When it comes to processors, today's PC buyer has never had it so hard: It's easy for system builders and would-be upgraders to be overwhelmed by the sheer breadth of CPU designs, sockets, formats, packages, and platforms. The old joke about the guy cutting off CPU pins to make his Athlon XP fit in a Pentium III motherboard may be an urban legend, but terms like Socket A, Socket 478, BGA, PPGA, and FC-PGA can be undeniably confusing.

Here's a short course in the physical differences between different chips, which should help make sense of today's processor terminology and tomorrow's product announcements. By the end of these pages, you'll be able to hear a phrase like "a new Socket 478 flip-chip CPU with IHS, in your choice of OPGA or Enhanced BGA packages" without resorting to a blank stare.

What's Past Is Prologue

The current multitude of CPU designs and socket formats makes it easy to forget it wasn't always this way. Back in the early days of personal computing, there was really only one x86 platform per era, with companies like AMD, Harris, and Cyrix sharing the limelight with Intel. Granted, the upgrade path on those early PCs ranged from limited to nonexistent. But as last week's guide to CPU upgrades mentioned, once motherboards moved to handy zero-insertion-force (ZIF) socket designs, users started happily trading their CPUs for faster versions. Even Intel got into the act with its OverDrive line of retail upgrade chips.

This all culminated in the Socket 7 platform used by Intel Pentium, AMD K6/K6-2, and other compatible CPUs, which offered consumers an unparalleled level of choice and forced the two processor powerhouses -- along with Cyrix, NexGen, and other hopefuls -- to compete on an equal playing field. CPU prices fell to unheard-of levels, and faster models appeared at a dizzying pace.

Socket 7 in some ways launched the CPU wars, as previously systems employed Intel, AMD, or Brand X 386 or 486 chips without much need for specific branding, but ever since Intel figured out the marketing game with the Pentium and AMD's K6 and Cyrix's x586/x686 followed, the brand-name fans and flames have been going strong.

But, like all good things, the days of a common CPU format came to an end when Intel, claiming that higher-speed chips required new technologies, introduced a new Slot 1 interface for the Pentium II. AMD, Cyrix, and others moved to Super 7; Intel promoted Slot 1 for some time before mysteriously deciding that a conventional pin-and-socket design was fine for the Celeron and Pentium III; and we were on the way to the morass of innovative but incompatible processor designs we see today.

The PGA Tour

There are two basic types of processor packages. The vast majority of current CPUs are Pin Grid Array (PGA) designs -- chips whose undersides bristle with a grid pattern of pins, which plug into a socket with scores of tiny holes. PGA processors, like the Pentium 4 shown below, feature a pin-out design that allows the CPU to be installed only one way (usually indicated by a notch or mark on the corner where Pin 1 is located) to avoid electrical shorts or physical damage.

The initials PGA are often preceded by other letters, indicating the many subdivisions or variations of the basic design. These include early Celerons' PPGA (Plastic Pin Grid Array); the Pentium III's FC-PGA (Flip-Chip Pin Grid Array); AMD's Athlon and Duron CPGA (Ceramic Pin Grid Array); and even the mobile Pentium 4's mFC-PGA (Micro Flip-Chip Pin Grid Array). Many of these labels, like the Pentium 4's FC-PGA2 (Flip-Chip Pin Grid Array 2), are derivative terms that mainly add a new wrinkle or simply redefine what's already been done.

An example is OPGA (Organic Pin Grid Array) -- AMD's switch from ceramic to organic substrates to house the Athlon XP processor core. The organic substrate is usually made of some form of fiberglass or resin, and offers lower impedance, a more durable surface, and lower production costs than the older Athlon/Duron ceramic package. The latter is easily differentiated by its blue-gray color, compared to the brown or green of the organic package.