PC 4GB RAM limit

what motherboard is it? my Asus A8V has some option somewhere in the bios to
"enable" 4gig or disable (ie "steal" memory for itselfs) i think.

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"Tim Anderson" wrote in message
...
The cheaper motherboards (ie. most of them) for P4 or Athlon 64 support a
maximum of 4GB RAM, even if the motherboards and processors support the
AMD64 extensions that can address sqillions of GB. On the other hand, more
upmarket boards support more RAM - up to 24GB or maybe more - for Xeon and
Opteron.

If you buy one of these 4GB boards and install 4GB RAM, you don't get the
use of all of it. The top of the 4GB address space gets shadowed by system
functions such as PCI Express addressing. This is not just a small detail
- typically you lose 1GB of your 4GB. See:

http://www.itwriting.com/blog/?postid=152

I can't at the moment find a clear explanation of this. I understand about
the shadowing, but the question of course is why a modern board can't use
a higher range of addresses to make the full 4GB available to the OS. The
manufacturers mutter about "PC Architecture", but then again they also
make boards that *do* overcome this limit.

With PAE, PC processors have been able to address more than 4GB for years.
So why are we still running into this limit?

Tim

Al Dykes wrote:
In article ,
David Maynard wrote:

Tim Anderson wrote:


"Al Dykes" wrote in message
...



The decision to limit user's physical memory is a cost and marketing
decision on the part of the mobo manufacturer.


I have no problem with that. I do have an issue with marketing a board as
"4GB" when it is in effect 3GB.

Tim



Unless there's something odd about the motherboard it *is* 4GB.

What happens is the BIOS should remap the top 1G above the I/O area so you
end up with 3GB RAM - 1GB I/O - 1GB RAM.

XP/2000 Pro won't see it, however, because they're limited to a 4 gig space
and remapping the top 1G above 4 puts it 'out of range'. Win2k Advanced
Server and the Win2k3 variants can use it, though, as they support 8 and
16GB address spaces (with PAE switch).

With Linux you have to make sure the kernel (2.4.x or higher) is compiled
with the PAE option enabled or else you have the same 4GB situation as XP.



This sounds like a 32 bit system.

Yes

I assume that PAE doesn't exist in
a 64 bit system (or for backward compatibility the OS simply maps PSE
segemnts into normal chucks of 64 bit address space. )

PAE is slow becasue of the context switches. A package like Oracle
can make efficient use of it but it's no replacemnt for a flat 64 bit
address space.

Tim Anderson wrote:

"David Maynard" wrote in message
...


Unless there's something odd about the motherboard it *is* 4GB.

What happens is the BIOS should remap the top 1G above the I/O area so you
end up with 3GB RAM - 1GB I/O - 1GB RAM.


This it is *not* doing.

How did you determine that?

I tend to agree with you though.


XP/2000 Pro won't see it, however, because they're limited to a 4 gig
space and remapping the top 1G above 4 puts it 'out of range'.


I'm running 64-bit Operating systems

I see.

(Windows x64

I don't know what capabilities and bugs might exist in a pre beta O.S.

and Linux) that have no
problem addressing this range.

I'm not sure but what you may be jumping to premature conclusions. By that
I mean there are still configuration issues and 'limits' in addition to the
'theoretical maximums'. For example, it may be that certain assumptions the
O.S. is making about memory allocation are different than the ones the
motherboard is doing.

I'm afraid I can't debug your situation as I don't have that motherboard
nor 4 gig of ram but I've seen various problems mentioned about 4gig and
'64-bit' linux. One person, for example, complained that 4 gig was fine
till he 'upgraded' to the 64 bit kernel and, in that case, it seemed he
ended up with a mix of 64 bit addressing and PAE which, of course, are two
different things.

Not saying that's your problem but it's an example of how things are
sometimes not as they seem nor as simple as the 'theory'.

You sure something isn't causing a 32 bit 'compatibility' mode?





