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From: cet1@cl.cam.ac.uk (C.E. Thompson)
Newsgroups: comp.arch
Subject: Re: 64 bits
Message-ID: <1990Aug18.140853.12305@cl.cam.ac.uk>
Date: 18 Aug 90 14:08:53 GMT
References: <1990Aug17.215527.5822@nlm.nih.gov>
Reply-To: cet1@cl.cam.ac.uk (C.E. Thompson)
Organization: U of Cambridge Comp Lab, UK
Lines: 17

In article <1990Aug17.215527.5822@nlm.nih.gov> states@tech.NLM.NIH.GOV (David States) writes:
>Sure for physical memory and we are all spoiled by workstations with
>enough RAM that nothing ever page faults anymore (:-), but DEC started
>shipping VAXes with 128k of RAM, a 70 MB disk and a 4GB virtual address
>space (and it *did* page fault).  Virtual space > physical space can be
>useful for some problems.  

The VAX had (has) a 4GB virtual address space, but you can't spread objects
throughout the full range. To keep the page tables manageable you have to
use P0 space from the bottom, P1 space from the top, and S space from the
bottom. (Interchange top and bottom if you draw your pictures the other
way up...) What sort of virtual memory architecture are the 64-bit address
proponents envisaging? Reverse map?

Chris Thompson
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