Sorting out PC memory's letter jumble

August 29, 2002|By MIKE HIMOWITZ

THERE ISN'T another gadget that comes with more alphabet soup than a computer. By that I mean the tangle of abbreviations and acronyms that techies were born knowing but the rest of us have to wade through when we buy a PC or fix one.

Lately I've been hearing questions from buyers perplexed by the alphabet soup of computer memory. They see PC's advertised with SDRAM, DDR RAM, RDRAM, SIMMs, DIMMs, RIMMs and other terms that make no sense at all.

One reason is that memory chips - which were fairly standard for a couple of years - have branched off in different directions. PCs with the same processor can perform much differently if they're designed for different types of memory. As a result, it's worth wading through the soup and learning a bit about memory before buying.

The term "memory" refers to chips that store programs and data while your computer is running. These chips are known collectively as RAM, an acronym for random access memory. RAM is typically measured in megabytes (MB), or millions of bytes of data storage. Most PCs today come with 128 or 256 megabytes.

Unfortunately, many users confuse memory with hard disk storage. They're quite different, although they work together.

RAM is volatile - anything stored in it disappears when you turn off your computer. Your hard drive, on the other hand, stores programs and data permanently. Your computer transfers programs and data from your hard disk into memory when it's turned on. Hard disk storage is measured in gigabytes (GB), or billions of bytes of data. Today's PCs typically come with 20- to 40-gigabyte hard drives.

Obviously, you have a lot more hard disk space than memory, and here's where it gets a bit complicated. If you run out of physical memory because you're working with several programs or large documents, your computer stores the overflow temporarily on your hard drive, creating something called virtual memory. Unfortunately, reading and writing to a hard drive is much slower than using real memory, and the process is more prone to glitches.

As a result, your computer will run faster and more reliably when you add memory. In fact, if you have a computer with 128 MB or less, upgrading to 256 MB will give it a real kick in the pants. If a new PC you're considering has less than 256 MB, have the memory upgraded before you leave the store. For most work, 256 MB is plenty, but if you're an avid gamer or plan to edit digital photos or video, having 512 MB or even a gigabyte of memory can pay off.

Memory chips come on modules - small printed circuit boards that fit into slots on your computer's main circuit board, or motherboard. Depending on your computer's design, it may have two, three or four memory slots.

Modules in common use today are generally known as SIMMs (single inline memory modules) or DIMMs (dual inline memory modules). SIMMs are an older design - they usually have 72 pins - while the more recent DIMMs have 168 pins. Both come with varying memory capacities, typically 128 or 256 megabytes today, with older designs available in 32 or 64 megabyte modules.

Over the years, the industry has improved both the capacity and speed of memory chips, which is good. Unfortunately, those improvements have often required changes in motherboard design, so that many older computers can't use the latest memory chips, and vice versa. The design of your motherboard - particularly the chipset that transfers information from the processor to memory - determines what kind of memory and how much your computer can handle.

Almost all main memory is "dynamic," which means it has to be refreshed thousands of times per second. For that reason, you'll often see main memory referred to as DRAM.

About five years ago, the industry came up with a system that improved memory speed by "synchronizing" the reading and writing of data with the computer's internal clock. That led to the acronym SDRAM, the basis for most types of memory today. If you see a PC with plain SDRAM, it's the bottom of the line. It doesn't mean the computer won't run well - it may be perfect for your needs. But it's not going to provide top-level performance.

A newer technology allowed computer makers to double the rate at which memory chips could exchange data. These chips tack the abbreviation "DDR" in front of SDRAM, for "double data rate." A computer with DDR memory can theoretically perform better than a computer with traditional SDRAM, even if the processors are running at the same speed.

Finally, you'll see computers with memory labeled RDRAM. This was an entirely new design from Rambus Inc., which Intel adopted two years ago for its Pentium 4 computers. RDRAM required yet another motherboard design and a different packaging module called a RIMM.

RDRAM was also considerably more expensive than SDRAM, and early on it failed to provide enough performance enhancement to justify the price increase. As a result, many computer manufacturers, and eventually Intel itself, developed Pentium 4 machines that could run with earlier types of memory. But many of the early performance problems with RDRAM have since been solved.

That's why you'll find all that alphabet soup on the shelves, machines with SDRAM at the low end, DDR SDRAM in the middle, and RDRAM at the top. Unless you crave top-of-the line performance and you're willing to pay a premium for it, RDRAM certainly isn't necessary. But the type of memory installed is a good indicator of the general class of machine you're buying.

Class dismissed.

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