CPU Buying Guide

Choosing the CPU

There are many different factors to consider when selecting a new CPU and the PC it runs. (See "CPUs Up Close" on page 23 for more information.) Following are some key factors.

Clock speed. Perhaps the single most important indication of a CPU's performance is its speed, expressed in terms of clock frequency in megahertz. The CPU clock speed can be set at up to five times faster than the motherboard's bus speed. The faster a CPU (and motherboard), the better the system performance. For example, Intel's 450MHz Pentium II must be paired with a 100MHz motherboard.

Note that you may encounter something called a P-Rating. Based on Ziff-Davis' Winstone benchmark, the P-Rating ostensibly helps manufacturers rate processor performance objectively. For example, a processor with a P-Rating of P266 should perform as well as (or better than) a Pentium 266MHz processor, even if the chip is running at a slower clock rate.

Cache.

Your CPU may or may not have built-in L2 cache. With built-in cache you get a bit more speed, but cache size can't be increased. Intel Pentium II and Celeron chips fall into this category. Cache for Socket 7/ Super 7 processors from AMD, Cyrix, and others is usually on the motherboard next to the CPU, which means that you can often pick cache size when you buy the computer. Get at least 512K, and 1MB if you can.

Slot/Socket. CPUs are held in place using various incompatible schemes. For example, Pentium IIs and Celerons use the cartridge-style Slot 1. (See our cover for an example.) Pentium and Pentium MMX CPUs, as well as AMD and Cyrix chips, use the Socket 7. (The Pentium II Xeon uses the slightly larger Slot 2.) Neither approach affects performance, but it does limit your upgrade options. For example, if your PC has a Slot 1 connector, you can only replace the existing CPU with another Slot 1 chip.

MMX and 3DNow. Two other features to consider when picking the CPU inside are MMX and 3DNow support. MMX is the well-established instruction set that adds powerful 2D- (and some 3D-) graphics processing. Today, virtually all modern CPUs employ some variation of the MMX instruction set, and all of these implementations are fully compatible. The notable exception is Intel's Pentium II Xeon, which omits MMX because its server/work station role isn't graphics oriented.

In an effort to beat the Pentium II family, AMD, Cyrix, and IDT have adopted AMD's 3DNow instruction set, which adds significant 3D- processing power directly to the CPU. 3DNow is supported by Windows 98/DirectX 6, and a program written to the 3DNow API will zoom. Intel chips don't support 3DNow, but rumor has it that Intel is planning its own 3D instruction set (dubbed MMX2) for future Pentium IIs.

In the Chips

The CPU story doesn't end here. With the introduction of Intel's Pentium III slated for the first quarter of 1999, Intel promises clock speeds of up to 500MHz, along with faster 3D graphics and MPEG-2 decoding performance. But it won't be alone for long: AMD's 500MHz K7 (due the first half of '99) opens the door to 200MHz system bus speeds. In short, the future of CPUs has never been brighter--or faster.

Trouble-free CPU Upgrades

The age of the OverDrive chip has passed. These days, if you want to boost your CPU speed, yank out the old processor, plug in a faster one, set a few motherboard switches, and get a full speed boost. That's assuming, of course, that your system is built around a Pentium II or equivalent CPU. If it is, the motherboard within can probably accommodate CPUs of varying speeds.

For example, if you have a motherboard that accepts processors from 266MHz to 350MHz and the system comes with a 300MHz CPU, you could pop out the 300, pop in a 350, change a few jumpers, and that's it. (Of course, if you wanted anything faster you'd have to install a new motherboard--not a task for the uninitiated.)

Pick the Right Chip

Swapping CPUs can be pretty straightforward. But before you run out and buy that megahertz monster, figure out what you need.

The documentation that accompanied your PC and/or motherboard should be the best source of information for planning your CPU upgrade. It should list all the CPUs your motherboard is compatible with, and it should point out the location of critical jumpers or switches you'll need to reconfigure to support a new CPU.

