Karbosguide.com - Module 3d.2

About Cooling and Over clocking (continued)

The contents:

What is clocking?
Two frequencies to clock on..
What is over clocking?

Next page
Previous page

What is clocking?

[top]

All Pentium CPUs run with clock doubling. That's the way they are built. The PC works with two frequencies, which the user can adjust. The clock doubling is set on small jumpers on the motherboard. You simply set a clock doubling factor, to make the CPU work – but who says that you must use the factor listed in the manual?

If you are brave, you try to set your CPU to run faster than it is designed to run. Often it works. If you "cheat" the CPU in this manner to work faster, it is called over clocking. Over clocking is kind of a PC tuning, which can be fun to fool with – if you are interested in the technicalities of PC-hardware. Otherwise - skip it!.

If you are lucky, you can make a medium speed CPU run as fast as the top of the line version! Please note, I accept no responsibility for the result of your experiments. I will now try to explain the technologies in the over clocking phenomenon. The interesting part is that, like much of the theory I tried to describe in in the modules 3a, 3 b and 3c, it all comes together here in the clock doubling technology.

Two frequencies to clock

[top]

The CPU works on two frequencies: An internal and an external.

The external clock frequency (the bus frequency) is the speed between the CPU and RAM. In the Pentium CPUs it is actually the speed between L1 and L2 cache. In the Pentium II it is the speed between L2 cache and RAM.

The internal clock frequency is the speed inside the CPU, that is between L1 cache and the various CPU registers.

For practical reasons you let these two frequencies depend on each other. In practice you choose a given bus frequency (between 60 and 153 MHz) and double it up a number of times (between 3½ and 8). The latter frequency become the CPU internal work frequency.

Here I show a number of theoretical CPU frequencies, resulting form different clock doublings: Many of these frequencies will actually never be used, but they are possible because of the system structure:

Bus
frequencies Clock doubling
factors Examples of resulting CPU frequencies

60 MHz
66 MHz
75 MHz
83 MHz
100 MHz
117 MHz
133 MHz
153 MHz 1½,
3½,
4,
4½,
5,
5½,
6
6½,
7
7½,
8 233 MHz, 266 MHz, 333 MHz
333 MHz, 366 MHz, 400 MHz,
433 MHz, 466 MHz, 500 MHz,
300 MHz, 338 MHz, 375 MHz,
375 MHz, 416 MHz, 458 MHz,
468 MHz, 527 MHz, 585 MHz
533 MHz, 600 MHz, 667 MHz,
612 MHz, 688 MHz, 765 MHz

Note an important point: The CPU frequency is the result of the the bus frequency multiplied with a factor. If you increase the bus frequency, it affects the CPU frequency, which is also increased.

Look here at a page from the manual to a ASUS P2L97 motherboard. It has a clear instruction about how to set the two values (bus frequency and clock factor). This (old) motherboard accepts bus frequencies up to 83 MHz with a clock factor up to 5:

What is over clocking?

[top]

Since clock doubling and bus speed can be freely adjusted on the motherboard according to your desires, you can in principle make the CPU run at 600 MHz. You set the bus to 133 MHz and the clock factor to 4½. Then the CPU runs at 600 MHz – if it runs. The question is whether the chip will tolerate that - and if it will give a stable performance, since clock doubling means more than added heat.

We have now seen that there are two frequencies which can be manipulated, if you want to re-clock the CPU:

The bus frequency can be increased, let's say from 133 to 153 MHz.

The CPU frequency can be increased. That can happen as a result of an increased bus speed, which also affects the CPU frequency, or it can happen by using a greater clock factor. The latest is not possible anymore.

Both techniques result in a faster PC. If the bus frequency is increased, it affects all data transport to and from RAM. It will work faster, to the joy of all work done on the PC. However, the RAM has to cope with the increased speed.

When the CPU internal frequency is increased, many applications will be happily affected.

More cooling

The tuning will often work, but it requires good cooling of the CPU, the more cooling the higher you can have the clock frequency. CPUs are built in CMOS technology. That is a type chip which works better the cooler it is. See this relationship between temperature and performance:

You can see that the performance drops drastically with increased CPU temperature.

This problem caused the Kryotech company to manufacture coolers utilizing the Danish Danfoss compressors, just like in refrigerators. See this cooling unit on a CPU:

It is fed from the compressor in the bottom of the cabinet:

This form of cooling is extreme, but it works. Kryotech can make a standard CPU work at 400-700 MHz! But it requires that it is kept constantly cooled to -40 degrees F or C. (it is the same). The Kryotech setup is efficient, but it is expensive, noisy and power consuming.

If you like, look at Kryotech's Home Page http://www.kryotech.com/

Another company in this business is Asetek.

This was to demonstrate that over clocking can be a serious issue....

However, the CPU speed has become less important. To most users it really does not matter whether you have a CPU running at 300 or at 600 MHz.

Learn more
[top]

Also see Module 3e - about the latest CPUs.

Read more about the boot process and system bus in Module 2b

Bus frequencies	Clock doubling factors	Examples of resulting CPU frequencies
60 MHz 66 MHz 75 MHz 83 MHz 100 MHz 117 MHz 133 MHz 153 MHz	1½, 3½, 4, 4½, 5, 5½, 6 6½, 7 7½, 8	233 MHz, 266 MHz, 333 MHz 333 MHz, 366 MHz, 400 MHz, 433 MHz, 466 MHz, 500 MHz, 300 MHz, 338 MHz, 375 MHz, 375 MHz, 416 MHz, 458 MHz, 468 MHz, 527 MHz, 585 MHz 533 MHz, 600 MHz, 667 MHz, 612 MHz, 688 MHz, 765 MHz