Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!sun-barr!texsun!pollux!killer!chasm
From: chasm@killer.DALLAS.TX.US (Charles Marslett)
Newsgroups: comp.arch
Subject: Re: 386/486 Virtual Memory Question...
Summary: There really is no exit!
Message-ID: <8313@killer.DALLAS.TX.US>
Date: 8 Jun 89 04:32:16 GMT
References: <MYX2SzW00XoF42d1lf@andrew.cmu.edu>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 24

In article <MYX2SzW00XoF42d1lf@andrew.cmu.edu>, hs0l+@andrew.cmu.edu (Hugh Brinkley Sprunt) writes:
> 
> The 386 and 486 architectures claim a virtual address space size of
> 2^46 bytes.  The virtual address is formed from a 14 bit selector
> and a 32 bit offset.  What does this really mean?  . . .

[stuff omitted]

> 	a) This scheme gives us 2^14 different ways to map into
> 	   the same 32 bit address space.
This is it:  the chip has 32 address pins so everything has to be mapped
down into the 2^32 address space before it can go to memory.  Unlike some
architectures, with tag fields for caches and such, the Intel design does
not provide for anything else.  On the other hand, in a virtual memory
environment, the software can use those extra bits of information so the
virtual address space (on the very, very big swapping disk) can be IMMENSE.

> Brinkley Sprunt
> Elecetical & Computer Engineering
> Carnegie Mellon University
> sprunt@maxwell.ece.cmu.edu

Charles Marslett
chasm@killer.dallas.tx.us