Tim

In message David Maynard
wrote:

(Windows x64

I don't know what capabilities and bugs might exist in a pre beta O.S.

You must not get much news up whereever it is you live, Windows x64 has
been in beta for over a year and went RTM several weeks ago.

In short, it hasn't been pre-beta for many many months.

--
Prayer has no place in the public schools, just like facts
have no place in organized religion.
-- Superintendent Chalmers

"David Maynard" wrote in message
...

How did you determine that?

Because the BIOS reports "1048 MB consumed by system" on boot up, before the
OS loads. The problems is also documented in the tech. spec.

(Windows x64

I don't know what capabilities and bugs might exist in a pre beta O.S.

Full release build.

and Linux) that have no problem addressing this range.

Suse Linux 64-bit can't see the RAM either - not surprising, as it it lost
before the OS boots.

Tim

"David Maynard" wrote in message
...

How did you determine that?

Because the BIOS reports "1048 MB consumed by system" on boot up, before the
OS loads. The problems is also documented in the tech. spec.

(Windows x64

I don't know what capabilities and bugs might exist in a pre beta O.S.

Full release build.

and Linux) that have no problem addressing this range.

Suse Linux 64-bit can't see the RAM either - not surprising, as it it lost
before the OS boots.

Tim

"Adam Webb" wrote in message
...
what motherboard is it? my Asus A8V has some option somewhere in the bios
to
"enable" 4gig or disable (ie "steal" memory for itselfs) i think.

This is an Intel board unfortunately.

But yes, it sounds like that would fix it.

Tim

DevilsPGD wrote:
In message David Maynard
wrote:


(Windows x64

I don't know what capabilities and bugs might exist in a pre beta O.S.


You must not get much news up whereever it is you live, Windows x64 has
been in beta for over a year and went RTM several weeks ago.

In short, it hasn't been pre-beta for many many months.


Right.

The problem was me doing a half dozen things at the same time and thinking
of something other than XP/64.

Tim Anderson wrote:

"David Maynard" wrote in message
...


How did you determine that?


Because the BIOS reports "1048 MB consumed by system" on boot up, before the
OS loads. The problems is also documented in the tech. spec.

I see. Well, since you hadn't mentioned it again I threaded back to see if
you said what board it was and from the docs it appears to not be a '64
bit' board. By that I mean, it apparently doesn't have anything to support
64 bit specific features and, in particular, no more than the traditional 4
GB address space, which could explain why it doesn't remap HiMem.

The literature targets it at 'desktops' and, specifically, XP/2000 as the
supported O.S. and neither of them would be able to use remapped memory
even if the board did it, which might also explain why they didn't bother.

I was thinking in terms of a board that actually supports (part of) the 64
bit address space, in some form and if, for example, you look at the
documentation for the Intel server board that was posted earlier in this
thread you'll see it has a BIOS option for mapping, or not, that space to
above installed memory. (It also supports 8 GB.)


(Windows x64

I don't know what capabilities and bugs might exist in a pre beta O.S.


Full release build.


and Linux) that have no problem addressing this range.


Suse Linux 64-bit can't see the RAM either - not surprising, as it it lost
before the OS boots.

Tim

"David Maynard" wrote in message
...

I see. Well, since you hadn't mentioned it again I threaded back to see if
you said what board it was and from the docs it appears to not be a '64
bit' board. By that I mean, it apparently doesn't have anything to support
64 bit specific features and, in particular, no more than the traditional
4 GB address space, which could explain why it doesn't remap HiMem.

Not entirely true. The board specifically supports processors with EM64T
extensions.

The literature targets it at 'desktops' and, specifically, XP/2000 as the
supported O.S. and neither of them would be able to use remapped memory
even if the board did it, which might also explain why they didn't bother.

OTOH Intel offers downloads *for this board* for WinXP x64. So apparently
Intel does expect this to be an OS that you might use with this board.

So this isn't the explanation.

Tim