Every few months, fashion magazines tell women that their clothes are hopelessly out of style. Almost as often, computer magazines and enthusiast sites tell PC owners the same is true of their system memory.

But as last month's study of the CPU/memory interface explained, it's not the whims of fashion but the attempt to keep pace with ever-faster processors that makes PC memory a minefield of new technologies and tech jargon. One day EDO is all the rage; the next it's SDRAM, then RDRAM, then DDR -- but should that be DDR333 or PC2700? Trick question -- they're the same thing.

Let's take a step back and sort out the morass of memory types, specifications, and standards. This article will explain what all the acronyms and numbers really mean, and how they affect your PC-buying or -upgrading decisions.

It's All About the RAM

Every computer relies on Random Access Memory (RAM), a data storage medium that provides equally fast access to data in different locations (plus even faster "burst" retrieval from sequential or adjacent locations). Unlike read-only memory (ROM) chips or hard disks, RAM is volatile storage; it loses all data when the power is turned off.

As long as the power's on, static RAM (SRAM) can maintain its data with no need for constant refreshes; it's used most commonly for the CPU cache. Most PC memory, however, is dynamic RAM (DRAM), which needs constant rewriting or updating.

Since about 1998, older asynchronous types such as fast-page-mode (FPM) and extended-data-out (EDO) DRAM have yielded the market spotlight to synchronous DRAM (SDRAM), which runs in sync with the system bus -- i.e., is ready with fresh data every time the CPU asks for some. Michael Schuette, technical consultant for memory vendor Mushkin Enhanced Memory Systems, says, "What it all boils down to is that running the memory synchronous with the CPU bus gives the best performance by quite a margin."

As system bus speed increased, PC66 SDRAM (for use with a 66MHz bus) was rapidly succeeded by PC100 and PC133 memory. PC133 SDRAM is far from the performance cutting edge nowadays, but it can still be found in new entry-level or value-priced PCs.

DDR

The current generation of SDRAM is double-data-rate synchronous DRAM, usually known simply as DDR. It effectively doubles the speed -- delivering twice the bandwidth or throughput -- of SDRAM without increasing the clock frequency, because it transfers data on both the rising and falling edge of each clock cycle (the moments when a bit is changed from a 0 to a 1 and back again). The double-data-rate equivalent of 133MHz PC133 -- and staple of today's affordable, mainstream PCs -- is DDR266. Like SDRAM, DDR runs on a 64-bit bus and can be installed in either single or pairs of modules.

DDR is the current industry standard and commonly ships in 200MHz, 266MHz, and 333MHz speeds -- DDR266, for instance, yielding the best performance with the 266MHz-system-bus AMD Athlon XP 2200+, while DDR333 is the ideal match for the new 333MHz-bus Athlon XP 2700+.

System overclockers and tweakers can also play with not-officially-ratified-standard 400MHz and faster modules. Pioneers have discovered, however, that as DDR clock speeds increase, stability suffers and severe limitations are placed on the number of modules installed. For example, DDR400 is usually restricted to a single module on most platforms, which translates to a 512MB system maximum.

This has led to a less-than enthusiastic response to DDR400 and higher speeds, as any practical memory standard must span both ends of the PC market spectrum -- system vendors like performance as much as anyone, but not at the sacrifice of keeping costs low and stability high. Lately, the emphasis has shifted to pairing lower-speed DDR with dual-channel memory controllers; using two relatively low-cost DDR266 modules at once offers more effective bandwidth than DDR400. Dual-channel DDR chipsets are just now reaching the market, and are likely to proliferate and gain mass acceptance in 2003.

New Label, Same Contents

Before discussing other types of memory, let's pause for silicon identification, or the fact that DDR266 is also known as PC2100 -- the numbers referring not to clock speed, as is the case with PC133 SDRAM, but to memory bandwidth. The latter term is heavily used in product reviews, manufacturers' specs, and water-cooler debates, but exactly how these numbers are derived is still an arcane science to many.

Memory bandwidth is represented numerically by the theoretical maximum amount of data that can be passed in a second, such as 2.1 gigabytes for PC2100. To do the math, take the memory bus size, divide by 8 to convert bits to bytes, and multiply by the actual memory speed.