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Actually, the limit is 4GB. Windows hides 0.5gb, because this RAM is essential for the operational system tasks, and hides something else depending on how much RAM your VGA has, also how much of your RAM exceeds the 4GB limit. Only 32bits OS have 4GB limits, it's happens due 32bits architecture. That's why people using 64bits Vista doesn't have this problem. Wikipedia quotes: By the way, readyboost isn't a program that uses flash drives - like pendrives - to increase RAM? Or it enables the use of unseen RAM, on the real RAM hardware? Because the main problem here isn't the limit, it's wasting the RAM that you already have, but the OS can't "see" or address. Enabling PAE allows the system to see this ram, but not address yet due the architecture limitation, and that's why something like ramdisk is needed. If you can't use that RAM like "Physical memory" you still having the option to set it as "Virtual memory". In theory it should be as fast as physical memory, because you still acessing your RAM. Flash drives are slower than your RAM, paging file at hard disk also gives you lag.

32bits Operational System address a maximum of 4GB of RAM. But... There's a problem. Any 32 bit OS can only address 4 GB of total memory, and so when booting into the OS, it first addresses I/O devices with their own onboard memory like VGAs and things like that first. If you have a 512mb VGA, it will be less 512mb of avaliable RAM. So, what can be done to avoid this waste of RAM? It's simple. Use a RAM disk. What is that? A RAM disk works like a new partition, but this partition isn't created in your Hard Disk... it's created in your RAM. We can say that RAM memory is a really fast HD used to archive temporary data used by running programs. So, it's possible to create a partition using that. Enough of explanation, let's go to the Tutorial. Step 1 First of all, you're going to need to do some calculations. For example, if I have: 6GB of total RAM and 1GB VGA my system will adress only 4GB. So, it wastes 2GB of unseen RAM plus 1GB of RAM due VGA's RAM, giving total of 3GB of unseen RAM. (Remeber, 32bts OS address 4GB of total RAM). If you have 4GB it's easier, yours VGA RAM will be the amount of wasted RAM. Now you know your wasted RAM, we're going to the next step. Step 2 You need to download Gavotte's RAMdisk. There's many options, but usually they are paid options, or they aren't able to make use of the unadress RAM. It's not the case of Gavotte's Ramdisk, it's a freeware that can use that unseen RAM. http://www.crocko.com/1907118935/Gavot5_200811130.zip Step 3 Unzip the folder and double-click "ram4g.reg" file. You need this Registry before doing anything else. Step 4 Go to C:\ and open your boot.ini with Notepad. Here you're going to see something like that: [operating systems] multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect Add /pae ate the end of the line: multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect /pae If you aren't seeing any file with this name, select the Tools menu and click Folder Options. After the new window appears select the View tab. Here you need to put a checkmark in the checkbox labeled Display the contents of system folders. Under the Hidden files and folders section select the radio button labeled Show hidden files and folders. Remove the checkmark from the checkboxs labeled Hide file extensions for known file types and Hide protected operating system files.Now, press the Apply button and then the OK button. Step 5 Reboot your system. Now, at your System Properties you can see "Physical Address Extension" under system's specs. You can run now "ramdisk.exe" inside the RAMdisk folder. Now Install your RAMdisk... Check "Fixed Media" and type your wasted RAM, apply and check ok. Reboot again. If you enabled your PAE before install RAMdisk and set the right amount of RAM, it will appear as RamDisk-PAE (R:). If not, redo the steps from 3 and try to adress less memory on the RAMdisk's installation. Step 6 You can ask me "Ok, I've done every thing. Now, what can I do with this thing?!". Simple. You can use it to your browser cache for example, or do what I do... use it for my system's virtual memory. To do that open My Computer, right click and select System Properties, go to Advanced tab, under Performance click on Settings button. Here click on Advanced again, and go to Virtual Memory. Here select your RAMdisk, write the amount and click in "set". Enjoy. But Attention! Any data stored on RAMdisk disappears upon restart. It's not a good idea try to use it like a normal partition... And it isn't a good idea to do that on 64bits OS. There aren't any proofed benefits. More about the subject: https://en.wikipedia.org/wiki/Memory_address https://en.wikipedia.org/wiki/Physical_Address_Extension