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If Memory Serves

Do It Yourself?

Determining which type of RAM you need, however, isn't easy. With so many types, in many speeds, sizes and capacities, it's easy to buy the wrong kind.

The type of RAM you'll be upgrading is called DRAM (Dynamic RAM). If you own a brand-name PC, upgrades will be relatively straightforward. Call a reputable memory vendor, and tell them your PC's brand and model. The company should be able to find RAM modules that fit your system.

But if you have a no-name clone or want to buy your memory at a computer superstore, you'd better know what you're doing. Here are the factors you need to consider:


  • SIMMs and DIMMs: First, you need to know whether your computer uses SIMMs (single in-line memory modules) or DIMMs (dual in-line memory modules). A SIMM or DIMM is a plug-in board with a number of RAM chips that fits into a socket on the motherboard. SIMMs come in 30- or 72-pin varieties, while the newer DIMM design has 168 pins. If you have a 486 PC, you're probably using SIMMs with either 30 or 72 pins. Pentium PCs usually use 72-pin SIMMs or 168-pin DIMMs.


  • Pins: These are tiny vertical gold bars at the bottom of the memory module. The easiest way to find out how many you have is to remove the PC's cover and measure the size of the module. A 30-pin SIMM is just over 3 inches long, a 72-pin SIMM measures a bit over 4 inches long, and DIMMs are about 5 inches long.


  • Memory speed: Most of today's PCs use either 60- or 70-nanosecond (ns) RAM. To determine which speed your computer uses, consult your PC's manual or technical specifications, or ask your vendor. If your PC requires 70ns RAM, you can use the faster 60ns chips, although you won't get any speed advantage. Don't buy RAM that's slower than required because it won't work in your system.


  • Memory banks: When you add RAM modules to your PC, you'll need to fill an entire memory bank. On some PCs, one bank can hold four SIMM slots; on others, two SIMMs or one DIMM fill up a bank. If you can't tell by looking at the motherboard, check its documentation to determine what constitutes one memory bank in your PC. If your memory banks are full, you'll need to replace your existing RAM with higher-capacity SIMMs or DIMMs.

Remember, you can't mix memory of different capacities, such as 1MB and 4MB SIMMs, in the same bank. You can, however, mix them in terms of having, say, one bank full of 1MB SIMMs and another bank full of 4MB SIMMs.

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This article was originally published in the April 1997 print edition of Entrepreneur with the headline: If Memory Serves.

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