If you want express video, take a ride on the local bus


My friend Dave is looking for a new computer. This is not his favorite occupation because it means a visit to the Land of Technobabble. As usual, he returned from this trip far less enlightened than he was when he embarked.

"I told the guy at the store I wanted a fast computer," he said. "So he told me to take the local bus, or something like that, because it would make the computer screen faster. I told him I didn't want any slow old local bus. I wanted a computer with an express bus. He looked at me like I was crazy. I figured he just didn't have any of those express bus computers in stock, so I left. You know where can I get one?"

Sorry, Dave, you've just run into one of the latest PC buzzwords, and it's more than a little confusing. The salesman was right. Local bus video is fast. In fact, it's really an express bus. They just call it a local bus, which must have made sense to the guys who invented it but doesn't make much sense to anybody else.

Whatever it's called, you'll be seeing more of it in the next year or so as manufacturers develop ways to deal with the huge power demands that graphical environments, like Microsoft Windows, put on systems.

First we'll deal with the bus issue. Most of us think of a bus as a large vehicle that we get stuck behind while it belches obnoxious fumes in the process of transporting people rather slowly from one place to another. The analogy isn't perfect in the computer world, because here the term bus refers to the highway, or wiring system, that's used to transport data from your computer's microprocessor to a variety of other important devices, including disk drive controllers, video controllers, printer ports and the like.

Those controllers plug into the expansion slots inside your computer. The slots are wired directly to the bus, but you can't see the bus itself very well unless you're willing to perform exploratory surgery on your machine, which I don't recommend.

Now the amount of traffic that any highway can handle in a given period of time is theoretically controlled by three factors -- the number of lanes, the speed limit and the number of 80-year-old guys cruising at 40 in their 1957 Nash Ramblers. Luckily, in the computer world, only the first two count.

When IBM set the current industry standard with its Advanced Technology (AT) bus back in the mid-1980s, it designed a highway that was 16-bits wide with a speed limit of 8 megahertz.

A bit is the smallest unit of data that a computer can process -- a binary zero or one. So a 16-bit bus allows 16 little zeros and ones to go marching down the road, side by side. The 8 mHz speed means the bus can handle 8 million little cycles each second.

The original IBM AT computer's microprocessor could handle only 16 bits of data at a time, and it had a clock speed of 8 mHz, too. So the bus wasn't a bottleneck.

Things are different now. The Intel 80386 and 80486 processors in most computers today can handle 32 bits of data at a time, and they run as fast at 50 mHz. So the old bus design, now known as the Industry Standard Architecture (ISA), can really slow things down.

In the late 1980s, IBM developed a new 32-bit bus design it called Microchannel Architecture (MCA). It could theoretically move twice as much data, but it wasn't compatible with any of the old-style controller boards for disk drives, video circuitry or other goodies. IBM also wanted a fortune from other manufacturers to license its technology. Not many people bought it.

To protect itself against IBM, the rest of the industry developed its own 32-bit bus, which it called EISA (Extended Industry Standard Architecture). EISA machines can accept all the old controller cards, which operate at the old data rate, or new, faster EISA controller cards.

EISA bus computers haven't been that popular, either, because they're more expensive and because the real problem with the old bus was its speed limit. MCA and EISA bus computers may have wider highways, but the traffic doesn't move much faster than it did on the old bus -- at least not enough to be worth a substantial price premium.

So to address one of the real speed problems -- the zillions of little screen pixels that computers have to move around in today's graphical environments -- manufacturers developed a special, additional bus to handle video controllers. Piped directly into the computer's processor, this "local" bus promised express video speed at a relatively small cost.

Early local bus video controllers were indeed faster, but they were proprietary.

Today, the Video Electronics Standards Association (VESA) has adopted a local bus design called VL that many manufacturers have accepted. The Intel Corp. is developing another, more sophisticated local bus standard, but the jury is still out.

Computers with VL-Bus slots are available today, and they can provide much faster screen displays. That's what you're probably getting when a dealer advertises a computer with "local bus" video. But be careful that you don't get a computer with a proprietary local bus.

With IBM-compatibles selling as commodities these days, some manufacturers are starting to add local bus video to their machines to differentiate them from the pack. They're good values, if you need a fast screen display. Over the next few years, we'll see local bus disk controllers, too. They may have an even greater impact on speed.

So there's today's lesson in technobabble. If you want express video, take the local bus.

(Michael J. Himowitz is a columnist for The Baltimore Sun.)

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