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

[Johnny Billquist]

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.

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.

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.

> 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.

> And X-11, too.  :-)

That would *not* happen without a larger virtual address space.

	Johnny