[Info-vax] OpenVMS.Org quick pool

[Stephen Hoffman]

On 2012-08-24 14:58:04 +0000, Johnny Billquist said:

> Hmm. I could see other languages do the same trick under the hood, 
> especially if you implement a virtual machine. In fact, I've done this 
> myself, for the Z-machine on RSX. Programs can be much larger than 64K, 
> and the Z-machine does memory remappings as needed, without the program 
> ever being aware of it.
> 
> Also, overlays in RSX allows you to get the same functionality without 
> your program noticing either, by using memory resident overlays. You 
> are never able to directly address more than 64K (as always), but 
> overlays can be memory resident, and just mean a memory remapping 
> instead of a disk read, if you want to. Much faster, but do require 
> that you have more physical memory available.


With a virtual memory demand-paging system, you're not necessarily even 
able to access what you're permitted to access within the address space 
without faulting, or without smacking into implementation limits.


> 
>> This same windowing was also trivial to do as far back as on an Apple II
>> box, though that required add-on expansion memory hardware designed to
>> permit windowing.  Those add-on boards were fairly common back then, too.
> 
> It's very trivial, yes. However, it don't really get you around the 64K 
> addressing limit. It gives you a way of getting more code/data into the 
> existing 64K by changing the contents at times in part of that virtual 
> address space.


And this differs from "true" virtual memory how?


> So, plenty of them still in steel mills, paper mills, automotive 
> industry, and various controller roles. All the ones I know about are 
> running RSX, and have track records that survives all the replacemnt 
> systems that have been mentioned.

And I've visited a number of these sites over the years, and am 
familiar with how those entities get themselves tangled, and how they 
justify this; for better or for worse.  Legacy hardware does eventually 
and inevitably die out, variously as the staff involved itself ages 
out.  And where I usually ask what the sites are missing by not moving 
forward; some sites have certainly considered that, and some have not.


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