Page 70 - A Practical Guide from Design Planning to Manufacturing

46 Chapter Two

can successfully run. Changes to the design or the manufacturing process
can improve the average processor frequency, but there is always some
manufacturing variation.
Like a sheet of cookies in which the cookies in the center are overdone
and those on the edge underdone, processors with identical designs that
have been through the same manufacturing process will not all run at the
same maximum frequency. An Athlon XP being sold to use FSB400 might
first be tested at 2.3 GHz. If that test fails, the same test would be repeated
at 2.2 GHz, then 2.1 GHz, and so on until a passing frequency is found,
and the chip is sold at that speed. If the minimum frequency for sale fails,
then the chip is discarded. The percentages of chips passing at each
frequency are known as the frequency bin splits, and each manufac-
turer works hard to increase bin splits in the top frequency bins since
these parts have the highest performance and are sold at the highest
prices.
To get top bin frequency without paying top bin prices, some users
overclock their processors. This means running the processor at a higher
frequency than the manufacturer has specified. In part, this is possible
because the manufacturer’s tests tend to be conservative. In testing for
frequency, they may assume a low-quality motherboard and poor cooling
and guarantee that even with continuous operation on the worst case
application the processor will still function correctly for 10 years. A
system with a very good motherboard and enhanced cooling may be
able to achieve higher frequencies than the processor specification.
Another reason some processors can be significantly overclocked is down
binning. From month to month the demand for processors from different
frequency bins may not match exactly what is produced by the fab. If
more high-frequency processors are produced than can be sold, it may be
time to drop prices, but in the meantime rather than stockpile processors
as inventory, some high-frequency parts may be sold at lower frequency
bins. Ultimately a 2-GHz frequency rating only guarantees the processor
will function at 2 GHz, not that it might not be able to go faster.
There is more profit selling a part that could run at 2.4 GHz at its full
speed rating, but selling it for less money is better than not all. Serious over-
clockers may buy several parts from the lowest frequency bin and test each
one for its maximum frequency hoping to find a very high-frequency part
that was down binned. After identifying the best one they sell the others.
Most processors are sold with the bus ratio permanently fixed. Therefore,
to overclock the processor requires increasing the processor bus clock
frequency. Because the processor derives its own internal clock from the
bus clock, at a fixed bus ratio increasing the bus clock will increase
the processor clock by the same percentage. Some motherboards allow
the user to tune the processor bus clock specifically for this purpose.
Overclockers increase the processor bus frequency until their computer
fails then decrease it a notch.