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Old 05-08-10, 12:55 AM   #26
Rakeesh's Avatar
Join Date: Jun 2004
Location: Sonoran Desert
Posts: 6,869
Default Re: Installed Ram Shows 4GB and it says 2.5GB are usable

Only one of two solutions work for you. One, as everybody has already stated, use 64-bit windows. Two, use physical address extension (PAE.) Note that PAE is less CPU efficient when it comes memory addressing.

The problem is this: 32-bit kernels can only address 2^32 bytes of memory, or 4GB. The easiest way I can think of to describe that problem in plain English is to compare memory addressing to a table of contents in a book. Suppose that the page numbers in a table of contents could only be two digits long. This means that the table of contents can't refer to, or "address" any page above 99. Let's also suppose that this same book has 120 pages, and you can't turn to any page that isn't addressed by the table of contents. This means that the extra 20 pages you have are useless.

Does that make sense? Now in a computer, anything that is memory takes up physical address space in this "table of contents." This includes your RAM (that "4GB of memory" number you throw around) your motherboards BIOS, your video card, and any memory on any PCI cards you might be using, etc. It all adds up to above 4GB. Your main "memory" goes last in this address space, e.g. it is what is at the end of the table of contents no matter what. So the actual memory that you can use is your 4GB minus everything else. Hence, you can't use all 4GB. How much you can actually use depends on what other components you have.

Physical Address Extension is basically saying instead of one table of contents, we can have two or more in additional volumes that are in the same book. Everything past page 99 goes into volume 2, and the page numbers start all over again. It takes more CPU operations per memory operation to do this however, thus it's slower to do it this way.

If we take this analogy from 32-bit to 64-bit, then we double the number of pages that the table of contents can address. So instead of a maximum of 99 pages, we can go up to 9999 pages.

Make sense?
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