Memory Holes?

NTDEV
1

I’m encountered a problem I didn’t expect.

We have a SuperMicro motherboard with dual Xeons. With two 1GB DIMMs,
we get 2GB of RAM in Windows XP SP2. Everybody happy.

In an attempt to explore other issues, we added two additional 1GB DIMMs
of the same type. The BIOS now shows 4GB of RAM, but Windows still
shows 2GB. I flashed the BIOS to the most recent version, and even
tried the /PAE switch, but Windows still shows only 2GB.

On a whim, I booted a 64-bit Linux CD. It finds 4GB of RAM. There are
2GB at physical address 0, then a 2GB gap, then 2GB running from 4GB to
6GB in physical space. I have no doubt that a Win64 system could find
it as well.

Now, there has to be some kind of a hole in the lower 4GB to allow for
PCI devices, so I assumed that Windows would just deal with this
transparently. Are my expectations unreasonable? Is XP SP2 limited to
physical memory below 4GB?

–
Tim Roberts, xxxxx@probo.com
Providenza & Boekelheide, Inc.

2

Different SKUs of Windows are limited in how much memory they allow you
to use, this cap is implemented as a limit on the address space i.e. if
WinXP supports 4GB then all physical addresses over 4GB are ignored.

If you look at the memory layout of a typical system there are other
holes as well, but the PCI gap is the big one. Some BIOS will allow you
to adjust the size of the PCI gap to mitigate the memory loss.

It would make more sense for the OS to add up the fragments regardless
of where they exist and stop adding when you hit the “Limit” but
unfortunately that is not the case.

-Zach

-----Original Message-----
From: xxxxx@lists.osr.com [mailto:bounce-271182-
xxxxx@lists.osr.com] On Behalf Of Tim Roberts
Sent: Monday, November 27, 2006 4:43 PM
To: Windows System Software Devs Interest List
Subject: [ntdev] Memory Holes?

I’m encountered a problem I didn’t expect.

We have a SuperMicro motherboard with dual Xeons. With two 1GB DIMMs,
we get 2GB of RAM in Windows XP SP2. Everybody happy.

In an attempt to explore other issues, we added two additional 1GB
DIMMs
of the same type. The BIOS now shows 4GB of RAM, but Windows still
shows 2GB. I flashed the BIOS to the most recent version, and even
tried the /PAE switch, but Windows still shows only 2GB.

On a whim, I booted a 64-bit Linux CD. It finds 4GB of RAM. There are
2GB at physical address 0, then a 2GB gap, then 2GB running from 4GB to
6GB in physical space. I have no doubt that a Win64 system could find
it as well.

Now, there has to be some kind of a hole in the lower 4GB to allow for
PCI devices, so I assumed that Windows would just deal with this
transparently. Are my expectations unreasonable? Is XP SP2 limited to
physical memory below 4GB?

–
Tim Roberts, xxxxx@probo.com
Providenza & Boekelheide, Inc.

Questions? First check the Kernel Driver FAQ at
http://www.osronline.com/article.cfm?id=256

To unsubscribe, visit the List Server section of OSR Online at
http://www.osronline.com/page.cfm?name=ListServer

3

I seem to recall PAE requiring a different HAL and kernel. I’m wondering if this is because you are still using the non-PAE versions because the extra memory wasn’t there when you installed the OS?

If that’s the case, then there’s a probably a way around this (short of re-installing).

But I could be way off on this one- don’t have that kind of memory, myself…

4

Bob Kjelgaard wrote:

I seem to recall PAE requiring a different HAL and kernel. I’m wondering if this is because you are still using the non-PAE versions because the extra memory wasn’t there when you installed the OS?

If that’s the case, then there’s a probably a way around this (short of re-installing).

But I could be way off on this one- don’t have that kind of memory, myself…

Oooh, that’s a good point. I’ll give that a shot.

–
Tim Roberts, xxxxx@probo.com
Providenza & Boekelheide, Inc.

5

I have XP SP2 32-bit on an Asus MB with 4GB of memory. It only gives me
about 3.5GB visible (+/- 300KB).

