[Info-vax] OT: obscure PDP11 OSes (even more dinosaury)

[Bill Gunshannon]

In article <a5da$557f3c62$5ed4324a$14524 at news.ziggo.nl>,
	Dirk Munk <munk at home.nl> writes:
> moroney at world.std.spaamtrap.com (Michael Moroney) wrote:
>> Simon Clubley <clubley at remove_me.eisner.decus.org-Earth.UFP> writes:
>>
>>> On 2015-06-15, Dirk Munk <munk at home.nl> wrote:
>>>> bill at server3.cs.uofs.edu (Bill Gunshannon) wrote:
>>>>>
>>>>> Of course, all of this still doesn't explain how the VAX was
>>>>> "a best-seller" when compared to the PDP-11 when the PDP-11
>>>>> sold 50% more systems.
>>>>>
>>>>
>>>> Because one VAX could do the work of several PDP-11's?
>>
>>> In the early days of the VAX, wasn't there some literature floating
>>> around showing how the PDP-11 was faster at some things than the VAX ?
>>
>> IIRC, the PDP-11/74 (multiprocessor PDP-11/70) was killed because it was
>> faster than a VAX 780 and would have eaten into 780 sales.
>>
> That depends.
> 
> The PDP-11 was a 16 bit architecture. Writing larger programs on a 
> PDP-11 always involved using overlay. I've spend many hours trying to 
> construct my Cobol programs in such a way that I could use memory 
> resident overlay. Otherwise it would become disk-resident overlay, and 
> that involved lots of extra disk IO as the name suggests. That would 
> make an application a lot slower. Using RMS in supervisor mode also was 
> a must to keep a somewhat fast application.
> 
> The 32 bit VAX/VMS architecture didn't have those problems. The 
> application would fit in memory, no need for overlay.
> 
> So perhaps the PDP-11/74 would have been faster in pure processing 
> power, for real world applications the much larger memory footprint a 
> VMS application could have was far more important. Anything that can be 
> done in memory is much better than having to rely on disk IO.

The PDP-11's memory capability was expanded twice, to 18 and then 22 bits.
I see no reason why it could not have been expanded again instead of being
replaced by a totally different architecture.  The harder part would have
been the size of registers but even I can see ways to deal with that and
I am certainly not in the same class as people who came up with things
like the Alpha.

bill

-- 
Bill Gunshannon          |  de-moc-ra-cy (di mok' ra see) n.  Three wolves
billg999 at cs.scranton.edu |  and a sheep voting on what's for dinner.
University of Scranton   |
Scranton, Pennsylvania   |         #include <std.disclaimer.h>