Windows 64 Bit Compatibility Question

Theo wrote:

Sebastian G. wrote:
Theo wrote:

Windows XP SP2 and later by default on NX capable processors enables
PAE in order to enable NX, but limits physical address space to 32
bits for driver compatibility reasons.

That's why you have to enable it explicitly via the /PAE switch.

"but limits physical address space to 32 bits (4GB) for
driver compatibility reasons."

Yes, it's wrong and I didn't catch it. NX without PAE limits the *virtual*
address space.

Sebastian G. wrote:

John John wrote:

Sebastian G. wrote:

Theo wrote:

Windows XP SP2 and later by default on NX capable processors enables
PAE in order to enable NX, but limits physical address space to 32
bits for driver compatibility reasons.



That's why you have to enable it explicitly via the /PAE switch.


Enabling the /PAE on Windows XP will still not provide for memory
addressing above the 4GB boundary.



Actually I just didn't catch the meaning to notice that the above is
wrong: NX without PAE switch limits the *virtual address space* to 32
bits, which is 4 GB minus PCI address range, 1 MB lower memory, config
ROMs etc.

With the PAE switch the virtual address space is extended to 36 bits,
but the physical address range is limited to 32 bits. That is, you can
actually fully use the entire 4 GB, since the memory burned by the
reserved areas is remapped to addresses above the 4 GB virtual address
limit.

Don't confuse Virtual Address Space with Physical RAM. With Windows XP
32-bit enabling PAE does not permit the operating system to access RAM
addresses remapped above the 4GB boundary. If the OP has 4GB of RAM in
his machine he will not be able to use all of it. The Physical Address
Space on Windows XP 32-bit is limited to 4GB and using Physical Address
Extensions does not change that limit.

John

John John wrote:


Don't confuse Virtual Address Space with Physical RAM. With Windows XP
32-bit enabling PAE does not permit the operating system to access RAM
addresses remapped above the 4GB boundary. If the OP has 4GB of RAM in
his machine he will not be able to use all of it. The Physical Address
Space on Windows XP 32-bit is limited to 4GB and using Physical Address
Extensions does not change that limit.

Argh, now I got it wrong as well. But I found the conclusion: With the /PAE
switch, the kernel does not limit the physical address range at all.

See http://blogs.msdn.com/carmencr/archive/2004/08/06/210093.aspx

Sebastian G. wrote:

John John wrote:


Don't confuse Virtual Address Space with Physical RAM. With Windows
XP 32-bit enabling PAE does not permit the operating system to access
RAM addresses remapped above the 4GB boundary. If the OP has 4GB of
RAM in his machine he will not be able to use all of it. The Physical
Address Space on Windows XP 32-bit is limited to 4GB and using
Physical Address Extensions does not change that limit.


Argh, now I got it wrong as well. But I found the conclusion: With the
/PAE switch, the kernel does not limit the physical address range at all.

No, with Windows XP the use of the /PAE switch does not change the
available physical address space, it will still be limited to 4GB, the
XP PAE kernel will not make use of RAM addresses remapped above the 4GB
boundary, it is completely limited to the lower 4GB arena. The use of
the /PAE switch is different for 32-bit Server products than it is for
Workstation products.

John

John John wrote:


Don't confuse Virtual Address Space with Physical RAM. With Windows
XP 32-bit enabling PAE does not permit the operating system to access
RAM addresses remapped above the 4GB boundary. If the OP has 4GB of
RAM in his machine he will not be able to use all of it. The Physical
Address Space on Windows XP 32-bit is limited to 4GB and using
Physical Address Extensions does not change that limit.

Argh, now I got it wrong as well. But I found the conclusion: With the
/PAE switch, the kernel does not limit the physical address range at all.

No, with Windows XP the use of the /PAE switch does not change the
available physical address space,


Which is 36 bits, as long as the board is properly wired.

it will still be limited to 4GB, the

XP PAE kernel will not make use of RAM addresses remapped above the 4GB
boundary, it is completely limited to the lower 4GB arena.


As you may read up in the link I provided, the Technet documentation is
obviously not precise enough. When using PAE implied through the NoExecute,
these limits are applied - if you use PAE explicitly, this doesn't happen.

The use of the /PAE switch is different for 32-bit Server products than

it is for Workstation products.

This changed in XP SP2.

Sebastian G. wrote:

John John wrote:


Don't confuse Virtual Address Space with Physical RAM. With Windows
XP 32-bit enabling PAE does not permit the operating system to
access RAM addresses remapped above the 4GB boundary. If the OP has
4GB of RAM in his machine he will not be able to use all of it. The
Physical Address Space on Windows XP 32-bit is limited to 4GB and
using Physical Address Extensions does not change that limit.


Argh, now I got it wrong as well. But I found the conclusion: With
the /PAE switch, the kernel does not limit the physical address range
at all.


No, with Windows XP the use of the /PAE switch does not change the
available physical address space,



Which is 36 bits, as long as the board is properly wired.

it will still be limited to 4GB, the

XP PAE kernel will not make use of RAM addresses remapped above the
4GB boundary, it is completely limited to the lower 4GB arena.



As you may read up in the link I provided, the Technet documentation is
obviously not precise enough. When using PAE implied through the
NoExecute, these limits are applied - if you use PAE explicitly, this
doesn't happen.

The use of the /PAE switch is different for 32-bit Server products than


it is for Workstation products.

This changed in XP SP2.

No, it hasn't. Regardless of which switch is used or not used in the
boot.ini file Windows XP computers with 4GB of RAM installed cannot see
or use all of the installed RAM if one of the Service Packs is
installed. The information on the memory addressing and the use of /PAE
in the article that you point to applies to servers only, it does not
apply to Windows XP.

Physical Address Extension - PAE Memory and Windows
http://www.microsoft.com/whdc/system...AE/PAEdrv.mspx

How PAE X86 Works
http://technet2.microsoft.com/window....mspx?mfr=true

The system memory that is reported in the System Information dialog box
in Windows Vista is less than you expect if 4 GB of RAM is installed

Who ate my memory?
http://blogs.msdn.com/dcook/archive/...my-memory.aspx

The RAM reported by the System Properties dialog box and the System
Information tool is less than you expect in Windows Vista or in Windows
XP Service Pack 2
http://support.microsoft.com/kb/888137

John