5 May
2018
5 May
'18
3:06 p.m.
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
5 May
5 May
10:53 p.m.
New subject: /dev/drum
On 2018-05-05 15:06, Noel Chiappa wrote:
Aha! But then it is not actually giving programs direct access and
manipulation of the hardware. It is a software construct and service
offered by the OS, and the OS might fiddly around with various hardware
to give this service.
So the hardware is totally invisible after all. You might understand how
the OS realize the service using the PDP-11 hardware, but the program
itself cannot manipulate the hardware, and is in fact totally oblivious
to how the hardware works.
Ok. It wasn't totally clear, since you also use a different term for
demand paging...
Like I said. In the last few years, there has been a growing problem
with lots of people not understanding virtual memory. I blame it on the
monoculture we now have, where all people know, have been exposed to,
and read any code about is x86 and Linux. And thus, they are at a point
where they created some concepts in their heads, and anything that does
not match all their preconceived ideas becomes weird.
The wikipedia article on virtual memory on and off have had such
problems as well. It's not perfect now, but it has improved a lot from
how it was for a while. But it has also been more correct at times as
well. The discussions page is really enlightening.
But anyway, I certainly hope things have not in general degenerated
enough that virtual memory actually have taken on a different meaning.
There is also the constant issue that pops up with these "revisionist"
interpretations that there are plenty of old documentation and
references that contains definitions and usage of the term virtual
memory that does not fit with their very restrictive interpretation.
Right. :-)
Well. The PDP-11 chunks certainly, from your description above, would
appear to be invisible to programs running under MERT. MERT provides a
segment mechanism, which programs can use, but programs cannot play with
the hardware. Thus, the hardware is invisible to the program. The MERT
segment is visible, but such a segment might then involve one or more
hardware "chunks". The program don't know, and do not deal with them
individually or directly. In fact, the program would not even need to be
aware of what the hardware have, or do. It's really invisible to the
program. The MERT segment thingy is not a direct access or map to the
hardware.
Or at least it is not, if I understood you correctly.
Those MERT segments sounds identical to the PLAS functionality provided
by DEC OSes.
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'. 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.
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.
I might take a peek when I have some time.
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... :-) 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.
Right. But these are also not direct translations of the hardware
functionality. The fact that these segments can be more than 8K should
be enough of an argument showing that this is not the same as the
hardware, and that the program do not know, care, or need to understand
how the PDP-11 "chunks" actually appear or work.
(And I'll probably revert to saying pages after this reply, it's too
much work to try and use a separate name for them.)
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt(a)softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
2388
days inactive
2388
days old
1 comments
2 participants
participants (2)
-
jnc＠mercury.lcs.mit.edu -
Johnny Billquist