The Bus Speed Guide 


The bus speed of modern computer systems is still a fairly neglected subject. If you want to know about the performance of a system, you always look first at the CPU speed. The higher the CPU speed, the faster the system -- that's how most computer users judge system performance

Two variables determine the speed at which your CPU runs: the frontside bus speed and the clock multiplier. By manipulating these variables you determine the CPU's clock speed.

Understanding Bus Speed : The CPU uses the FSB (frontside bus, also called the system or external bus) to communicate with system memory and peripherals.

FSB Speed x Clock Multiplier = CPU Speed

(This is what publicised speed of CPU e.g. Pentium III 450 MHz here FSB speed is 100 MHz and internal speed of CPU is 450 MHz using the clock multiplier of 4.5 i.e. 100 x 4.5 = 450 MHz).

The motherboard chipset controls the clock multiplier, which, in conjunction with the FSB speed, determines the core speed of the CPU. Multiply the FSB speed by the clock multiplier to get the CPU speed. By manipulating the clock multiplier and/or the FSB speed, you can increase the core speed at which the CPU runs.


The Importance of the Bus Speed

First of all, I think I should explain to all those who are unsure what 'Bus Speed' means and what it does.

The 'Bus Speed' is the frequency a Pentium, Pentium II, or compatible is tacted with externally. Only in the first days of the Pentium did the CPU run with the same tact/speed externally as internally. These were the days of the P60 and P66. A Pentium 60 was clocked at 60 MHz from the motherboard and its core was running at the very same speed. Times were changing and faster CPUs were needed, so Intel came out with the P90 and P100. These two fellows were still running at 60/66 MHz externally, but the core was multiplied by a factor of 1.5, which made a 90/100 MHz CPU. A short while later, Intel released the P75 to replace the P60 and P66. The P75 also runs with a multiplied speed: 50 MHz x 1.5 = 75 MHz. You know what has happened from there. Now the current CPU's, which runs at 100 MHz externally and 600 MHz (100x6) internally.

So what does this 'runs externally' mean?

To start with, the external clock is obviously supplied by the motherboard. At this speed/frequency/tact the CPU is communicating with all of the system components that it's directly connected to. These are

the Chipset
the RAM
            the Second Level Cache

We know about the important performance components of a PC system?

  1. the CPU - well, that's covered, it's specified by it's clock speed
  2. the Second Level Cache - aha, determined by type, size and ... the bus speed
  3. the RAM - aha again, determined by the type, the size, the timing and ... the bus speed
  4. the Video Card - ugh, determined by the type and the PCI speed ... = bus speed/2
  5. the Harddisk(s) - allright, determined by the type, the bus type and of course the PCI Speed ... hence the bus speed

Well, it seems there's nothing in that box, which isn't influenced by the bus speed. That's why a Pentium II  @ 350 MHz is much  faster than Pentium II @ 333 MHz. None other cause but only of  the external frequency difference.   So the faster the external speed the least CPU has to wait for instructions to fetched in. Now in coming days after the arrival of althon from AMD the comparisons will not only be by the internal speed of the cpu but also how much the external speed the cpu supports.