Clock Frequencies and Performance

Increasing the bus frequency and the internal and external clock frequency of the processor can raise the performance of a computer. As mentioned earlier, the best and the simplest method of increasing the performance of the system is to raise the bus frequency.

However, this can only be done if the specific motherboard model allows it. If it does not, you must be content with only increasing the clock frequency of the processor. The internal processor frequency is derived using the multiplier of the external frequency.

Note that the internal processor frequency is the rate at which the processor executes operations. External frequency is the rate of the host-bus (FSB, SB, CPU Bus), that is, the speed at which the chipset, the cache memory, and the RAM operate.

Other components of the computer, device controllers for example, operate at the frequency of the buses that connect them to the system, for instance, through the PCI and AGP buses. For PCI buses, the operating rate, as a rule, is half of the host-bus frequency.

For certain motherboards, it is either half or a third, depending on the frequency. Other input-to-output frequency ratios of the host bus are also possible. Modern video adapters in computers with Pentium and similar processors are generally connected to the other components of the computer by an AGP bus, the frequency of which also depends on the frequency of the host bus.

This all means that any change in the frequency of the host-bus will lead to a change in the throughput of the PCI and AGP buses, and in the long run will change overall computer performance. To track the interdependence of these frequencies, let us use an Intel Pentium-166 processor.

The internal frequency of this processor, for which it is intended and at which it is recommended to perform internal data processing, is, as you can see from its name, 166 MHz. This frequency is determined by the host bus frequency and the multiplier.

If the host bus frequency is 66 MHz, the multiplier will be 2.5(2.5×66 MHz gives us 166 MHz). Thus, if the processor works at a clock frequency that is X times more than the host-bus frequency, the internal clock frequency of the processor is determined as follows:

Host-bus frequency × X = internal clock frequency.

where X is the multiplier.

In the above example, the frequency of the PCI bus is 33 MHz. For Intel Pentium 150 processors, the internal clock frequency is 150 MHz, the external frequency is 60 MHz, the multiplier is 2.5, and the PCI frequency is 30 MHz.

Special jumpers on the motherboard usually determine the multiplier for Intel Pentium and similar processors from other companies. Less often, it is set in BIOS Setup.

You should remember, however, that to attain maximum performance you must first raise the host bus frequency, or at least, after having set the parameters of the chosen mode, try not to lower its frequency.

For example, having changed the arrangement of the jumpers from assigning an internal clock frequency of 166 MHz (2.5×66 MHz) to one of 180 MHz (3×60 MHz), you risk lowering the real performance of the system.

It would seem that since the processor clock frequency has gone up from 166 to 180 MHz, you would obviously gain in performance. And truly, the performance of the processor seems to im-prove.

But don't forget about another very important parameter — the external frequency, or the host-bus frequency. This frequency plays one of the most important roles in transferring data between the processor and the memory (cache, RAM), and also defines the operating speed of the rest of the subsystems, that is, it has a significant influence on the overal performance of the entire computer sys-tem.

As for the growth in the processor performance, in this case it is reflected mostly in operations that don't demand intensive data exchange with the memory and with other computer subsystems.

When changing from a clock frequency of 133 MHz (2×66 MHz) to a clock frequency of 150 MHz (3×50 MHz) you also are risking loss of speed, and thus decreasing the actual performance of the system.

Thanks to the increase in the processor clock frequency it actually will work more intensively. However, taking into account the decreased bus frequency, the speed of data transfer will fall.

And, therefore, when performing tasks that require intense data transfer between the processor and the cache memory or the RAM, the performance of the computer will suffer as well.

Keep in mind that, nominally, Pentium, Pentium Pro, and AMD K5 processors use bus frequencies of 50, 60, and 66 MHz. The choice of bus frequency depends on the processor and the chipset. The official bus frequencies for 6×86 processors are 50, 55, 60, 66, or 75 MHz.

However, there are motherboards that allow you to attain bus frequency values higher than the generally accepted ones, so called “undocumented bus frequencies”. These are frequencies of 75 and 83 MHz.

It should be noted that there are some motherboards with a documented bus frequency of 75 MHz, that is, the manufacturer of the motherboard guarantees its capability to operate at that frequency. These are boards from, for instance, ASUSTeK.

Using the non-standard bus frequency of 75 MHz for Intel processors and chipsets, you can attempt to raise the performance of the computer by overclocking the Pentium processor even without increasing the internal clock frequency.

For example, let us take the Pentium-150, which can be overclocked from 150(2.5×60 MHz) to 150 MHz (2×75 MHz). Based on the facts provided above, the overall computer performance will increase, but without increasing the internal clock frequency of the processor and without practically any change in its heating rate.

You should note, however, that the load of the RAM and the cache memory is increased, and they will have to work at a higher clock frequency, which is es-sentially overclocking of the memory (and certain other subsystems). In order to change the clock frequency of the host bus, you must consult the documentation supplied with the motherboard.

There you will find all the information you need for this. More specifically, you'll need to know which jumpers are responsible for this clock frequency, which combination you must choose to set the required bus frequency, and if possible, the multiplier.

Using increased bus frequencies, like 75 and 83 MHz, can lead to certain consexquences that you should be aware of before attempting overclocking. When using frequencies of 75 or 83 MHz, the PCI bus will generally work at frequencies of 37.5 or 41.6 MHz, respectively.

The effect of these frequencies may be seen in the work of the video adapter, for example, which is set to the PCI bus, and in the work of the disk controller, which is connected through the same PCI bus. At higher frequencies (in overclocked modes), certain devices maintain their serviceability.

However, when functioning at these higher frequencies, they may become overheated. In such a case, you must implement proper cooling of such components. Certain other devices may become unstable.

If this is the case, you should either stop using a higher frequency, or replace the devices with ones that are better adapted to working at such high frequencies. The speed of the EIDE controller depends not only on the PIO or DMA modes, but also quite heavily on the PCI bus frequency.

This is exactly why it's advantageous to use a higher frequency. However, there are cases where a hard drive will work reliably and quickly at a frequency of 75 MHz, but at 83 MHz its performance abruptly declines to, for example, PIO 2.

The same can be said of the CDROM drive. Obviously, such a mode is undesirable, since the general performance of the system decreases. You may also expect to run into problems with the memory.

But there are exceptions when certain memory chips, against their type and nature, maintain the same level of efficiency at increased frequencies. Nevertheless, it's better to use those types of memory that are intended for work at high frequencies.