4GB Physical Memory Support

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Guest

So I understand both the PCI Memory Hole, the 4GB limitation for physical
memory within Windows itself, and the 4GB of total Virtual Memory Address
available to each process, etc. etc. However, I have run across a situation
that I find very odd.
A new build consisting of an Intel E6750 2.66GHz Core 2 Duo with a
1333MHz FSB, Intel DG33BUC motherboard, and 4GB of DDR2-6400 clocked down to
667MHz running Windows XP Professional first recognized only 3.23GB of
physical memory installed. Also, Windows had automatically loaded into PAE
mode to support DEP since it's built into the processor. My first impression
was that the missing .77GB of physical memory was a victim of the PCI hole.
But what confused me was that after tweaking the Page File size downward the
reported physical memory increased to 3.48GB. Out of curiosity I completely
removed the page file, but the reported physical memory did not change.
After returning the page file to the "System Managed Size" the reported
physical memory remained at 3.48GB. Finally, when reverting to the original
custom settings populated by XP during the initial setup of the system the
physical memory shrank back to 3.23GB.
This is clearly not an issue of the PCI Hole or 4GB limitation. So what
is the problem?
 
CR-Sac said:
So I understand both the PCI Memory Hole, the 4GB limitation for
physical
memory within Windows itself, and the 4GB of total Virtual Memory Address
available to each process, etc. etc. However, I have run across a
situation
that I find very odd.
A new build consisting of an Intel E6750 2.66GHz Core 2 Duo with a
1333MHz FSB, Intel DG33BUC motherboard, and 4GB of DDR2-6400 clocked down
to
667MHz running Windows XP Professional first recognized only 3.23GB of
physical memory installed. Also, Windows had automatically loaded into
PAE
mode to support DEP since it's built into the processor. My first
impression
was that the missing .77GB of physical memory was a victim of the PCI
hole.
But what confused me was that after tweaking the Page File size downward
the
reported physical memory increased to 3.48GB. Out of curiosity I
completely
removed the page file, but the reported physical memory did not change.
After returning the page file to the "System Managed Size" the reported
physical memory remained at 3.48GB. Finally, when reverting to the
original
custom settings populated by XP during the initial setup of the system the
physical memory shrank back to 3.23GB.
This is clearly not an issue of the PCI Hole or 4GB limitation. So what
is the problem?
Click to expand...

The 32 bit version of Windows XP only supports a maximum of 4 GB of memory
space. However, what you have overlooked is that all the memory mapped bits
of hardware also have to occupy this space. Normally with smaller amounts
of memory, this isn't a problem, the memory mapped hardware occupies
addresses for which no physical memory is present. Once 4GB is installed
the latter part of it isn't available because something else has already
pinched the address range. 3.2 to 3.4 GB is about the right ball park for
the memory that is available.

The only mystery that remains is: why you think you require 4GB of memory in
the first place?
 
M.I.5¾ said:
The 32 bit version of Windows XP only supports a maximum of 4 GB of memory
space. However, what you have overlooked is that all the memory mapped bits
of hardware also have to occupy this space. Normally with smaller amounts
of memory, this isn't a problem, the memory mapped hardware occupies
addresses for which no physical memory is present. Once 4GB is installed
the latter part of it isn't available because something else has already
pinched the address range. 3.2 to 3.4 GB is about the right ball park for
the memory that is available.

The only mystery that remains is: why you think you require 4GB of memory in
the first place?
Click to expand...
Obviously you have never tried to edit large (1 GB+) files. I have 3 GB
of RAM and have had it use all of it plus a large swap file when trying
to open it with Wordpad or some other programs. My normal memory usage
is 1.5 to 2 GB of memory with just my normal programs open.
 
Michael W. Ryder said:
Obviously you have never tried to edit large (1 GB+) files. I have 3 GB
of RAM and have had it use all of it plus a large swap file when trying to
open it with Wordpad or some other programs. My normal memory usage is
1.5 to 2 GB of memory with just my normal programs open.
Click to expand...

I regularly edit files much larger than that. What makes you think large
amounts of memory are required for editing large files? Never heard of a
hard disk? Windows will generally occupy whatever memory you install,
requiring less swap file as the memory gets bigger. 512 MB to 1 GB is
generally adequate for most purposes, but editing High Definition video is
often more comfortable with 2GB. I cannot think of anything that needs any
more than 2GB, but there may be some very specialist applications out there
that benefit from it. Otherwise there is seldom much performance gain going
from 1GB to 2GB let alone up to 4GB.
 
Hate to be the party pooper of the group here, but we're a bit off topic.
Whether or not 4GB of physical memory is necessary is a moot point, the
question remains why changing the page file size altered the amount of
physical memory recognized by Windows. M.I.5¾ points out the existence of
the "PCI Hole", which I already understand and expected. However, keep in
mind that the PCI Hole exists as a result of addressing done by the BIOS, not
by Windows. Also, since Windows is in PAE mode it can address much more than
4GB of RAM. Each application only has 4GB of virtual memory assigned (a
combination of physical memory and page file space), but Windows itself is
capable of managing and addressing a full 4GB of physical memory, so long as
the motherboard appropriately support memory remapping to workaround the PCI
Hole. The problem here still remains: the PCI Hole explains the missing
..52GB of memory, but it does not explain why this hole increases when the
Page File gets bigger. So my original question still remains.
 
You might want to check MSinfo32 before and after to see what might shed
some light on the issue.
 
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