FSB - Front Side Bus Computer Speed

Factual Questions
1

My system has an Epox EP-8KHAL motherboard. The specs say “266MHz Max FSB” but there are several things that don’t match:

The block diagram shows the bus between the processor and the KT266A as “133/100 MHz” and the memory modules as “266/200 MHZ DDR SDRAM”. The way I understand it is that the RAM clock speed is really 133/100 MHz but it transfers twice per cycle (Double Data Rate).

The jumper settings for the FSB state either 133 or 100 Mhz.

The jumper settings for the multiplier are for 133/100 Mhz and not 266/200 Mhz. For instance, I am running at 133 MHZ FSB and 11.5 multiplier for 1.53 GHz.

It all seems to point in the direction that 133 and 266 are really the same thing and 100 and 200 are also the same thing. Up until now I was saying my system’s FSB speed was 133 Mhz but I see everybody is talking about 266.

So what’s the Straight Dope on FSB speeds? Is the true speed here 133 or 266? Is it really 133 but they call it 266 on account of the DDR memory? What is the correct nomenclature? Even Epox seems contradictory.

2

Check this for a quick-n-dirty description of the FSB. Essentially, on a 200MHz FSB, the actual speed is 100MHz, and info is transferred on the rising and falling edge of each tick.

Hope this helps.

3

The 200/266 is mostly marketing, although not a false statement. True clock is 100 or 133, with the effective clock being 200/266 due to the double data rate.

The Pentium 4 is quad-pumped, with 4 bits of data transferred each clock cycle. That’s how they get the “533 MHz” FSB, 4x133 (actually it is 133.33333333, as 4x133=532).

/Markus

4

I see my suspicions were correct. Thank you both.