Should you rock the clock?



Q After reading about how you can increase your computer's performance by pushing the clock speed past the CPU's standard clock setting, I gave it a try. I reset the jumpers on the motherboard to make my Pentium 100 run at 120MHz. No problem. I ran a diagnostic that said I was running at 120 MHz. And I didn't change anything else. But rather than showing an improvement, my PC seems to be a tiny bit slower! Did I damage my machine, or does overclocking just not work?
- Peter Fly

A Probably neither. Overclocking is a much-debated technique for increasing system performance by increasing your CPU's internal processing speed or your motherboard's system bus speed beyond the manufacturer's recommended values. Some CPUs can be pushed to higher rates. And some motorcycles can be driven at 200kph. And in both cases, everything may turn out all right.
Overclocking may improve your PC's performance, sometimes substantially. Or, as in your case, it may help only a little or not at all. In the worst case, it can result in a dead CPU. Here's why.
Inside the CPU, data moves at the computer's designated speed -- originally 100MHz for your system. But this internal clock speed is actually a multiple of your motherboard's system bus speed. Your Pentium 100 motherboard's system bus runs at a speed of 66MHz, and its CPU multiplies that by 1.5 to get 100MHz.
Pentium machines typically support system bus speeds of 50MHz, 60MHz, and 66MHz (a few even go to 75MHz) and multipliers ranging from 1 to 3.5. You can select different CPU speeds by setting your motherboard jumpers for the appropriate combination of system bus speed and multiplier.

MATCH YOUR CPU SOCKET TO THE RIGHT UPGRADE
Socket
CPU
Maximum upgrade
Number of pins
4
60, 66MHz
Pentium 133
272
5
75MHz and higher Pentiums
Pentium MMX-200
320
7
latest Pentiums
Pentium MMX-200
321
8
Pentium Pro
Pentium II (mid-1998)
387
slot 1
Pentium II
n/a
n/a
Make sure your socket is the right one for the CPU upgrade you have in mind

CPU SPEEDS AND THE BUS SPEEDS THEY REQUIRE
CPU internal processing speed (MHz)
Motherboard system bus speed (MHz)
CPU multiplier
90
60
1.5
100
66
1.5
120
60
2
133
66
2
150
60
2.5
166
66
2.5
200
66
3
233
66
3.5
Is your motherboard running at full speed? With overclocking, sometimes not


You didn't see a boost in performance because to get your CPU running at 120MHz, you actually had to reduce the system bus speed to 60MHz and increase the multiplier to 2. Since lots of data moves around outside the CPU, the faster CPU was offset by the slower motherboard.
So why not set the motherboard to 66MHz with a multiplier of 3 and reap the benefits of a 200MHz system? Because your CPU might not be able to handle it. At higher speeds, CPUs generate a lot more heat. And if they're not cooled properly, they can fry. Also, bumping up the system bus speed past 66MHz can prevent some PCI cards and some types of memory from functioning properly.
In short, this is an inexact science. For more details, point your browser to http://www.sysopt.com/overc.html.
- Kirk Steers

Category: Hardware
Issue: Apr 1998

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