[Info-vax] RealWorldTech on Poulson

[Johnny Billquist]

On 2011-07-05 16.21, glen herrmannsfeldt wrote:
> Johnny Billquist<bqt at softjar.se>  wrote:
>> On 2011-07-04 17.54, Michael Kraemer wrote:
>
> (snip)
>>> That was the Mips R4000.
>>> And given the RAM constraints of that time,
>>> 64bit addressing was almost useless.
>
>> By the way, 64 bit addressing was not that useless. RAM size isn't the
>> only thing here. A VAX has 32-bit address space at a time when machines
>> shipped with 512K, and something like 4 Megs was considered huge. The
>> large virtual address space still made a big change on how you could
>> write your software.
>
> Actual 64 bit addressing is useless, and will be for some
> years now.  More than 32 bit is, but we are still many years
> from being able to use up a 64 bit virtual address space.
> (Terabyte disk drives are currently readily available, so 40 bits
> of virtual address space.)

Well, ok. I should have said "larger than 32 bits". But for the Alpha, 
it is really 64 bits of virtual address space. The fact that the OS will 
restrict you to allocating less before it gives you a error is another 
issue. And also the fact that Alpha implementations might not have a 
page table that covers all possible 64 bit address space. But the high 
bits are still checked, and in such a case needs to be 0. So you cannot 
use them for something else. It's a 64 bit address, even if you cannot 
actually use the whole address space in the current implementation.

	Johnny