First check the system specifications section of your PC's manual. It should note which CPUs are supported. Next, locate the section about jumper settings and find out which jumpers set the appropriate bus speed, multiplier, and CPU voltage. Newer motherboards (mostly those for the Pentium II) can often automatically select the appropriate CPU voltage, so you may not have to set a jumper. Note: Never attempt a CPU upgrade without the necessary documentation. If you don't have documentation for your PC or motherboard, you can usually find it at the vendor's Web site.

Making the Upgrade

Upgrading a CPU is normally a fast and straightforward process, but there are a few wrinkles that can make the going tough. Here are a few tips for a smoother ride:

Power off. Don't even think of swapping chips while your PC is on. Turn off and unplug the PC before opening the outer case.

Be static safe. CPUs are extremely complex and delicate chips, and they're sensitive to accidental damage from static discharge. Get an inexpensive antistatic wrist strap from your local computer store and make sure you ground yourself properly before handling a processor.

Mark the processor's orientation. Many people run into trouble because they forget where pin 1 is on the CPU socket. Pin 1 is always located where that little cutout corner is. Mark this corner with an indelible marker. This is not a problem for Slot 1 CPUs (like the Pentium II), because they can only be inserted one way.

Lock down the new CPU in its socket. If you're installing a Socket 7 CPU, make sure that the ZIF-socket (zero insertion force) lever is all the way down and locked. Otherwise the lever may pop up during operation and the CPU could fall out. If you're installing a Slot 1 CPU, make sure that the retention mechanism holding the CPU cartridge in place is secure.

Check and reattach the heat sink and fan assembly. CPU cooling is very important for reliable, long-term CPU operation. Slot 1 and Slot 2 processors (i.e., Intel's) typically build an appropriate heat sink/fan assembly into the processor cartridge, but available Socket 7 processors may not. Ask the vendor if you need a heat sink/fan assembly and if your old cooling unit is adequate for the new CPU. Once the new CPU is in place, double-check the heat sink/fan assembly and make sure it's attached properly.

If you're reattaching the heat sink/ fan to a new Socket 7 processor, consider applying a very thin layer of thermal grease between the top of the CPU and the bottom of the heat sink. Thermal grease is not an adhesive--it simply fills the microscopic gap between the CPU and heat sink, improving heat transfer away from the CPU. You can get a tube of thermal grease from your local computer store.

Configure the clock jumpers. Now's the time to set your motherboard's bus speed and multiplier jumpers to accommodate the new CPU's speed. Make sure that any appropriate CPU voltage jumpers are also set. (Com parable Intel, AMD, and Cyrix CPUs often use slightly different voltages.)

Check the BIOS. When you turn on the PC, it should boot normally. Check the BIOS banner information and see if the CPU is identified correctly. If not, you may need a BIOS upgrade for the motherboard. This happens frequently when new processors are introduced after a system is released. In most cases, the system will work properly even if the CPU is misidentified.

Save the old CPU. After you remove the old CPU, place it in the antistatic wrap your new chip came in. Hold onto this old CPU--you may need to reinstall it if there's trouble with the new chip, and you can always pass it along to someone with an even older CPU so that they can upgrade, too.

PUs on the Web

Interested in learning even more about CPUs? The Web is packed with informative sites from vendors and nonvendors alike.

alternativecpu.com
An alternative CPU site

come.to/overclock
Overclockers Workbench

cpu.simplenet.com
The Brotherhood of the CPU

infopad.eecs.berkeley.edu/CIC/announce/
Latest CPU announcements

kikumaru.ww.ne.jp/pc/celeron/index_e.html
The Dual Celeron System site

www.aceshardware.com
Ace's Hardware

www.affordablesystems.com/oc/index.html
Zane's Overclocking Q&A Page

www.altx86.com
Alternative x86 processor site

www.amd.com
AMD's company Web site

www.anandtech.com
AnandTech hardware's review site