All This PC Memory Stuff Is an Awful Lot to Remember
RAM. DRAM. VRAM. EDO RAM. Sam I Am. DIMM. SIMM. I’m not him. Sounds more like a Dr. Seuss story than a discussion of computer memory. What does it all mean?
Think of your computer as a small office. You store all your important documents in the file cabinet--the hard drive. When you want to work on a document, you take it out of your file cabinet and put it on your desk’s work area, which is the computer’s random access memory, or RAM.
In the early days of the PC, if you wanted to upgrade your computer memory, you had to pop in a series of memory chips. Bend one tiny pin and the entire chip was toast. Then somebody thought to put a row of chips on a small circuit card that could easily be popped into a slot on the motherboard. Thus was born the single inline memory module, or SIMM.
SIMMs made memory upgrades a snap, literally. They became the workhorses of personal computing. But as computers such as Pentiums and PowerMacs got faster, SIMMs couldn’t quite keep up. Because SIMMs are configured in a single row, the data path is limited to 32 bits.
The need for faster RAM access led to the invention of the dual inline memory module, or DIMM, which has a bit path of 64 bits--twice that of a SIMM. Like a V-8 engine with its efficient double row of cylinders, DIMM chips are organized into two rows on the circuit card. The layout enables DIMMs to process information much more quickly and efficiently than SIMMs.
When someone asks how much RAM you have in your computer, they’re usually talking about the dynamic RAM, or DRAM. DRAM is the most common--and least expensive--form of RAM. When you say, “I’ve got 65 megs of RAM,†you’d be more correct, technically, to say, “I’ve got 65 megs of DRAM.â€
Extended-data-out RAM, or EDO RAM, has been the staple in DRAM for some time. EDO RAM differs from older types of DRAM in that it has built-in smarts. As your computer’s central processing unit, or CPU, needs information, it accesses it from RAM. EDO RAM assumes that the next request will be for information located next to the data that was just accessed, so it holds its place in that spot. The result is memory performance up to 40% more efficient than with older types of DRAM.
While CPU speeds have been pushing past 250 megahertz, bus speed--the time it takes for data to move through the rest of your computer--has long been limited to 66 MHz. But with the Pentium II and PowerPC 750 processors have come motherboards with significantly faster bus speeds. EDO RAM--and the faster burst EDO, or BEDO RAM--can’t quite keep up. That’s why attention is shifting to synchronous dynamic RAM, or SDRAM, which is at home with bus speeds of up to 100 MHz.
SDRAM has been available for some time, but since it’s been more expensive than EDO RAM, it didn’t generate much interest--until bus speeds started increasing. The new interest in SDRAM is likely to drive its price down, making it more common in the super-high-speed PCs debuting in coming months.
There are two types of special-purpose RAM. The first is cache RAM--memory storage space not on the hard drive or in regular DRAM--which is used for data your computer thinks it might soon need again. It speeds up your system’s performance considerably.
Most modern CPUs have on-board cache. You can add external cache RAM, also called Level 2 cache, to most modern systems.
The other special-purpose RAM is video RAM, or VRAM. Today’s computers can display graphics in 8-bit color, also called 256 colors; 16-bit color, sometimes referred to as thousands of colors; and 24-bit color, also called millions of colors or photo-realistic color.
The higher your monitor’s resolution, the more VRAM you need on your video card to display the same color bit-depth. When you buy a video card, make sure you can upgrade its VRAM in case you later decide to buy a larger monitor.
