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

[Johnny Billquist]

On 2015-06-22 18:42, Bill Gunshannon wrote:
> In article <mm99ta$mue$1 at iltempo.update.uu.se>,
> 	Johnny Billquist <bqt at softjar.se> writes:
>> 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. :-)
>
> As usual, I think I am not being clear here.  Of course the VAX had new
> features.  It WAS a new feature.  :-)  But, it was not a PDP-11 by any
> stretch of the imagination.

Correct. The VAX is not a PDP-11, but it was designed with a subset of 
the PDP-11 in there.

>>>> 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.
>
> Umm...  Unless I greatly mis-remember my early VAX the reason they
> could run was much closer to todays VAX emulation than the VAX actually
> being a superset PDP-11.  I am fairly certain I can not run any kind
> of PDP-11 code on any of my curretn VAXen as that was hardware speci-
> fically put into the early VAX and removed after a very short time.
> Current x86 machines (even x86-64) can still run code written for the
> 8088.

That is because of a totally different reason. In order to shrink and 
simplify the chip design, a bunch of stuff was deleted from the VAX 
hardware, and instead trapped out to the OS, who had to implement the 
parts that early models did in hardware.
This is not only true of the PDP-11 bits, but quite a few parts of the 
VAX architecture itself as well.

And it just means you need some additional software in the OS in order 
to enable running of that code.

In addition, the PDP-11 bits were also moved out as a separate product 
for VMS, so that you didn't necessarily have that capability on your 
specific machine.

The VAX architecture itself did not change, and the PDP-11 compatibility 
bit is still in there in the PSL.

And so, yes, you could still run your PDP-11 code on your latest VAXen. 
You just need to install the AME product onto the system.

>>>>> 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.
>
> While it would definitely be different I don't see how it is a compatability
> issue at all.  I would expect a program written to run in a 64K PDP-11 to
> have no problem running in a machine that has more memory.  Now, the other
> way.......

Even that is hard. But that is what VAX/VMS did, although not for RSTS/E 
but for RSX. And you can still utilize that.

>>                                               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. :-)
>
> But the VAX is a totally different architecture.  Other than both being
> (technically) CISC and made by DEC, just what do they have in common?

The PDP-11 compatibility mode of the VAX. Obviously. Apart from that, 
no, they are not compatible, except in a cultural sense. The VAX is 
clearly based on the PDP-11, but the extension as such is not identical 
to a PDP-11.
Same as a x86-64 is not compatible with an 8086, but they have a clear 
heritage, and the x86-64 can be told to run 8086 code. (Or rather, if I 
remember right, you actually switch out from 8086 to x86-64 and not the 
other way.)

>>>>> 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.
>
> Well, obviuosly, it RSTS or any other OS would have to be modified to
> make use of any of the new features.  But a complete re-write?  Was
> VMS completely re-written when it went from VAX to ALPHA and then to
> Itanium?  Now, given the opportunity, I would likely re-write RSTS, yes.
> But mostly to make it more portable and thus written in a higher level
> language than MACRO-11.  But then, that has always been one of my stated
> objectives given the source and the opportunity.

Not necessarily a complete rewrite, but you would have to overhaul just 
about every part of it. Very few lines could be left untouched.
Modified is not a word I'd use, as I think that don't reflect the size 
of the work.

>>                           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. :-)
>
> Well, as also stated previously, this is all academic.  I doubt there
> would be anyone really interested, but who knows. I know people still
> playing with Primos and just this morning in another group I got an
> announcement about a DPS8 emulator running Multics that was being made
> avaialble.
>
> I guess there is just no way to explain the things we want to do for fun.

That is true... :-)

	Johnny