Access more memory

Microsoft Developer Network, all non-server 32-bit versions of Windows XP and Vista impose a memory limit of 4GB. Even if you have 4GB of memory installed in your PC, you may not be able to use it all. For example, if your video card comes with 512MB of memory and you have 4GB of RAM, your system actually has 4.512GB of memory physically installed. But Windows will use only 4GB of that total, regardless.


There are tweaks to improve the way Windows manages memory, freeing up more RAM for your applications.

One method is to use Physical Address Extension (PAE), a feature of x86 processors that lets 32-bit operating systems overcome the 4GB memory limit. 32-bit Windows operating systems support PAE. Even though XP and Vista still cling to the 4GB limit with PAE enabled, the feature may help you get back some of your unused RAM.

Windows relies on the technology to support the security feature known as Data Execution Prevention (DEP). ( See Data Execution Prevention & Boot.ini File) If a computer supports hardware-enforced DEP, then PAE is enabled as well. Here's how to check for it in Windows XP:

Step 1. Choose Start, Run.

Step 2. Type sysdm.cpl and press Enter.

Step 3. Click the Advanced tab. In the Performance box, click Settings and choose the Data Execution Prevention tab.

Step 4. Look for a status message at the bottom of the dialog box. If it indicates that your hardware does not support DEP, chances are PAE is not enabled.

To check your system's PAE status in Vista, do the following:

Step 1. Press Win+R to open the Run dialog box.

Step 2. Type SystemPropertiesDataExecutionPrevention and press Enter.

Step 3. If prompted by User Account Control, click Continue.

Step 4. If the status message at the bottom of the dialog box says your system supports DEP and the "Turn on" button is selected, then PAE is enabled as well.

If PAE is not already enabled on your system, here's how to activate it in Windows XP:

Step 1. Choose Start, Run.

Step 2. Type notepad c:\boot.ini and press Enter.

Step 3. Under the [operating systems] heading, look for a line that contains the /noexecute switch, which turns software DEP. For example, it may be /noexecute=optin, /noexecute=optout, or /noexecute=always on. Place the cursor directly after that switch and type a space followed by /pae. Save the file and reboot.

If you don't have DEP enabled on Vista (or you don't want it enabled), you can still activate PAE by following these steps:

Step 1. Click Start, type cmd.exe and press Ctrl+Shift+Enter.

Step 2. If prompted by User Account Control, click Continue. This opens a command prompt window with administrator privileges.

Step 3. At the prompt, type BCDEdit /set PAE ForceEnable and press Enter.

Some drivers will not load if PAE is enabled. After you make this change, keep an eye on your system. If you have problems with drivers or your system starts acting up, remove the /pae switch from boot.ini in XP, or enter the following command line in an administrator command prompt in Vista:

BCDEdit /set PAE ForceDisable

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