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From: alan@pdn.UUCP (Alan Lovejoy)
Newsgroups: comp.arch
Subject: Re: Definition of "virtual memory" (was Re: Free Software Foundation)
Message-ID: <1500@pdn.UUCP>
Date: Fri, 9-Oct-87 09:46:45 EDT
Article-I.D.: pdn.1500
Posted: Fri Oct  9 09:46:45 1987
Date-Received: Sun, 11-Oct-87 17:39:47 EDT
References: <738@hplabsz.HPL.HP.COM> <76700006@uiucdcsp> <1087@vaxb.calgary.UUCP>
Reply-To: alan@pdn.UUCP (0000-Alan Lovejoy)
Organization: Paradyne Corporation, Largo, Florida
Lines: 33

In article <1087@vaxb.calgary.UUCP> radford@calgary.UUCP (Radford Neal) writes:
>By this definition various old processors - e.g. the CDC 6600 - would
>be judged to have "virtual memory" - but we didn't say they did back then.
>It's a lot more consistent with established usage to call this a "memory
>management" facility. Unfortunately, I've noticed a tendency to redefine
>"memory management" recently so that it means anything that manages 
>memory - i.e. just about any program or machine whatsoever.
>

There is a significant difference between "Logical Address Space"
virtual memory and "True" virtual memory.  The confusion arises because
the former is a necessary implementation technique for the latter.
In common usage "virtual memory" can mean either one, unfortunately.
Perhaps we should seek to change this, and refer to the case where
the addresses generated by the program are mapped onto a separate
set of physical addresses as "Logical Addressing" and reserve the term
"Virtual Memory" for those systems whose logical address spaces are
larger than their physical address spaces.

Perhaps we should distinguish between systems where 

  1) the sum of all processes' (logical) address spaces must be less than or 
  equal to the physical address space (Logical Addressing),
  
  2) each process's logical address space must be less than or equal to the 
  physical address space, but the sum of all the address spaces can be
  greater than the physical address space (Inter-Process Virtual
  Memory), and
  
  3) each process's logical address space can be greater than the
  physical address space (Intra-Process Virtual Memory).

--alan@pdn