64 bit Windows on '06 C2D 4GB RAM?

[2ms]

I have a late 06 20" iMac with 2.16 GHz C2D. I have 4 GB of ram in it, but I seem to recall that the Intel chipset or something was not able to make more than 3 GB or something actually available to the system.

Is this true?

If so, that sucks, because I hear that the main speed improvements in using 64-bit Windows over 32-bit come when you have 4GB ram. I just installed Windows 7 RC 64-bit in hopes of it making my CAD work go a little smoother/faster, which is what other CAD people say it indeed does, at least if you have 4GB ram.

 

[madog]

From Microsoft:

Physical Memory Limits: Windows XP

Limit in 32-bit Windows**********Limit in 64-bit Windows
********4 GB**********************128 GB

Also:

PAE, 4-gigabyte tuning (4GT), and Address Windowing Extensions (AWE) serve different purposes and can be used independently of each other:

* PAE allows the operating system to access and use more than 4 GB of physical memory.
* 4GT increases the portion of the virtual address space that is available to a process from 2 GB to up to 3 GB.
* AWE is a set of APIs that allows a process to allocate nonpaged physical memory and then dynamically map portions of this memory into the virtual address space of the process.


Physical Address Extension (PAE) enables x86 processors to access up to 64 GB of physical memory and x64 processors to access up to 1024 GB of physical memory. PAE is supported by 32-bit Windows Server systems to enable access to up to 128 GB of physical memory. However, note that each process on 32-bit Windows is still limited to a 4 GB virtual address space. For details, see Memory Limits for Windows Releases.

The problem is, that with 4GB of RAM loaded up in XP, the system will only recognize ~3.25GB, so people don't think it can utilize a full 4GB. However,

In the 32-bit Windows world, each application has its own “virtual” 4GB memory space. (This means that each application functions as if it has a flat 4GB of memory, and the system's memory manager keeps track of memory mapping, which applications are using which memory, page file management, and so on.)

This 4GB space is evenly divided into two parts, with 2GB dedicated for kernel usage, and 2GB left for application usage. Each application gets its own 2GB, but all applications have to share the same 2GB kernel space.

So the OS itself has 2GB, and other applications share that 2GB, but then also get their own "virtual" 2GB as well. The 4GT option noted at the top of the page changes that from 2GB to 3GB "blocks".

Also, I don't believe 64bit XP is supported natively with Bootcamp drivers, but Vista 64bit is.

P.S. PAE was enabled on my MBP by default.

Edit: It's also due to MS changing PAE in XP SP2, and could be referred to as a "bug". I couldn't find the link before, but it's at the top of the references.

MS: Incorrect RAM reported

the-4gb-windows-memory-limit-what-does-it-really-mean.aspx

MS: Windows Memory Limits

MS: PAE

 

[2ms]

Is there a RAM utilization limit in my hardware though? I have confirmed that Windows 7 64-bit (what I'm using) will utilize 4 GB of RAM.

However, I seem to recall that my 2.16GHz iMac (that generation of Intel chipsets) is NOT able to utilize more than 3 GB ram. It's just a vague recollection though.

Is there in fact this hardware limitation, or was there only ever a Windows software limitation?

 

[TBi]

Is there in fact this hardware limitation, or was there only ever a Windows software limitation?

It can be both. It depends on your hardware. The easiest way to find out is to check Intel's website for the chipset you have. They have all the datasheet's detailing the max ram you can put on it.

There could be a hardware limit imposed by Apple if they don't use all the memory address lines. Basically if you can see 4GB in OSX then you can see it all in 64 bit windows.

 

[madog]

The PAE feature is limited by hardware, but I believe the C2D Intels that Apples uses can utilize it.