Most PC-compatible computers for sale these days come with more than the 640K of memory that MS-DOS, the basic operating software, can use. And even more memory can easily be added. The question for many ordinary users is what to do with the extra memory.

Numerous major applications, especially spreadsheet programs, know how to use "expanded" memory, which was developed to get around the 640K limit. Operating environments such as Microsoft Windows and the software designed for it also automatically use extra memory.

But even if you don't use such software, additional memory above the 640K barrier can make your computer more efficient. The first thing you need, though, is some utility software to manage the memory. If you have extra memory, it probably came with software to configure it as "expanded" memory. (There's no need to dwell here on the distinction between "expanded" and "extended" memory. Suffice it to say that any PC-compatible can use "expanded" memory, which is not the case with "extended" memory.) But configuring your added RAM as expanded memory is only the first step.

To go farther, you need a program such as QRAM, $59.95 from Quarterdeck Systems. QRAM is actually a package of utilities whose best feature is the ability to load RAM-resident programs into expanded memory. That may sound highly technical, but the concept is simple enough.

Nearly everyone who owns a PC has some favorite program that stays out of sight in the background of his system's memory, waiting to be summoned at the touch of a "hotkey." The most famous such program is Borland's original Sidekick, a set of handy tools, including a calendar, a calculator and a modest word processor that could be popped up at any time in the midst of doing something else. It used about 64K of your computer's main memory.

The success of Sidekick brought a rush of such software to market. You can now find a pop-up program to do just about anything your computer is capable of. The trouble with these programs is that, as handy as they are, they each consume part of your system's main memory, sometimes slamming into each other and causing your computer to lock up.

QRAM permits these pop-up programs to reside in expanded memory, leaving your application software more operating headroom in your system's main memory. And it's not just pop-up utilities that can be installed in expanded memory. If you use a mouse, the software that drives it can be kept out of the way in expanded memory. So can print spoolers, RAM disks and disk caches, all of which enhance your computer's performance and do it better the more memory they have.

A print spooler is a waiting room in your computer's memory where data intended for your printer are held and fed to your printer as it is ready to receive the information. This keeps you from having to wait while one document is printed before you can start to work on another.

A disk cache performs a similar function with data that have been read from a disk drive. If you need it again, the computer first looks in the cache. If the data are there, it will be retrieved much faster than if the computer has to get it again from disk. A RAM disk is a section of memory fenced off by your computer and treated as if it were a physical disk drive. The only practical difference is that when the computer is turned off, the data are lost unless it has first been copied to a physical disk.

QRAM enables you to establish any or all of these software devices in expanded memory, enhancing your system without reducing the memory available to your programs. With enough extra RAM, you can also load all your favorite pop-up programs without crowding your main memory.

QRAM is not a program for the faint-hearted user. Installing Sidekick in expanded memory, for example, could require adding a line to AUTOEXEC.BAT file that reads "C: 1/4QRAM 1/4LOADHI R:1 SK." Because many users don't even know what their AUTOEXEC.BAT file is, this sort of thing is truly daunting. There is a utility included with QRAM called OPTIMIZE which analyzes your system, then automatically makes some modifications to your AUTOEXEC.BAT and CONFIG.SYS files. There also is a useful program included called MANIFEST, which lets you peer into your system's configuration. But you will still need to edit your configuration files to get the most out of QRAM. The instructions are reasonably clear.

Brit Hume is a contributor to the Washington Post Writers Group. He is chief ABC News White House correspondent and the founding editor of a computer newsletter.