[Simh] pdp11 i/o addressing

[Larry Baker]

Sorry Clem and Bob,

I think you are mixing apples and oranges and adding confusion with your use of VM to describe two different things.  Relying on my own recollections of the early days of virtual memory...  What the PDP-11 does is memory mapping with an MMU.  Virtual memory is a concept, described in a seminal ACM article by Peter Denning (https://en.wikipedia.org/wiki/Peter_J._Denning <https://en.wikipedia.org/wiki/Peter_J._Denning>).  Memory mapping and virtual memory both require that whatever code or data that a program is permitted to access must actually be resident in memory.  While there is hardware in an MMU to trap unmapped memory accesses, in a true VM architecture there is no aliasing of memory addresses to multiple code or data objects.  Whether overlaying is done by overwriting already mapped program memory from storage or by remapping program address space to another memory block, the same memory addresses are reused to reference different objects.  The MMU cannot tell whether the object being referenced is the "correct" object intended by the programmer.  Other methods must be employed, such as an indirect "entry-point table" that can perform the MMU remapping on-the fly.  In a true VM architecture, different objects can be distinguished by their program addresses.  This is a key distinction between memory mapping and virtual memory.

That's my 2¢.

Respectfully,

Larry Baker
US Geological Survey
650-329-5608
baker at usgs.gov



> Date: Fri, 16 Feb 2018 14:51:27 -0500
> From: Clem Cole <clemc at ccc.com <mailto:clemc at ccc.com>>
> To: Ethan Dicks <ethan.dicks at gmail.com <mailto:ethan.dicks at gmail.com>>
> Cc: mikestramba at gmail.com <mailto:mikestramba at gmail.com>, SIMH <simh at trailing-edge.com <mailto:simh at trailing-edge.com>>
> Subject: Re: [Simh] pdp11 i/o addressing
> Message-ID:
> 	<CAC20D2NgAew9CbkitU6sA14hSNsaA1iaNHRCKdt-q0oikbd8Og at mail.gmail.com <mailto:CAC20D2NgAew9CbkitU6sA14hSNsaA1iaNHRCKdt-q0oikbd8Og at mail.gmail.com>>
> Content-Type: text/plain; charset="utf-8"
> 
> curmudgeon warning below.....
> 

> Date: Fri, 16 Feb 2018 20:23:05 +0000
> From: Bob Eager <rde at tavi.co.uk <mailto:rde at tavi.co.uk>>
> To: simh at trailing-edge.com <mailto:simh at trailing-edge.com>
> Subject: Re: [Simh] pdp11 i/o addressing
> Message-ID: <20180216202305.07e199d3 at raksha.tavi.co.uk <mailto:20180216202305.07e199d3 at raksha.tavi.co.uk>>
> Content-Type: text/plain; charset=ISO-8859-1
> 
> When I teach virtual memory, I start by talking about the 'old days'
> and overlays. I remember Overlay Description Language on the PDP-11!
> 

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