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

[Dirk Munk]

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.