Hardware Reference
In-Depth Information
can change their frequency outputs via software, which results in the system running at different
speeds. Because all CPUs are based on the speed of the CPU bus, when you change the CPU bus
speed generated by the frequency synthesizer chip, you can change the speed of your processor.
Because the PCI, AGP, and memory buses are often synchronized with the speed of the processor bus,
when you change the processor bus speed by a given percentage, you also change the speed of those
other buses by the same percentage. The software to accomplish this is built into the BIOS Setup
menus of most modern motherboards.
Overclocking Tips
Most modern motherboards automatically read the CPU and memory components to determine their
proper speed, timing, and voltage settings. Originally, these settings were controlled by jumpers and
switches, but in most modern boards you can enter the BIOS Setup to change these settings to manual
and then use the menu options in the BIOS Setup to alter the speed of the system. Because such
alterations can make the system unstable, most systems are designed to boot into the BIOS Setup at a
default low speed so you are not locked out from making changes in the future. This makes
overclocking as simple as changing a few menu options and then rebooting to test the selections
you've made.
The concept for overclocking is simple: You change the settings to increase the speed of the
processor, memory, buses, and so on, until the system becomes unstable. Then you can go back in and
reduce the settings until the system is stable again. In this manner, you find the maximum sustainable
speed for a system. Because each processor is different, even ones with the same ratings can end up
allowing different maximum stable speeds.
Why can some chips be clocked faster than others? The reason is in how they are manufactured and
marked. As an example, the first Pentium 4 chips based on the Prescott core used die that were 112
square mm on 300mm wafers, resulting in a maximum of 529 full die per wafer. Due to defects, many
of those die wouldn't work, but let's say that 423 (about an 80% yield) were good. Intel initially sold
the Prescott core processors at speeds from 2.4GHz through 3.4GHz, which meant that all the die on
each wafer were designed to potentially run at the highest rated speed. However, out of the good
(working) die, testing would show that although some of those would indeed run at the highest
3.4GHz rated speed, others would work reliably only at lower speeds. The finished chips would
have been sorted into bins according to their speed test results.
Early in manufacturing a given processor design, the sorted bins of chips at the end of the line would
contain more that passed only the lower speed tests, and fewer that ran at the highest speeds. This is
why the fastest chips are the most expensive—generally fewer of the chips produced on a given day
will pass the testing at that speed. Eventually, however, as the manufacturing processes and chip
design are tweaked, more and more of the finished chips end up passing the higher-speed tests. But
because lower-speed chips are priced less and sell more, the manufacturer might have to dip into the
faster bins and mark those chips at the lower speeds to fill the larger number of orders.
Essentially what I'm saying is that chipmakers such as Intel and AMD make all the chips on a wafer
identically and try to make them so they will all run at the highest speeds. If you purchase one of the
lesser-rated chips, you really have the same chip (die) as the higher-rated versions; the difference is
the higher-rated ones are guaranteed to run at the higher speeds, whereas the lower-rated ones are not.
That is where overclockers come in.
