[Simh] Simh Digest, Vol 169, Issue 58

[Johnny Billquist]

On 2018-02-17 21:56, Larry Baker wrote:
>> On 17 Feb 2018, at 5:20:13 AM, simh-request at trailing-edge.com 
>> <mailto:simh-request at trailing-edge.com> wrote:
>>
>> Date: Sat, 17 Feb 2018 14:20:00 +0100
>> From: Johnny Billquist <bqt at softjar.se <mailto:bqt at softjar.se>>
>> To:simh at trailing-edge.com <mailto:simh at trailing-edge.com>
>> Subject: Re: [Simh] pdp11 i/o addressing
>> Message-ID: <4b366ace-3b40-7137-ed99-f55beb7c6124 at softjar.se 
>> <mailto:4b366ace-3b40-7137-ed99-f55beb7c6124 at softjar.se>>
>> Content-Type: text/plain; charset=utf-8; format=flowed
>>
>> 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.
> 
> My first experience with overlays—the old fashioned kind—was on an IBM 
> 360.  I have much experience on RSX.  I gave presentations at DECUS 
> about the easy way to construct overlays.

:-)
Constructing overlays is a big topic. Depending on the program it can be 
(maybe) easy, but sometimes it can become rather complex, no matter how 
much you try.

>> (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.)
> 
> RSX offered more than just memory-mapped overlays.  (Which, gained us 
> much speed—not a waste of memory at all.)

Memory mapped overlays potentially wasted lots of space. The "problem" 
is that each overlay level must be page aligned, which means 8K aligned. 
It also means that you cannot have more than 7 levels, since you only 
have 8 pages. Compare to disk based overlays, which can pack the levels 
right next to each other, and it should be obvious that you potentially 
loose a lot of memory with memory mapped overlays.

>  RSX Fortran had VIRTUAL 
> arrays.  For us, enough program address space for data was usually the 
> limitation.  I would say that RSX's VIRTUAL arrays come closer to the 
> true virtual memory concept than memory-mapped overlays.

Good point. Virtual arrays is yet another mechanism, which is really 
nice, but with a few limitations.
But virtual arrays really is also a concept which is more akin to demand 
paging than virtual memory. Required data is read from disk into memory 
when referred to, and written back to disk again when other parts of the 
array is needed. But all managed behind the back of the user program. 
But also at the user level, just like overlays. The OS is not at all 
involved in this at all, and from the OS point of view, it all lives 
within the virtual memory space of the process.

> I suppose it I am being pedantic, but what you describe is really just 
> the difference between program or logical address space and physical 
> address space.  The former defined by the architecture, the latter 
> defined by the implementation.  Virtual memory is more than that.  See 
> Denning's http://denninginstitute.com/pjd/PUBS/ENC/vm08.pdf, which 
> mentions the Manchester Atlas invention of paged virtual memory.  Some 
> of what you described is known as the working set, a concept which 
> Denning first described.

What you talk about now is demand paging. That really is not virtual 
memory itself, but a way to more efficiently implement virtual memory 
under some circumstances. But we need to separate the concept from the 
implementation.
RSX have virtual memory. But it does not deal with demand paging, 
working sets, paged virtual memory, or any such concepts. But it still 
have virtual memory. So clearly you cannot claim that virtual memory 
requires these concepts.
Demand paging is not virtual memory.

	Johnny

-- 
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