[Info-vax] [OT] PDP-11 address space, was: Re: HP adds OpenVMS Mature Product Support beyond the end of Standard Support

[Johnny Billquist]

On 2014-02-02 10:30, Bill Gunshannon wrote:
> In article <lclbf5$hs1$1 at dont-email.me>,
> 	Simon Clubley <clubley at remove_me.eisner.decus.org-Earth.UFP> writes:
>> On 2014-02-02, Johnny Billquist <bqt at softjar.se> wrote:
>>> On 2014-02-01 07:41, Bill Gunshannon wrote:
>>>>
>>>> Of course, being as they are still being made, it would be real
>>>> interesting to see a new PDP-11 with more address lines, too. :-)
>>>> There I go dreaming again.  :-)  But, you have to admiit, it could
>>>> be a lot of fun.
>>>>
>>>> Hmmm..   Let's see...  How many overlays would it take to get Open
>>>> Office running on a PDP-11?   :-)
>>>
>>> Eh.. What? Any "modern" PDP-11 already have 22 address lines...
>>> My PDP-11 at home have 4 megs of ram.
>>> Have you never seen a PDP-11 with that? Under which rock have you been
>>> the last 40 years, Bill? :-)
>>>
>>
>> In fairness to Bill, I think he's talking about program space address
>> lines, instead of machine space address lines (ie: he wants to be able
>> to run larger programs without getting stuck into reading the overlay
>> section of the TKB manual yet again :-)).
>>
>> Simon.
>>
>> PS: I absolutely do _not_ miss TKB. :-)
>>
>
> I only did RSX once, for a project when I was with Martin Marietta.
> But I was real good at doing overlays with Ultrix-11.  Especially
> for larger kernels.

The design of overlays in Unix/Ultrix is rather primitive compared to 
RSX, but doing overlays is a complicated business.

> But, yes, I would love to be able to do bigger programs on PDP-11s.
> But if nothing else develops with the OSes, I guess it won't make
> much difference except for Ultrix-11.

If we're talking about getting a larger virtual address space, when is 
needed is not more address pins, but larger registers, a redesign of the 
instruction set, and so on. That is pretty much what the VAX did.

And yes, sometimes I really wish I had a larger address space for my 
programs, and so on. But in many cases the need for that become rather 
small with the introduction of I/D space, and supervisor mode libraries, 
which means I can do RMS with almost no memory space lost, and have 64K 
of code and 64K of data at the same time. Really, I very seldom need to 
go play with overlays nowadays.

Ultrix do not have that kind of fancy stuff, so Ultrix is in a worse 
position, I guess.

TKB is slow as molass, but very capable. And if you have a fast machine, 
and if you use a modern TKB, which also exploits I/D space and 
supervisor mode libraries, it is much faster. So life nowadays in RSX is 
really not that bad.
The only thing that still really is black magic is when you still need 
overlays. Getting your overlays in a nice shape in RSX is complicated.

And of course, overlays only help for large code. If you need large 
amounts of data, you need to do other tricks.

	Johnny