Choosing, buying and learning to use an IBM-compatible personal computer are daunting enough. But as computers have grown more powerful, the issue of memory has become complicated to the point of bewilderment.

Recently, there was only one issue: How much memory you could afford. The rule was simple: The more the better. An IBM-PC or XT could use as much as 640 kilobytes of memory, which was more than anybody seemed likely ever to need.

Before long, though, spreadsheet power-users began to complain that 640K didn't allow them to fit their increasingly massive data files into memory. The problem was that MS-DOS, the operating software developed for the original IBM-PC, could only address a total of 1 megabyte (1024K) of memory. Once DOS had dealt with the memory required by such things as the computer's display, only 640 kilobytes could be devoted to so-called "user RAM."

Many power users had migrated to the newer IBM-AT or compatible systems using the Intel 80286 microprocessor. IBM-ATs can address as much as 16 megabytes of memory. But DOS, developed for the first-generation PCs and XTs using the Intel 8088, was still confined to 640K.

That need for more headroom led Lotus, Intel and Microsoft to develop software and hardware to create something called "expanded" memory. It got around the 640K barrier by creating a window or "page frame" in the 640K base. Data stored in memory above the 1 megabyte limit was then swapped in and out of this window, thus fooling DOS into thinking it was never dealing with more than 640K.

It was an ingenious, if inelegant, solution and expanded memory remains in wide use today. Although it was developed to work on PCs and XTs, it does much better on ATs and other fast systems because their greater processing speed makes the "swapping" of code in and out of memory much smoother.

As if to complicate matters, though, manufacturers began increasingly to ship ATs with a different kind of expansion memory. Since ATs had the potential to address many megabytes, motherboards for these systems were made with room for extra memory -- so-called "extended" memory.

DOS can't do anything with such memory, so its utility is limited. But it's cheaper to add memory by plugging chips into the motherboard than it is to put it on an expansion board. What's more, there was widespread expectation that Microsoft, creator of DOS, would soon have a second-generation operating system on the market that could fully use this extended RAM.

That operating system, OS2, has arrived. But so far almost nobody uses it. It consumes staggering amounts of memory just to load and so far there's no application software good enough to force people to switch to it. So it languishes, and extended memory remains, by and large, less useful than its stopgap cousin, expanded RAM.

Some uses have been found for extended memory. It can be used for a RAM disk, which is a section of memory fenced off and treated by the computer as if it were a physical disk drive. The advantage is that data flows in and out of a RAM disk at mach speed.

Extended memory also can be used as a disk cache. This is similar to a RAM disk, except that it holds data recently retrieved from disk, on the theory that such data is likely soon to be used again. When it's needed, it pops into use much faster than if it had to be loaded from a disk drive.

Making the picture even more complex is that some memory can be configured as either extended or expanded memory, or partly both. And software is available to convert extended memory into expanded memory. And now, along comes Microsoft Windows 3.0, which actually can use extended memory the way it was intended to be used, as straight user RAM, an extension of the base 640K.

So what kind of expansion memory should you get? It depends on your computer and your usage. If you have a PC or XT, get expanded memory.

If you have an AT, you might be able to get some mileage out of extended memory using Windows 3.0. But you're probably better off with expanded memory.

Extended memory works best on computers using the 80386 or 80486 processor. They have memory-management capability built into them that the 80286 lacks. What's more, they've been helped by some ingenious utility software designed to help them get the most out of memory.

Indeed, memory-management software has become a hot item in the PC industry. You don't need a 386 or 486 system to benefit from it. More about that in a future column.

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.