Which CPUs can be over clocked?
The first CPUs which were dramatically over clocked were AMD's 5x86 series. That was a 486 CPU, which could be forced up to an excellent performance at 160 MHz.
Since then especially Intel's Pentium CPUs have been over clocked. Many of those seem to be sold with specs far from their optimum performance. Actually it was so easy that as a result many P133s were sold in 1996 as fake P166s. They worked fine, and the users did not know it. But Pentium MMX and Pentium II can also be re-clocked.
It appeared that Intel were aware of this activity, and they don't seem to care. Unfortunately their CPUs came in two groups:
You cannot guarantee that it always will work. But let me show a couple of examples, which I have made work with good results:
|CPU||Manufacturers spec||Tuning result|
|Intel Pentium||2½ X 60 MHz = 150 MHz||3 X 66 MHz = 200 MHz|
|Intel Pentium Pro||3 X 66 MHz = 200 MHz||3½ X 66 MHz = 233 MHz|
|Intel Pentium II||3½ X 66 MHz = 233 MHz||4 X 75 MHz = 300 MHz|
|Intel Pentium II||4½ X 100 MHz = 450 MHz||4½ X 117 MHz = 527 MHz|
Looking at the three examples, number 1 and 3 show the best results, where both bus frequency and clock factors are increased. That simply moved the CPU up one class in performance.
Here is a table of the clock factors, which the CPU's theoretically can accept (according to my studies):
|Intel Pentium (P54C)||1½, 2, 2½, 3|
|Intel Pentium Pro||2½, 3, 3½, 4|
|Cyrix 6x86||2, 3|
|Cyrix 6x86MX (M2)||2, 2½, 3, 3½|
|Intel Pentium MMX (P55C)||2, 2½, 3, 3½|
|AMD K5 PR75 - PR133||1, 1½|
|AMD K5 PR150 and PR166||2|
|AMD K6-2 and K6-3||4, 4½, 5|
|Intel Pentium II, Celeron and Pentium III||Up to 8 and 12 (latest models)|
Some AMD and Cyrix chips were special, in that they did not always respond to motherboard settings. It is like they determined their own frequencies.
All modern Intel processors are locked at fixed clock factors (Multiplier Locking). They only operate with one specific multiply factor.
The Celeron 533
will only work with clockfactor 8, so if you want to overclock it, you
have to go for a motherboard with adjustable system bus frequencies. This
could be 8 X 100 MHz instead of 8 X 66 MHz increasing the CPU speed from
533 MHz to 800 MHz. Many users have found this in-expensive way to get a higher performance.
Disadvantages and risks in over-clocking?
Many factors need to be considered, when you start tampering with these system settings. Watch out for:
The last two problems are associated with increased system bus speed. This kind of over clocking gives the best results. However those also create the biggest problems, at least in my experience.
|RAM type||Speed||Maximum bus frequency|
|FPM||60 ns||66 MHz|
|EDO||50 ns||75 MHz|
|SD||10 ns||100 MHz|
|SD||7 ns||133 MHz|
Finally you could say that with cheap CPUs running at 900 MHz and above - you really do not need any overclocking. Most users will not experience any benefit from shifting from say 700 MHz to 1000 MHz.
See Module 3e - about the latest CPUs.
Read more about the boot process and system bus in Module 2b
Read more about I/O buses in module 2c
Read more about the motherboard chip set in module 2d
Read more about RAM in module 2e
Read about EIDE in module 5b
Copyright (c) 1996-2005 by Michael B. Karbo. www.karbosguide.com.