“Tim Roberts” wrote in message news:xxxxx@ntdev…
> Bob Kjelgaard wrote:
>> I seem to recall PAE requiring a different HAL and kernel. I’m wondering
>> if this is because you are still using the non-PAE versions because the
>> extra memory wasn’t there when you installed the OS?
>>
>> If that’s the case, then there’s a probably a way around this (short of
>> re-installing).
>>
>> But I could be way off on this one- don’t have that kind of memory,
>> myself…
>>
>
> Oooh, that’s a good point. I’ll give that a shot.
>
> –
> Tim Roberts, xxxxx@probo.com
> Providenza & Boekelheide, Inc.
>
>

6

Hi Tim,

PAE is used to support memory bigger than 4GB so technically it makes no
sense to add PAE to Windows XP Sp2 because it only supports 32 bit
physical address ranges. In fact, PAE switch is added to XP Sp2 for
quite other purposes, DEP(Data Execution Prevention) if you care.

64 bit Os doesn’t need PAE because they are inherently capable of
support larger physical address spaces.

The only 32 bit OS that is capable of using PAE and successfully
identified the missing holes I know so far is some version of Windows
Server 2003.

I will extend on this a little bit more if you are interested. The
Memory Controller will usually map part of the physical memory address
space (that has been occupied by PCI IO or else) to above 4GB. The net
result is, although you believe your 4GB memory is contiguous, it is in
fact not. So your physical address apace may be something 0~2.5G,
4~5.5G. And because XP is only limited to 4GB physical memory, you lost
those higher than 4GB.

For capabilities of different version of OS, you may refer to the table
at the bottom of this article:
http://www.microsoft.com/whdc/system/platform/server/PAE/pae_os.mspx

Best regards,
Cody

-----Original Message-----
From: xxxxx@lists.osr.com
[mailto:xxxxx@lists.osr.com] On Behalf Of Tim Roberts
Sent: Tuesday, November 28, 2006 8:43 AM
To: Windows System Software Devs Interest List
Subject: [ntdev] Memory Holes?

I’m encountered a problem I didn’t expect.

We have a SuperMicro motherboard with dual Xeons. With two 1GB DIMMs,
we get 2GB of RAM in Windows XP SP2. Everybody happy.

In an attempt to explore other issues, we added two additional 1GB DIMMs
of the same type. The BIOS now shows 4GB of RAM, but Windows still
shows 2GB. I flashed the BIOS to the most recent version, and even
tried the /PAE switch, but Windows still shows only 2GB.

On a whim, I booted a 64-bit Linux CD. It finds 4GB of RAM. There are
2GB at physical address 0, then a 2GB gap, then 2GB running from 4GB to
6GB in physical space. I have no doubt that a Win64 system could find
it as well.

Now, there has to be some kind of a hole in the lower 4GB to allow for
PCI devices, so I assumed that Windows would just deal with this
transparently. Are my expectations unreasonable? Is XP SP2 limited to
physical memory below 4GB?

–
Tim Roberts, xxxxx@probo.com
Providenza & Boekelheide, Inc.

Questions? First check the Kernel Driver FAQ at
http://www.osronline.com/article.cfm?id=256

To unsubscribe, visit the List Server section of OSR Online at
http://www.osronline.com/page.cfm?name=ListServer

7

Make that 3GB visible under ‘My Computer’. My pagefile is about 2.1GB, but
XP recommends 4606MB.

“David J. Craig” wrote in message
news:xxxxx@ntdev…
>I have XP SP2 32-bit on an Asus MB with 4GB of memory. It only gives me
>about 3.5GB visible (+/- 300KB).
>
> “Tim Roberts” wrote in message news:xxxxx@ntdev…
>> Bob Kjelgaard wrote:
>>> I seem to recall PAE requiring a different HAL and kernel. I’m
>>> wondering if this is because you are still using the non-PAE versions
>>> because the extra memory wasn’t there when you installed the OS?
>>>
>>> If that’s the case, then there’s a probably a way around this (short of
>>> re-installing).
>>>
>>> But I could be way off on this one- don’t have that kind of memory,
>>> myself…
>>>
>>
>> Oooh, that’s a good point. I’ll give that a shot.
>>
>> –
>> Tim Roberts, xxxxx@probo.com
>> Providenza & Boekelheide, Inc.
>>
>>
>
>
>