The Real Front Side Bus
So you think you have a P4 with an 800MHz FSB or an Athlon with a 400MHz FSB. Well how wrong can you be. For the ones that don't care about the details, just divide the 400/533/800 by 4 and you'll come up with the real FSB speed of the P4. For AMD, divide by 2 and you'll get the real speed. For those that want to know the details of why these advertised FSB speeds are bogus, read on.
If you don't know what a bus is, and I don't mean the kind that takes your kids to school, then do a little searching on the web for "define front side bus". I'm basically lazy and see no reason to reinvent the wheel here. Now down to basics.
All bus speeds have always been defined by the clock speed in MHz. A. Hertz was honored for his work in electrcal magnetics fields by designating his name for 2 units of measure. One was the standard clock cycle of elctricity and the other was for the cycle of radio waves. That's it. Period. Now since a bus is electrical and has cycles it's obvious that it is defined in Hz just like the common electricity that comes out of the wall socket here at 60 cycles per second (60Hz). Seems simple enough right. So a bus that has 200 million cycles per second has a speed of 200MHz. Seems simple right. It is.
Now along comes AMD with a new interface to the cpu bus that will accept data at a rate of 2 bits per cycle. I'm not going to explain how as it's all over the web too. So they desigante their new technology as a 200FSB bus. Note that MHz is not in the designation. That's because the actual clock speed is 100MHz. The data rate is twice the clock speed though, which brings us to a new term, Data Rate.
I'm sure most of you know what data rate is. You've seen it on modems as 56Kbps or maybe even erroneously as 56KBaud when they really mean 56Kbps. Some old timers may remember the misuse of baud when they started semding more than one bit per baud with the 600/1200bps modem. I think the misuse there came to an end some time back. And while it's a different story, but it's pretty much identical to what has happened to the FSB. In the modem incident, they used the data rate numbers and turned them into the carrier term of Baud, which on a 1200bps modem is about 600Baud if I recall correctly. Anyway, data rates are desinated in 2 basic forms, bps (bits per second), and Bps (Bytes per second). And the combination of all the lines on the bus is designated as bandwidth. AMD and Intel have now done the same thing with the bus speed. They've taken the amount of data bits per clock cycle and called it MHz, which it obviously isn't. I think AMD stuck pretty much without the MHz designation until Intel blatantly added it to the data rate speed, but I could be wrong. In any case, it's hard to tell now what anyone is talking about in terms of FSB speeds since the erroneous numbers are being used by so many. Will it ever clear up? I hope so. We're already at the point where AMD has a real FSB of 200MHz and a bogus 200MHz for their older Athlon/Duron cpu's that run on a 100MHz real FSB speed. And AMD is tlking about real FSB speeds of 266MHz and higher. Now how confusing is that going to get. Some one says they have a 266MHz FSB and you won't know if they really mean 266Mhz or 133Mhz.
Just one last comment on AMD's explaination of calling it the effective FSB speed. Effective compared to what? Without that information, it's just another worthless number. If they had only left it alone.:-(