[Info-vax] OpenVMS I64 V8.1 "Evaluation Release"?

[Johnny Billquist]

On 2012-03-26 14.48, glen herrmannsfeldt wrote:
> Johnny Billquist<bqt at softjar.se>  wrote:
>
> (snip)
>> I'm still ready to claim that no machine produced today can connect 2^64
>> bytes of ram to it. I just don't see it as even probable that anyone
>> would be putting out 64 address pins.
>
> Yes. It would be easy enough to add them when they are needed.
> The architecture should be designed to allow for them, and even
> should check that they are zero for now, but no need to implement
> them yet.

Maybe, maybe not. Look at the VAX. They had to introduce an incompatible 
change to the PTE to go beyond 30 bit physical addresses, and then they 
went to 34. There is absolutely no need, and no point in having much 
relationship between how a virtual address looks like, and a physical.

Even more, for the VAX, the actual memory limit was actually 2^29, if I 
remember right, since half the space was defined/reserved for the I/O space.

>> And that is ignoring possible architecture limits, where the machine
>> very well cannot form 64 bit physical addresses. After all, it is not
>> really required that a machine can form a 64 bit physical address, even
>> if you can form a 64 bit virtual address.
>
> There should at least be the possibility. One could have alternate
> forms for page tables, and use a smaller, more efficient form for now.

Possibly, but it all depends on how the architecture have defined them. 
Sometimes it's not easy to extend. And this I definitely do not know for 
the s360 in 64 bit mode. Which is why I asked... :-)

>> It would be interesting to hear if anyone with deep knowledge about the
>> architecture could comment on what the theoretical limits are on how
>> large physical addresses can be formed.
>
> The z/ Principles of Operation is on the IBM web site. Last I knew,
> they were reasonably priced, for the size, if you wanted to buy one.
> Way over 1000 pages long.

Lets see if someone who knows speaks up...

	Johnny