What about the fine print?
Depending on the type of motherboard you have, memory modules can also be of a certain type, speed, or in some cases, a specific metal lead type.
The types of RAM used in PCs over the past few years include, from fastest to slowest: SDRAM (used only in DIMMs), EDO (extended data out), old-fashioned FPM (fast-page mode) DRAM (found only in SIMMs), and DRAM (used in both DIMMs and SIMMs). Most motherboards require that you use only one of these four types. Some motherboards can handle more than one type, but still don't allow you to mix them. Consult your manual to find out which type of RAM you need, and whether you can use two different types at the same time or not.
The speed of SDRAM produced today is measured in megahertz (MHz). Your SDRAM must match the speed of your system bus or it won't perform as well as it's rated. Most new computers with SDRAM use PC100 or PC133 RAM. In other words, the RAM communicates with the CPU via a 100MHz or 133MHz bus. PC66 RAM was once de rigueur, but Intel popularized the PC100 standard with its 400BX motherboard, which supports Pentium II and Pentium III CPUs.
Older DRAM (EDO and FPM) speeds are measured in nanoseconds (ns). The lower the number, the faster the RAM. You can install DRAM with a faster speed rating than your system requires, but it won't improve performance. You cannot, however, install slower RAM or mix EDO and FPM DRAM in the same system and expect it to work.
SIMM socket contacts, or leads, are plated with tin or gold. Most manufacturers will insist that you buy SIMM modules that are made out of the same metal as your motherboard, or else run the risk of oxidising the module contacts over time. Manufacturers of DIMMs addressed this issue by making all contacts out of gold.
Until a few years ago, virtually all PCs depended on parity RAM, which supported a primitive form of error checking through the use of a ninth parity bit for each 8-bit byte stored. In the early days of the Pentium era, when memory was hard to come by and prices were high, many PC manufacturers switched to non-parity RAM. Most systems you'll encounter don't support parity at all; on others, it's an option at setup. If you're upgrading an older machine, make sure you don't mix parity and non-parity SIMMs.
Error Correction Code (ECC) RAM not only checks for errors, but also corrects some of them along the way. ECC RAM is more expensive than other types of RAM, and it's typically used only in machines that used for mission-critical applications, such as file servers. ECC and non-ECC RAM cannot be mixed.
