Vista & accessing > 3g RAM - YMMV

Full article at http://www.ocmodshop.com/ocmodshop.aspx?a=989

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You can tweak Vista's core to add another 4-bits of addressing capability, but you must have a 64-bit capable processor (pretty much any processor made within the past two years like Intel's Core 2 Duo or AMD's Athlon 64). Even though the math works out to address way more than 4GB (236 = 68,719,476,736), the operating system still has a cap (Windows Server can address anywhere from 8GB to 128GB depending on the flavor).

To force this new addressing method, you have to tell Vista to boot using this new parameter. Vista no longer uses a BOOT.INI file as previous versions of Windows did, so you must modify the boot file using a built-in Vista tool called BCDedit.

Also note that using PAE forces Vista to run theoretically slower, so you need to disable PAE if you go back to a lower amount of RAM. Vista runs slower with PAE because of the new page-translation system being used. By default Vista uses 2 cycles to address memory, and will use 3 when Physicall Address Extension is enabled. PAE also supports advanced procesor features such as Data Execution Prevention (no execute), Non-Uniform Memory Architecture (NUMA), and hot-add memory. PAE is automatically disabled when DEP (Data Execution Prevention) is disabled, so you must force PAE when DEP is disabled by running BCDedit again:

1. Open a command prompt (Press Window key + R to open the Run dialog, and then type cmd).
2. Press Ctrl + Shift + Enter to execute (this allows you to run cmd in administrative mode if you haven't already setup up a permanent admin mode).
3. Type BCDedit /set nx AlwaysOff
4. Type BCDedit /set pae ForceEnable
5. reboot

 

PAE is nothing new, it's been around since Windowns 2000. It's not a magic solution to the 4GB problem, and I would not advise enabling it on a single-user PC, least of all a notebook. The performance hit will generally outweigh any benefit of access to extra RAM.

PAE was a CPU "hack" Intel added years ago, to allow servers running 32-bit OSes to access more than 4GB of RAM. Windows supports PAE through Address Windowing Extensions (AWE), one of several ways to exploit the feature. As servers move more to 64-bit OSes (as they should), it is less and less relevant. Servers need a LOT of RAM to serve many users and function properly, so the performance hit was an acceptable trade-off to gain the increased capacity.

But in a single-user workstation scenario, it only hurts you. PAE/AWE works by allowing the OS to page RAM in and out of the 4GB addressable space (because no matter what, an x86 processor can NEVER address more than 4GB at a time, since it uses 32-bit pointers, and processes under Windows are limited to 2GB of application space). As more RAM is needed, it is swapped into the physical address space so the CPU can get to it -- very similar to virtual memory being swapped to disk, just faster. But it's still a huge performance hit over native memory addressing, and unless you have an active need of more than 4GB, you aren't gaining anything but rather losing. Even if you aren't doing anything to exceed 3GB of memory use, a performance hit remains because of the constant background overhead that Windows uses to manage the feature.

In order for a single process to access more than it's 2GB of address space, it must actually include its own AWE calls to do so, something that virtually no single-user applications do either.

Here's a Microsoft article describing their implementations of PAE support, along with the pros/cons.

http://www.microsoft.com/whdc/system/platform/server/PAE/pae_os.mspx

 

thanks for this great info