[TUHS] /dev/drum

Johnny Billquist bqt at update.uu.se
Sun May 6 06:53:36 AEST 2018

On 2018-05-05 15:06, Noel Chiappa wrote:
>      > 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'.

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.

>      > 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":

Ok. It wasn't totally clear, since you also use a different term for 
demand paging...

>      >>> 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.

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.

> 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... :-)

Right. :-)

>      > 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'.

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 

Or at least it is not, if I understood you correctly.

Those MERT segments sounds identical to the PLAS functionality provided 
by DEC OSes.

> 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 Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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