Q. I have seen several ads for screen-saver programs. Are they necessary?

A. Yes. If left on too long, a screen's phosphor coating may be damaged. This can result in a ghostlike image known as burn-in. Whatever was displayed at the time is permanently etched into the screen.

Until recently, screen savers have been popular mostly with the Apple Macintosh line of computers. But with the growing popularity of Microsoft Windows for IBM PCs and compatibles, this is changing. Windows's display is similar to that of the Macintosh. As Windows grows in popularity, so will screen savers for these machines.

A screen-saver program senses whether any activity, such as typing on the keyboard, is taking place. After a certain period of inactivity, the screen saver blanks out the screen to prevent burn-in. When activity resumes, the saver restores the screen's display. But a blank screen can mislead you into thinking your computer is turned off. To avoid this, screen savers display an animation to remind you that the computer is on.

But what kind of animation do these programs display? Therein lies an interesting tale of two screen savers. Pyro! from Fifth Generation and Berkeley System's After Dark, both popular with Mac users, are competing to see which can come up with the most elaborate screen displays.

Pyro was the first. For years, it was happy displaying a simple fireworks show. Then Berkeley came up with After Dark, which features 32 animation "modules" that allow the user to select from different animations.

Not to be outdone, Pyro was updated and got 12 modules. One of After Dark's more popular modules, called "Fish!," has fish swimming around on the Mac's screen. Pyro also has a fish module called "Aquarium."

Pyro's fish have more personality than After Dark's because they sometimes stare at you or devour each other. But After Dark's Fish! module is superior in that it includes bubble sound effects, sea plants, prettier fish, an editor to modify and add your own fish, and an optional sea floor. I can hardly wait to see what Fifth Generation is going to come up with to compete with that. Perhaps the sequel to "Jaws IV?"

Fifth Generation has announced its IBM PC and compatible screen saver, "Pyro! PC" ($49.95), but it currently does not work with Windows. And so the battle of the screen savers continues.

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