From: jnc@mercury.lcs.mit.edu (Noel Chiappa)
Subject: [TUHS] /dev/drum
Date: Sat, 5 May 2018 09:06:53 -0400 (EDT) [thread overview]
Message-ID: <20180505130653.8D85618C079@mercury.lcs.mit.edu> (raw)
> From: Johnny Billquist
>> in MERT 'segments' (called that) were a basic system primitive, which
>> users had access to.
> the OS gives you some construct which can easily be mapped on to the
> hardware.
Right. "A logical segment is a piece of contiguous memory, 32 to 32K 16-bit
words long ... Associated with each segment are an internal segment
identifiern and an optional global name." So it's clear how that maps onto the
PDP-11 memory management hardware - and a MERT 'segment' might use more than
one 'chunk'.
>> I understand your definitions, and like breaking things up into
>> 'virtual addressing' (which I prefer as the term, see below),
>> 'non-residence' or 'demand loaded', and 'paging' (breaking into
>> smallish, equal-sized chunks), but the problem with using "virtual
>> memory" as a term for the first is that to most people, that term
>> already has a meaning - the combination of all three.
Actually, after some research, it turns out to be only the first two. But I
digress...
> It's actually not my definition. Demand paging is a term that have been
> used for this for the last 40 years, and is not something there is much
> contention about.
I wasn't talking about "demand paging", but rather your use of the term
"virtual memory":
>>> Virtual memory is just *virtual* memory. It's not "real" or physical
>>> in the sense that it has a dedicated location in physical memory
>>> ... Instead, each process has its own memory, which might be mapped
>>> somewhere in physical memory, but it might also not be. And one
>>> processes address 0 is not the same as another processes address
>>> 0. They both have the illusion that they have the full memory address
>>> range to them selves, unaware of the fact that there are many
>>> processes who also have that same illusion.
I _like_ having an explicit term for the _concept_ you're describing there; I
just had a problem with the use of the _term_ "virtual memory" for it - since
that term already has a different meaning to many people.
Try Googling "virtual memory" and you turn up things like this: "compensate
for physical memory shortages by temporarily transferring data from RAM to
disk". Which is why I proposed calling it "virtual addressing" instead.
> I must admit that I'm rather surprised if the term really is unknown to
> you.
No, of course I am familiar with "demand paging".
Anyway, this conversation has been very helpful in clarifying my thinking
about virtual memory/paging. I have updated the CHWiki article based on it:
http://gunkies.org/wiki/Virtual_memory
including the breakdown into three separate (but related) concepts: i) virtual
addressing, ii) demand loading, and iii) paging. I'd be interested in any
comments people have.
> Which also begs the question - was there also a RK11-A?
One assumes there much have been RK11-A's and -B's, otherwise they wouldn't
have gotten to RK11-C... :-)
I have no idea if both existed in physical form - one might have been just a
design exercise). However, the photo of the non-RK11-C indicator panel
confirms that at least one of them was actually implemented.
> And the "chunks" on a PDP-11, running Unix, RSX or RSTS/E, or something
> similar is also totally invisible.
Right, but not under MERT - although there clearly a single 'software' segment
might use more than one set of physical 'chunks'.
Actuallly, Unix is _somewhat_ similar, in that processes always have separate
stack and text/data 'areas' (they don't call them 'segments', as far as I
could see) - and separate text and data 'areas' too, when pure code is in
use; and any area might use more than one 'chunk'.
The difference is that Unix doesn't support 'segments' as an OS primitive, the
way MERT does.
