I need my 4GB!

Four Gigabytes is a lot of RAM for a workstation or gaming machine, but the industry has pretty much forced us into using this much (especially if you're a heavy user of Virtual PC).  Most applications today are 32-bit and only address 2GB of RAM at a time (like Photoshop), but we need more memory for all the other running goodies that are memory-starved.  Even though Vista runs on 1GB of RAM, 2GB is alot better, and 4GB will unleash its potential.

Unfortunately, users of the 32-bit flavors of Vista have to run through some hoops to see all of the new RAM they've upgraded to.  Even then, they may not see all of the RAM available because of various hardware issues.  This article addresses various methods so you can utilize that massive amount of memory.

All 32-bit Operating Systems can only physically address 4 gigabytes of RAM, due to the math involved (2³² = 4,294,967,296).  Depending on your hardware, Vista may only see 3GB or 3.5GB of the total RAM installed, because there are some devices that are memory mapped.  The most common culpret of this type of device is a video card, which can use up to 512MB of memory for mapping.

Enable PAE mode
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. 

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 PAE forceenable.

This PAE flag (Physical Address Extension) tells the Vista core to use an additional 4 bits of addressing, which in theory allows the OS to see all of the RAM you have available.  We're not out of the woods yet, because once you reboot you may find that Vista still doesn't see all of your RAM.  You can turn PAE off again by typing BCDedit /set PAE forcedisable or BCDedit /set PAE default.

Enable DEP mode
Also note that using PAE forces Vista to run theoretically slower, so you need to disable this feature 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

You can disable DEP by typing BCDedit /set nx AlwaysOn

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