Answer:
When your computer boots, it runs a variety of hardware tests and will often list the amount of RAM it detects. This screen may go by quickly on some systems or be completely masked on others. Often, you can press a key (such as ESC) to display the testing information. The key will vary among systems, so consult your system documentation for more information.
Sometimes you can add more RAM than the system can support. A motherboard with three DDR SDRAM sockets, for instance, may be able to support only 1024MB of DDR SDRAM. If you install three 512MB modules, you’ll have 512MB more than the system can support. In some instances, a BIOS (Basic Input/ Output System) update may help (consult your manufacturer’s Web site), but in most cases you’ll have to keep your system’s memory limits in mind when adding memory.
Keep in mind, also, that your system utilizes some memory for itself for 'housekeeping' and for devices such as video cards. This fact can account for why your system shows slightly less memory than is actually installed.
In some instances your PC may not be properly recognizing one or more memory modules. If you’ve recently installed new memory, check to be sure it’s properly seated in its socket and the module is compatible with your system.
If the module is properly secured and appears to be compatible with your system, try cleaning the pins using rubbing alcohol and a cotton swab. Clean each socket on the motherboard, as well.
If problems persist, try changing the order of your memory modules. DIMM sockets on the motherboard are labeled (DIMM0, DIMM1, etc). You should always start installing memory in the lowest numbered socket available. In many cases, the BIOS will probe the memory module in DIMM0 and address all memory according to the specifications of the module in DIMM0. If you’ve installed faster memory in DIMM0, the system may not be able to communicate with slower modules in other slots. Placing a slower module in DIMM0 may fix your problems; however, the faster modules will then, of course, run at a slower speed).
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