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.
- Open a command prompt (Press Window key + R to open the Run dialog, and then type cmd).
- 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).
- 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.