Re: perspectives on 32 bit vs 64 bit
On Fri, Oct 14, 2005 at 12:18:50PM -0400, Adam Skutt wrote:
> That means it's logically impossible to have a 48-bit pointer, at all
> period.
Sure. You have 32bit pointers.
> Yes, but it's obvious now you didn't understand what I said.
>
> You /cannot/ have more than 32-bits of virtual address space. Period.
> There is no way to do it.
That is true. However you could have an OS that provided overlay style
replacements of blocks of data or code at the request of the
application. This of course requires custom applications to take
advantage of such a feature.
> What you can do is remap the same virtual space to different physical
> addresses. Which is different from having extra v.a.s.
Certainly.
> This isn't a 48-bit pointer, because descriptor selectors aren't pointers.
>
> And it won't work anyway. How do I get a base offset higher than
> 0xFFFFFFFF? And if I add to it, what behavior is yielded?
>
> Not what is desired, to say the least.
>
> You can't have more than 32-bit v.a.s. Anytricks to get around that
> don't really get around that, they just have the same addresses the
> user-space code sees point to different physical addresses.
>
> I really don't see how this is possible leafing through the IA-32 System
> Programming Guide so links or text would be preferred.
Certianly no more possible than using more than 640k on an 8086 without
swapping out code using overlays and loading different code in the same
memory space and continuing there. It worked, but it was a pain the,
and given we now have 64bit capable x86 cpu's, lets not even contemplate
implementing such a mess. :)
Len Sorensen
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