Memory comes in a variety of packages, pin configurations, types, and formats. Do you need SIMMs, DIMMs, or RIMMs? Do you need parity or ECC? Confused yet? If terms such as PC100 SDRAM Non-ECC make your eyes glaze over, we're here to help.

	168-pin DIMM

There are two basic types of RAM available today: SDRAM (synchronous dynamic RAM) and RDRAM (Rambus dynamic RAM). And there are three incompatible forms of SDRAM memory modules currently in widespread use. The form you can use depends on the RAM sockets installed on your motherboard. Most systems available on the market today take SDRAM in 168-pin DIMMs (dual in-line memory modules). Older Pentiums and later 486 PCs use FPM and EDO RAM in 72-pin SIMMs (single in-line memory modules).

RAM modules are usually a little more than an inch high and four to six inches wide; look for a row of slots projecting from the motherboard with at least one module plugged in. The number of pins refers to how many gold (or tin) contact strips are on the edge connector side of the SIMM or DIMM.

72-pin SIMM

You can buy DIMMs that hold up to 256MB or more, but note that not all motherboards support all module sizes. To determine the limitations of your system, check your motherboard or PC manual. On the expensive side are 30-pin SIMMs, which are rarely produced anymore. If you're thinking of getting 30-pin SIMMs to speed up an older machine, shop around first. In the long run, it may be cheaper to purchase a whole new system or to upgrade the motherboard.

Glossary terms Need help sorting through all the acronyms? These definitions will help. SIMM (single in-line memory module): A memory-chip module with 72 or 30 pins that connect the module to the PC motherboard. SIMMs are normally found in older systems. They support 32-bit and 16-bit data transfers, respectively. DIMM (dual in-line memory module): A memory-chip module with 168 pins that connect the module to the PC motherboard. DIMMs are the most popular memory module available today. They support 64-bit data transfers. DRAM (dynamic RAM): This older, "classic" form of RAM has been supplanted by SDRAM. DRAM stores each bit in a storage cell as an electrical charge that must be refreshed every few milliseconds to retain the information. Although less expensive than SDRAM, DRAM is also slower. EDO RAM (extended data-out RAM): A faster form of DRAM, EDO extends the time that output data is valid, which eases timing issues between the CPU and RAM and enhances performance. EDO has now been replaced by the even faster SDRAM. SDRAM (synchronous DRAM): The replacement for DRAM, FPM, and EDO, SDRAM synchronizes memory access with the CPU clock for faster data transfer. Basically, one block of data can be sent to the CPU while another is being prepared for access.