Noel
next reply other threads:[~2018-05-05 13:06 UTC|newest]
Thread overview: 105+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-05 13:06 Noel Chiappa [this message]
2018-05-05 20:53 ` Johnny Billquist
[not found] <mailman.1.1525744802.16322.tuhs@minnie.tuhs.org>
2018-05-08 22:39 ` Johnny Billquist
-- strict thread matches above, loose matches on Subject: below --
2018-05-07 15:36 Noel Chiappa
2018-05-06 13:07 Noel Chiappa
2018-05-06 15:57 ` Johnny Billquist
2018-05-03 11:39 Noel Chiappa
2018-05-03 21:22 ` Johnny Billquist
2018-04-30 15:05 Noel Chiappa
2018-04-30 16:43 ` Paul Winalski
2018-04-30 21:41 ` Johnny Billquist
2018-05-03 2:54 ` Charles Anthony
2018-04-28 20:40 Noel Chiappa
2018-04-29 15:37 ` Johnny Billquist
2018-04-29 16:34 ` Steve Nickolas
2018-04-29 16:48 ` Warner Losh
2018-04-28 0:19 Noel Chiappa
2018-04-28 10:41 ` Johnny Billquist
2018-04-28 12:19 ` Rico Pajarola
2018-04-27 23:01 Noel Chiappa
2018-04-27 23:10 ` Johnny Billquist
2018-04-27 23:39 ` Warner Losh
[not found] <mailman.1.1524708001.6296.tuhs@minnie.tuhs.org>
2018-04-27 22:41 ` Johnny Billquist
2018-04-26 1:53 Noel Chiappa
[not found] <mailman.143.1524696952.3788.tuhs@minnie.tuhs.org>
2018-04-25 23:08 ` Johnny Billquist
2018-04-25 22:55 Noel Chiappa
[not found] <mailman.139.1524690859.3788.tuhs@minnie.tuhs.org>
2018-04-25 22:54 ` Johnny Billquist
2018-04-25 22:46 Noel Chiappa
[not found] <mailman.137.1524667148.3788.tuhs@minnie.tuhs.org>
2018-04-25 21:43 ` Johnny Billquist
2018-04-25 22:24 ` Johnny Billquist
2018-04-26 5:51 ` Tom Ivar Helbekkmo
2018-04-25 22:37 ` Johnny Billquist
2018-04-24 7:10 Rudi Blom
[not found] <mailman.125.1524526228.3788.tuhs@minnie.tuhs.org>
2018-04-23 23:44 ` Johnny Billquist
2018-04-23 23:57 ` Steve Nickolas
2018-04-24 0:24 ` Ronald Natalie
2018-04-24 0:25 ` Warren Toomey
2018-04-24 0:31 ` Dave Horsfall
2018-04-24 1:02 ` Lyndon Nerenberg
2018-04-24 4:32 ` Grant Taylor
2018-04-24 4:49 ` Bakul Shah
2018-04-24 4:59 ` Warner Losh
2018-04-24 6:22 ` Bakul Shah
2018-04-24 14:57 ` Warner Losh
2018-04-24 6:46 ` Lars Brinkhoff
2018-04-23 22:01 Noel Chiappa
2018-04-23 22:09 ` Warner Losh
2018-04-23 18:41 Noel Chiappa
2018-04-23 19:09 ` Clem Cole
2018-04-23 23:01 ` Dave Horsfall
2018-04-23 23:49 ` Dave Horsfall
2018-04-24 0:26 ` Ronald Natalie
2018-04-22 18:06 Norman Wilson
2018-04-20 16:45 Noel Chiappa
2018-04-20 16:53 ` Charles Anthony
2018-04-20 17:16 ` William Pechter
2018-04-20 23:35 ` Dave Horsfall
2018-04-22 11:48 ` Steve Simon
2018-04-20 15:02 Tim Bradshaw
2018-04-20 15:58 ` Clem Cole
2018-04-20 16:00 ` David Collantes
2018-04-20 16:12 ` Dan Cross
2018-04-20 16:21 ` Clem Cole
2018-04-20 16:33 ` Warner Losh
2018-04-20 19:17 ` Ron Natalie
2018-04-20 20:23 ` Clem Cole
2018-04-20 22:10 ` Dave Horsfall
2018-04-22 17:01 ` Lars Brinkhoff
2018-04-22 17:37 ` Clem Cole
2018-04-22 19:14 ` Bakul Shah
2018-04-22 20:58 ` Mutiny
2018-04-22 22:37 ` Clem cole
2018-04-22 21:51 ` Dave Horsfall
2018-04-25 1:27 ` Dan Stromberg
2018-04-25 12:18 ` Ronald Natalie
2018-04-25 13:39 ` Tim Bradshaw
2018-04-25 14:02 ` arnold
2018-04-25 14:59 ` tfb
2018-04-25 14:33 ` Ian Zimmerman
2018-04-25 14:46 ` Larry McVoy
2018-04-25 15:03 ` ron minnich
2018-04-25 20:29 ` Paul Winalski
2018-04-25 20:45 ` Larry McVoy
2018-04-25 21:14 ` Lawrence Stewart
2018-04-25 21:30 ` ron minnich
2018-04-25 23:01 ` Bakul Shah
2018-04-23 16:42 ` Tim Bradshaw
2018-04-23 17:30 ` Ron Natalie
2018-04-23 17:51 ` Clem Cole
2018-04-23 18:30 ` Ron Natalie
2018-04-25 14:02 ` Tom Ivar Helbekkmo
2018-04-25 14:38 ` Clem Cole
2018-04-23 20:47 ` Grant Taylor
2018-04-23 21:06 ` Clem Cole
2018-04-23 21:14 ` Dan Mick
2018-04-23 21:27 ` Clem Cole
2018-04-23 22:07 ` Tim Bradshaw
2018-04-23 22:15 ` Warner Losh
2018-04-23 23:30 ` Grant Taylor
2018-04-24 9:37 ` Michael Kjörling
2018-04-24 9:57 ` Dave Horsfall
2018-04-24 13:01 ` Nemo
2018-04-24 13:03 ` Arthur Krewat
2018-04-23 23:45 ` Arthur Krewat
2018-04-24 8:05 ` tfb
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