfn) * 512(page size) = 1GB. As sometimes occurs, both are correct. The VAX architecture features a split instruction and data space, and there are page tables for each of the spaces. 1 GB of instruction space + 1 GB of data space = 2 GB of virtual address space. This is not to mention the other bit (of course) that indicates that you are accessing the 2 GB of virtual space reserved for the system, where all the real interesting stuff is -- the real memory, the i/o devices and the page tables themselves. -- Gregory G. Woodbury Sysop/owner Wolves Den UNIX BBS, Durham NC UUCP: ...dukcds!wolves!ggw ...dukeac!wolves!ggw [use the maps!] Domain: ggw@cds.duke.edu ggw@ac.duke.edu ggw%wolves@ac.duke.edu Phone: +1 919 493 1998 (Home) +1 919 684 6126 (Work) [The line eater is a boojum snark! ]