On 2018-02-16 22:35, Larry Baker wrote:
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.
Yes. We have a conflation of two different things. A rather common
mixup, unfortunately. Even more, though, while your comments below are
accurate, overlays are commonly not done through the MMU at all, so that
is yet another aspect. (That said, under RSX, there is the option of
having overlay mapping done through the MMU, which is faster, but
potentially waste a lot of memory space.)
Johnny
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). 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
ba...@usgs.gov <mailto:ba...@usgs.gov>
Date: Fri, 16 Feb 2018 14:51:27 -0500
From: Clem Cole <cl...@ccc.com <mailto:cl...@ccc.com>>
To: Ethan Dicks <ethan.di...@gmail.com <mailto:ethan.di...@gmail.com>>
Cc:mikestra...@gmail.com <mailto:mikestra...@gmail.com>, SIMH
<simh@trailing-edge.com <mailto:simh@trailing-edge.com>>
Subject: Re: [Simh] pdp11 i/o addressing
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curmudgeon warning below.....
Date: Fri, 16 Feb 2018 20:23:05 +0000
From: Bob Eager <r...@tavi.co.uk <mailto:r...@tavi.co.uk>>
To: simh@trailing-edge.com <mailto:simh@trailing-edge.com>
Subject: Re: [Simh] pdp11 i/o addressing
Message-ID: <20180216202305.07e19...@raksha.tavi.co.uk
<mailto:20180216202305.07e19...@raksha.tavi.co.uk>>
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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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--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: b...@softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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