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From: aglew@urbsdc.UUCP
Newsgroups: comp.arch
Subject: Re: Is the Intel memory model safe from
Message-ID: <28200148@urbsdc>
Date: 18 May 88 13:49:00 GMT
References: <353@cf-cm.UUCP>
Lines: 36
Nf-ID: #R:cf-cm.UUCP:353:urbsdc:28200148:000:1720
Nf-From: urbsdc.Urbana.Gould.COM!aglew    May 18 08:49:00 1988
Posted: Wed May 18 09:49:00 1988


>Hang on a sec--doesn't the fact that the VAX uses the first two address
>bits to determine which page table to use mean that it has Intel-ish
>segmentation?  Seems to me that 30 bits has been big enough so far.
>Has anyone out there ever written a program that used up a VAX's
>virtual memory?
>
>       Duke Robillard
>       AT&T Bell Labs           m10ux!rgr@att.UUCP                 
>       Murray Hill, NJ          {backbone!}att!m10ux!rgr

(1) Use of address bits to divide memory into sections is not the same
    as segmentation. In a segmented architecture, the segment address
    and the offset are logically separate. In machines that divide
    the addresses into fields, the implementation distinguishes between
    the fields, but the instruction set itself does not - using the 
    normal instructions that do arithmetic on addresses, one can 
    increment an address pointer and change the high order bit fields.

    eg. char *p; for(p=(char*)1;p;p++);
       will eventually step over all memory addresses, as long as it 
       doesn't fault. This is not true on a segmented machine, and is
       both the weak and strong point of segmentation.

(2) Yes, there are programs that can use more than 4GB of virtual 
    memory. 


Andy "Krazy" Glew. Gould CSD-Urbana.    1101 E. University, Urbana, IL 61801   
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