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

[Johnny Billquist]

On 2015-06-22 16:46, Bill Gunshannon wrote:
> In article <mm954o$766$1 at iltempo.update.uu.se>,
> 	Johnny Billquist <bqt at softjar.se> writes:
>> On 2015-06-15 15:47, Bill Gunshannon wrote:
>>> In article <2f70d$557ed38b$5ed4324a$44068 at news.ziggo.nl>,
>>> 	Dirk Munk <munk at home.nl> writes:
>>>> Because one VAX could do the work of several PDP-11's?
>>>
>>> Maybe in later times, but the first VAX I worked on was a real dog
>>> and I worked on several PDP-11's that outperformed it.  And, just
>>> like the debates here over what Alpha might have become, IMHO the
>>> PDP-11 could easily have kept up with the VAX.  All that would
>>> have been needed, really, was a bigger address bus and being as
>>> they had expanded it in the past, I don't think that was undoable.
>>> As time went on they could have made a 32bit PDP-11 that maintained
>>> 16bit compatability at the hardware level.  Intel did it twice,
>>> 8088 to 80386 and 80386 to x86-64.  Things like Floating Point and
>>> CIS could have been incorporated into the base architecture without
>>> breaking anything.
>>
>> DEC sortof did that. It was called the VAX. The early VAXen did have the
>> ability to run PDP-11 code in userspace.
>
> Yes, but with no new features.   To get new features you had to totally
> rewrite your MACRO-11 program in MACRO-32.  Iw as merely saying that
> rather than totally changing the architecture I see no reason why they
> could not have continued the architecture with enhancements at each
> step, just like other architectures did.

The new features were, by definition, not PDP-11 compatible, but to say 
that the VAX didn't have any new features is a bit of an odd view. :-)

>> And no, you would not have been able to extend the PDP-11 to 32 bits
>> without making it incompatible with a PDP-11, except as a special mode,
>> which is exactly what the VAX did.
>
> Not sure I agree with this.  I expect that if it was desired they
> could have done incremental modifications while maintaining backwards
> compatability.  But then, this is all academic, isn't it.

Try it, if you don't believe me. Suggest how you would extend the PDP-11 
without breaking the backward compatibility.

>> Adding a larger physical address space would have had very little value.
>> Extending the virtual address space was crucial, and it could not be
>> done without being incompatible. And if you go that way, you might as
>> well make some general improvements anyway, which is exactly what DEC
>> did with the VAX.
>
> The original x86 architecture had 64K segments.  It later added a mode
> with a large flat address space while maintaining compatabiltiy with
> "Small Model" programs.  It could be done.

They "maintained" backward compatibility in that you can grab an old 
program and still run it on the new hardware. No different than what the 
VAX did. You cannot take a new program and run it on the old hardware.

>>> Sorry, as much as I like the VAX, I think the PDP-11 was better.
>>
>> I prefer the PDP-11 too, but let's not try to deny the obvious.
>>
>>> Using the same argument we have heard here, what do yo think the
>>> capabilites of a PDP-11 with no new features but made using current
>>> technology for shrinks and clocking would be?  One screaming CPU.  :-)
>>> How many RSTS/E users do you think could be supported on a 2.5Ghz
>>> PDP-11/93 with 16GB of memory.  :-)
>>
>> You could easily, from a CPU performance point of view, support 64 users
>> on an 11/9x today. At that point, it becomes a question of memory again.
>> And it's not the physical memory that you start running out of first.
>> It's virtual memory. Even the kernel have limitations based on this. In
>> RSX, one crucial part is system pool. It all sits in the kernel virtual
>> memory space, and thus cannot grow much larger than it currently is, no
>> matter what you do. You need a larger virtual address space. Anything
>> else essentially just means you are trying to do magic with your knees,
>> and you spend more and more time running code to just try and move
>> around the address space limitations.
>
> Or, a big flat address space.  :-)

Which would be incompatible with the PDP-11. Which is exactly what the 
VAX did. You are merely suggesting exactly what DEC did with the VAX, 
you just don't seem to realize that this is what you are saying. :-)

>>> And X-11, too.  :-)
>>
>> That would *not* happen without a larger virtual address space.
>>
>
> Exactly.  :-)
>
> As I have stated in the past, one project I would love to do, just
> for the fun of it, was port RSTS to other architectures, both large
> and small.  I can envision RSTS/86 with X-11 and large dataspace so
> things like Postgres and otehr cool toold cold be ported.  Does it
> have some commercial value?  Even i seriously doubt it, but as one
> who really liked RSTS back in  the day, it wold be a lot of fun
> doing it.  And who know who else might have fun working on and
> enhancing it.  Even Minix is still being worked on.

If you were to extend the address space of the PDP-11, it would not be 
compatible, and thus you would have to rewrite RSTS/E as well, in order 
to use the new hardware. And then, when you're at it, you might want to 
change and enhance a number of things you have in RSTS/E, as you now 
could, since some of the designs in RSTS/E were dictated by the hardware.
And then you'd end up with something that in some ways looks like 
RSTS/E, but in some ways are different. And then you'd have people 
asking why you didn't just make a larger PDP-11 and run "pure" RSTS/E 
instead of the monster you created. :-)

	Johnny