[Info-vax] OpenVMS.Org quick pool

[Johnny Billquist]

On 2012-08-24 18:01, Stephen Hoffman wrote:
> 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.

Lets just first make clear that virtual memory and demand paging are two 
things, and one does not necessitate the other. However, they do work 
nicely in combination.
Operating systems on PDP-11s normally implement virtual memory, but they 
do not implement demand paging.

And overlays are a form of demand paging, that does not necessarily 
imply that you have virtual memory.

That said, yes, I see nothing to comment on your statement. Totally 
agree. Not sure what the point was, though.

>>> 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?

What do you mean by ""true" virtual memory"? What is in this case 
""false" virtual memory". As far as I know, there is only virtual memory.

"True" smells of snobbish, arbitrary definitions without any true 
meaning. :-)

	Johnny