Karbosguide.com - Module 3d.4

About Cooling and Over clocking (continued)

The contents:

I/O speed and experiments with that
Side effects
Fake Pentium IIs
Jumpers.

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Experimenting the I/O speed

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The area which has given me the most problems is the increased PCI speed. At 66 MHz the PCI bus runs at half the system bus frequency. At 100 MHz it runs at one third and at 133 MHz one quarter of this frequency. Thus if we increase the system bus, it also affects the PCI bus:

System bus speed Bus factor Resulting PCI speed

66 MHz The half 33 MHz

75 MHz The half 37.5 MHz

83.3 MHz The half 41.6 MHz

100 MHz One third 33.3 MHz

112 MHz One third 37.3 MHz

133 MHz One quarter 33.3 MHz

153 MHz One quarter 38.25 MHz

Side effects

When we increase the PCI bus speed, a number of units are affected. They may not always agree with the faster pace. This includes:

The EIDE hard disk

The video card

The network controller and other I/O cards.

My own experiment with Pentium II

In 1997 I experimented with a very early Pentium II, which was bought as a 233 MHz model.

First I made it run at 3½ X 75 MHz. It worked fine with CPU, RAM (10 ns SD) and hard disk (IBM DHEA). But the net card (a cheap 10/100 Ethernet card) refused. When I copied large volumes of files on the net, it froze up - stopped. It was quite obvious that the problem was in the net card.

I had to accept the traditional 66 MHz. But to soothe the pain, it turned out to run excellently with a clock factor of 4 - thus at 266 MHz.

Within a couple of weeks I was in the mood to experiment again. I now found an adjustment in the setup program. It is called PCI latency. It is not explained anywhere, but it has a default value of 32. I increased it to 36 and increased the bus frequency to 75 MHz – it works. Now the net card runs without problems.

Then I hoped to speed the system bus up to 83 MHz, which should give a significant performance improvement for all RAM transport. My 10 ns SD RAM can certainly handle 83 MHz. But no, it did not work. Regardless of the PCI latency, the PC would not start. This indicates that the PCI latency setting does not work like I expected. Maybe it has nothing to with this - I do not know.

My explanation is, that the video card could not tolerate the 41.5 MHz PCI frequency. Nothing appeared on the screen.

Now the PC runs fine at 4 X 75 = 300 MHz. There can be an occasional unexplained break-down in Windows 95 (that happens under other circumstances also), which I blame on the drastic over clocking. However, the advantages of the significant performance improvement far exceed the annoyance of these small interruptions, which happen far from daily.

Problems with NT 4.0

Windows NT 4.0 does not install with over clocked CPU. The program tests for "genuine Intel", and seems to register the change in clock frequency. And then it will not work. But if you install NT first, then you can over clock afterwards and NT will work. Actually NT is quite sensitive. One of my friends experienced some peculiar errors. The solution turned out to be moving the RAM module from one socket to another!

Fake Pentium IIs

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Since some Pentium II-233 perform very well at 300 MHz, they have been sold as such ones. To test your own Pentium II, you can download this test program from C't, which can check your Pentium II. Here is the interface of the Windows 95 version, which correctly detected my CPU to be over clocked:

Jumpers on the motherboard

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To set the clock doubling, some small switches (called jumpers) have to be reset. They are located on the motherboard, as you see here:

You can read in the motherboard manual how to set them. Or you can look at the motherboard! In the picture below you can see some of the printed information on the motherboard (this is an ASUS TX97 with a Socket 7).

Here you can read which jumpers to set to select clock doubling 1, 1½, 2, 2½, 3, 3½ and 4 for 6 types of processors:

P54C and K5

P55C, K6 and M2 (Cyrix 6x86MX)

M1 (Cyrix 6x86)

On modern motherboards you may find a software solution to the settings, and that is a lot better.

Learn more
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Also see: Module 3e - about the latest CPUs.

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

System bus speed	Bus factor	Resulting PCI speed
66 MHz	The half	33 MHz
75 MHz	The half	37.5 MHz
83.3 MHz	The half	41.6 MHz
100 MHz	One third	33.3 MHz
112 MHz	One third	37.3 MHz
133 MHz	One quarter	33.3 MHz
153 MHz	One quarter	38.25 MHz