Hardware Tips: Choose the Right Kind of Memory for Your System

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Adding RAM to your PC usually delivers the most bang for your upgrade buck, but only if you buy the right kind of memory module for your PC. There are more types of PC RAM than there are lattes at Starbucks: Do you want SDRAM, PC100, nonparity, or unbuffered DIMM? Why not enjoy a refreshing DDR SDRAM, PC2700, CL2.5, or registered DIMM? Here are the ins and outs of PC memory. (See "Step-By-Step: Add RAM for a Faster PC" for installation instructions.)

Begin by checking your system's user manual to identify the types of RAM your PC's motherboard supports. If you don't have the manual, visit the manufacturer's Web site and search for downloadable manuals or other tools that might help you find the information you need. Memory vendors Crucial Technology and Kingston Technology include utilities on their sites for identifying the right RAM for your PC. Enter the make and model of your PC or motherboard to generate a list of compatible RAM types. The configurator maintained on Memory Stock's site indicates your model's standard and maximum memory, and its total number of memory sockets, among other things (see FIGURE 1

FIGURE 1: Find out what type of RAM your PC uses, and other information, with Memory Stock's online configurator.

Before you buy, ascertain the following:

Maximum module size: Find out the maximum size of memory module that your PC supports. Don't buy a module larger than what your motherboard's memory slots can each accommodate.

RAM and connector types: Determine which of the four types of RAM your system uses: DRAM (EDO or FPM), SDRAM, DDR SDRAM, or RDRAM. All four types are mounted on one of three module types: SIMM, DIMM, or RIMM.

Most machines support only one type of RAM and have one type of module or connector, so mixing types isn't an option. The few motherboards that do accept two types of RAM allow only a single type to be used at any one time.

Memory speed: SDRAM, DDR SDRAM, and RDRAM are rated to match or exceed the PC's frontside bus speed, which is the speed at which data moves between the CPU and RAM. If your system comes with PC66 SDRAM, you can use PC100 SDRAM to replace it and get the faster speed, as long as your PC's frontside bus supports the higher rate. But if you mix RAM of different speeds, all RAM will operate at the speed of the slowest chip.

Memory banks: On some PCs, the memory slot closest to the CPU--usually called bank 0--must be filled before the motherboard's other memory slots. On other systems, bank 0 must have the largest RAM module (if you are using modules of different sizes). There's no fixed rule, so check your PC's documentation.

Nonparity or ECC: If your system supports error-correcting code (ECC) and has more than 512MB of RAM, buying ECC memory may be worth the added cost. Large amounts of RAM are more likely to experience occasional, random errors (which may be caused by cosmic rays, among other sources). However, unless your current RAM is ECC, forget it; you can use nonparity and ECC modules together, but error correction will be disabled.

To determine your type of memory, count the number of chips on the memory module. If the number is divisible by three, you have ECC or parity memory.

Column address strobe: The lower the CAS rating--or the CL rating--is, the better. SDRAM comes in CL2 or CL3 types, and DDR SDRAM comes in CL2 or CL2.5. Unless your motherboard requires a specific CAS or CL rating, get the lower (faster) rated module. Cost differences should be negligible. Again, if you mix modules of different speeds, they'll all operate at the slowest module's speed.

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