Problem:

I bought 4 GB of RAM! Why does the computer say I have only 3?



Solution:


Currently, there are limitations to using 4GB of RAM or more, especially in older notebooks. The first thing to keep in mind is that 32-bit operating systems (like the most common versions of Vista and XP) cannot use the full 4GB. Even though 32 bits can be used to compose addresses in a 4GB range, some of those addresses will be reserved for memory-mapped devices like the GPU and the HDD.  The net effect is that you will see between 3 and 3.5 GB of RAM, depending on the particular type of computer. Note that recent patches to 32-bit Windows XP and Vista enable them to recognize that the computer has 4GB of RAM, and display 4GB in the system properties; that doesn't mean they will use the whole amount, however. They are still limited to 3.5 GB, and there is no way around this limitation, so the information in the system properties is simply misleading.

Before the 32-bit limitation kicks in, there are several other things that can go wrong with older notebooks:
The hardware (chipset) might not be able to address the entire range. If so, there's nothing you can do except upgrade the motherboard (which in the notebook world is equivalent to buying a new computer).
Even if the hardware is able to address 4GB, the BIOS might not support it. A BIOS update might fix this, if the mainboard manufacturer included memory support in the BIOS fixes (check the list of changes).

Newer notebooks (at least the Penryn and Montevina generations) should not suffer from these problems.

If your hardware supports 4GB of RAM, one way to make use of all of them is to upgrade to OS to Vista 64 (if you're willing to use Vista). Make sure you have the necessary drivers before you take this step, though! There also exists a 64-bit version of Windows XP, but driver support for that version is very bad, so I don't recommend it. One (not so good) alternative, with which you can keep a 32-bit OS, is to enable PAE, if your hardware supports it; but PAE implementations are fraught with problems on many consumer machines.

Even if your computer doesn't use all 4GB of RAM, there's no big reason to worry: the difference from 3GB to 4GB of RAM is very difficult to feel in real life with current OS-es and applications, except in isolated cases e.g., working with graphical tools and very high-resolution images.