Re: [boost] Interest request for pointer+bit compression optimization

Subject: Re: [boost] Interest request for pointer+bit compression optimization
From: Edouard A. (edouard_at_[hidden])
Date: 2009-09-07 17:19:25

>You appear to be assuming that the OS guarantees that it uses the lowest
>available virtual addresses. Would you please provide a link to the
>documentation? I was under the impression that the 8TB limit is merely a
>practical one since this was the maximum hardware specification available
>for testing.

The OS isn't floating around in virtual memory space! The layout is strict
and doesn't change and of it's precisely and correctly documented in the
driver developer's manual.

There is some randomness added for security, but the whole user space
process is very strictly bounded. That's true for all OSes I've programmed
drivers for, which is, I agree far from being exhaustive.

For Windows x86 anything between 0x0 and 0x7fffffff is for user mode, with
the first pages made inaccessible to help debug null pointers errors. I also
remember that there's some clutter near the 2GB boundary (not sure about it,
look it up if you're interested). Of course that goes up a bit if you extend
the user space to 3GB.

For Windows 64 running on IA64 anything between 0 and 0x6fbfffeffff is for
user mode. It goes up to 0x7fffffeffff for AMD64.

Again the first pages are inaccessible for the same reasons. Must something
like a 32 kb locked range.

>I prefer using the least-significant bits when the alignment allows such an
>optimisation. This seems less error-prone and better documented as a safe
>idiom.

I believe in freedom.

-- 
EA 
 
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