How Bus Speed Affects Overall Performance, Changing The Speed Of A Computer

How Bus Speed Affects Overall Performance

The speed of the CPU is determined by multiplying the bus speed by a factor such as 1.5, 2, 2.5, or 3. This is called the multiplier. Studies have shown that when the multiplier is large the overall performance of the system is not as good as when the multiplier is small. This is a reasonable result because you are interested in the overall speed of the computer, which includes the CPU and the buses, not just the speed of the CPU.

For example, a bus speed of 60MHz and a multiplier of 5 yield a relatively fast CPU but a relatively slow bus. It’s better to have a bus speed of 80 MHz and a multiplier of 3 so that the bus is running fast enough to keep up with the CPU.

See the systemboard documentation to learn how to set these speeds using jumpers, DIP switches, or CMOS setup. The following figure shows the documentation for one systemboard that uses one bank of jumpers to set the CPU-to-bus multiplier and another jumper bank to set the bus frequency.

The steps to use one bank of jumpers to set the CPU-to-bus multiplier and another jumper bank to set the bus frequency are:

1. Read the documentation of your CPU to determine its recommended frequency. In our example, we are using a Pentium II rated for 350MHz. Find the row labeled Pentium 350MHz in the table of the previous figure.

2. Read the multiplier from the selected row, which is 3.5x. Find the jumper settings for a multiplier of 3.5 in the possible jumper combinations for the CPU Core: BUS Frequency Multiple (fourth entry in first row).

3. The CPU type and speed also determine the bus frequency, which is 100MHz. To set the bus frequency to 100 MHz, find the jumper combination for the second jumper bank, which is the fourth entry in the list of selections for the CPU External Clock (Bus) Frequency Selection.

4. Set the jumpers in the two jumper banks. The following figure shows the jumper group for the multiplier set to 3.5.

Changing The Speed Of A Computer

You can, to some extent, control the speed of your system. The table below lists how the CPU and several bus speeds are controlled. There are two ways you can change the speed of a computer:

1. Change the speed of the memory bus. Whatever the memory bus speed is, the PCI bus speed is half or one third of that.

2. Change the multiplier that determines the speed of the CPU. The choices for the multiplier normally are 1.5, 2, 2.5, 3, 3.5 and so forth.