Earlier types of RAM were rated in nanoseconds (ns), while SDRAM is rated in megahertz (MHz) because its speed is somewhat dependent on your system's bus. Your motherboard's bus rating will determine whether you can use 66MHz, 100MHz, or 133MHz SDRAM. You can use faster RAM on a slower motherboard, but the RAM will run only as fast as the motherboard allows.
PC100 is standard issue for Intel processors matched to motherboards with a 100MHz bus. A 100MHz RAM module without PC100 on the label may not run that fast in your system, since it is required to peak at 100MHz only and not necessarily sustain it. The same principles apply to 133MHz modules and the PC133 standard.
Don't just pick RAM at random. The simplest way to ensure you get the right kind of memory is to visit a memory vendor's Web site and look for a memory configuration page that will match your system up with the correct memory type. When in doubt, check your motherboard manual or call its manufacturer and ask. If you can't get anyone on the line, give the serial number and model name of your computer to the memory supplier. If you have a generic PC or if you built your own, jot down your motherboard model and serial number and contact your memory supplier. It should be able to provide the exact memory type you need (and possibly even help you identify which modules are compatible with your system).
If all else fails, take a memory module out of your PC to a local PC repair shop. (Make sure you ground yourself before removing the module. Then, put it in an antistatic bag; if you don't have one, get one from the repair shop or your company's IS department.) If you see more than one kind of RAM installed, take one of each type. The repair shop should be able to identify the kind of RAM your PC uses and help you to upgrade or replace it.
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.
DDR SDRAM (double-data-rate SDRAM): DDR SDRAM is similar to SDRAM, but it transfers data on the rising and falling edges of the system clock, theoretically doubling the speed of SDRAM. Though DDR SDRAM was originally developed for graphics cards, desktop PC manufacturers are beginning to use it as a faster alternative to SDRAM.
DRDRAM (direct Rambus DRAM; formerly RDRAM): A type of SDRAM developed by Rambus, DRDRAM is used with high-end Intel CPUs. Like DDR SDRAM, it transfers data on the rising and falling edges of the clock cycle. DRDRAM also uses two data channels to improve transfer rates. In a Pentium 4 system, DRDRAM is often called PC800 and is based on doubling the Pentium 4's 400MHz bus. Note: DRDRAM is more expensive than DDR SDRAM.
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