Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!cmcl2!rutgers!iuvax!pur-ee!uiucdcs!uxc.cso.uiuc.edu!ccvaxa!aglew
From: aglew@ccvaxa.UUCP
Newsgroups: comp.arch
Subject: Re: Definition of "virtual memory"
Message-ID: <28200054@ccvaxa>
Date: Sun, 11-Oct-87 22:41:00 EDT
Article-I.D.: ccvaxa.28200054
Posted: Sun Oct 11 22:41:00 1987
Date-Received: Tue, 13-Oct-87 04:45:32 EDT
References: <1500@pdn.UUCP>
Lines: 43
Nf-ID: #R:pdn.UUCP:1500:ccvaxa:28200054:000:1643
Nf-From: ccvaxa.UUCP!aglew    Oct 11 21:41:00 1987


>  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 saces 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).
 	This isn't necessarily a good distinction.
	There have been, and will be again (as we pass the 32 bit limit)
	machines that came with more physical memory than could be
	addressed by a process (even with separate I/D, and so on).
	Yet these machines could take a page fault
>--alan@pdn

I think that the operative distinctions are:

    Memory Mapping (Logical Addressing)
	The address that a process generates is not the same as the address
	that the memory system sees.

	Sub-issue: how is mapping done?
	    By adding an offset to all addresses? (with bounds)
	    With multiple pages or segments?

	Sub-issue: are synonyms allowed?

    Non-resident memory (virtual memory) (faulting)
	Can memory not be resident?
	Can the machine take a fault?
	Sub-issues: how do you recover from a fault?
	    Instruction continuation/restart?
	    Checkpoint?
	    Imprecise faults? [*]


* By the way, it is possible to have imprecise page faults, and still
  be able to restart with sequential semantics, even without a checkpoint
  restore mechanism. I think I've already made a post to this newsgroup
  about it, but it may have gotten lost.