[Simh] Simh Digest, Vol 169, Issue 58

[Larry Baker]

> On 17 Feb 2018, at 5:20:13 AM, 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.

> (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.)  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.

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.

> 
>   Johnny
> 

Tim left unnamed a machine that considered everything memory, even I/O.  I bet that was Multics.  I admire some of Multics' ambitions, but practicalities doomed the effort.  Kind of like micro kernels, ala Mach and OSF/1.  A good idea for designing a system, but not necessarily for implementing one.

Larry Baker
US Geological Survey
650-329-5608
baker at usgs.gov

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.trailing-edge.com/pipermail/simh/attachments/20180217/8b7c18ac/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 1455 bytes
Desc: not available
URL: <http://mailman.trailing-edge.com/pipermail/simh/attachments/20180217/8b7c18ac/attachment-0001.bin>