Hardware Reference
In-Depth Information
If you are intent on overclocking, there are several issues to consider. One is that most processors
sold since 1998 are multiplier-locked before they are shipped out. Processors that are locked ignore
any changes to the multiplier setting on the motherboard. Although originally done to prevent re-
markers from fraudulently relabeling processors (creating “counterfeit” chips), multiplier locking has
impacted the computing performance enthusiast, leaving tweaking the motherboard bus speed as the
only easy way (or in some cases, the only way possible) to achieve a clock speed higher than
standard.
Tip
Intel's K-series Core i7 and i5 processors have unlocked clock multipliers, as do AMD's
Black Edition FX, Phenom II Phenom, and Athlon X2 processors, as well as AMD's A-series
K-class processors. Choose these processors along with a motherboard that offers adjustable
clock settings in its BIOS for easy overclocking.
You can run into problems increasing motherboard bus speed as well. Most older Intel motherboards,
for example, simply don't support clock speeds other than the standard settings. Some newer
enthusiast-oriented Intel boards have “burn-in” or “override” features that allow you to increase the
default processor bus speed (and the speed of the processor core), voltages, and multiplier (for
unlocked CPUs). Most other brands of motherboards also allow changing the bus speeds. Note that
small incremental changes in clock multiplier speeds, rather than large jumps, are the best way to
coax a bit more performance out of a particular processor. This is because a given chip is generally
overclockable by a certain percentage. The smaller the steps you can take when increasing speed, the
more likely that you'll be able to come close to the actual maximum speed of the chip without going
over that amount and causing system instability.
For example, say you have a Socket 775 motherboard running a 2.4GHz Core 2 Quad processor at a
CPU FSB speed of 1,066MHz. The motherboard permits 1MHz adjustments of the CPU bus clock
speed (which is multiplied by 4 to obtain the FSB) to enable you to fine-tune your processor speed.
The base clock frequency is 266MHz and is multiplied by 4 to obtain the motherboard bus (FSB)
speed, which is then further increased by the CPU multiplier:
800MHz × 3.5 = 2,800MHz, or 2.8GHz
By adjusting the CPU clock frequency, you can change the FSB and CPU core clock speeds as shown
in
Table 3.28
.
Table 3.28. Core Clock, FSB, and CPU Speed Relationships
