Newbie question - FSB multi and dividers

[p3matty]

Can someone more knowledgeable than me please help me out with how FSB works? Specifically, the multipliers and dividers of that number and how it affects processor speeds and memory speeds.

There are a few things I understand, so maybe you can help fill in the gaps. I'm looking to build a core 2 duo computer here at the end of the year or the start of next year. They are "quad pumped" or whatever, so the FSB is technically a quarter of their 1066 rated speed, so it's really 266?? Is that right? That 4x multiplier can never be changed, right? Just like the multiplier on the CPU itself (7x for the 6300 I'm thinking of getting - 7x266=1.8gg gHz). Do I have it so far?

Here's where I start to lose it...when it comes to memory, and to overclocking. Lets start with memory, which has it's own multiplier of the 266 figure. So, a 2x multiplier would be 533 memory, and a 3x would be 800 mhz. Since 800 is obviously better (read faster) than 533, why buy 533 when 800 isn't that much more.

Now to overclocking..when someone says they have taken their 6300 to 3 gHz, they must have taken the FSB to 429 or so (429*7=3.003 ghz). If the FSB is that high, and the memory multiplier is at 2, it's at 858; or at 3 it's 1.3 ghz!! How many options do you have with these multipliers and dividers and such, and do I have it right so far?

Sorry for the newbie post, but I'm still using a pentium 3 on a daily basis!!

 

[pat]

.
AMD has a 200 MHz system clock and the Hypertransport clocks in at 5x not 4x like Intel's

HT speed = 4 x system clock = 4 x 200 = 1000 MHz -- however, it is bidirectional, thus multiply that by 2 to get effective speed of 2000 Mhz
CPU Speed = multplier x system clock = 10x200 = 2000 Mhz or 3800+ speed.

in my math, 4x200=800, not 1000

so, the system clock is muliplied by 5 to get 1000 Mhz. Bidirectional speed is still 1000Mhz. And even if you can go forth and back on a road that is limited at 50 MP/h don't means you are driving 100MP/H. The 2000 MHz is only a marketing number to impress noob and too look better than intel's 1066 FSB.

Then, the 2.0 Ghz is socket 754 3000+, socket 939 3200+ and X2 3800+..

 

[neiroatopelcc]

HT can be set between 4x and 5x so he's not completely wrong above .... in fact most is perfectly right.
As for the 2000mhz HT speed - you're right it is 1000mhz, but it still has a theoretical max thruput that'd equal a 2000mhz non-bidirectional link.... so the marketing stunt isn't as irratic as could've been.

 

[mr_fnord]

Bidirectional speed is still 1000Mhz. And even if you can go forth and back on a road that is limited at 50 MP/h don't means you are driving 100MP/H. The 2000 MHz is only a marketing number to impress noob and too look better than intel's 1066 FSB.

And DDR2-533 still runs at 133Mhz. If you care about data rates and not cycles then the 2Ghz label is just as accurate as DDR2-533.

 

[mr_fnord]

Options on these different speeds depends on the motherboard. If you read the recent review of 965 mobos <<http://www.tomshardware.com/2006/11/13/shootout_at_the_core_2_corral/>> you'll see that 'low end' consumer targetted boards have few system clock options, like 200, 266, and 333. They also usually only offer the most typical memory multipliers, 1:1(533), 3:2(800), and 2:1(1066). Boards designed for 'enthusiasts' (talk about a euphamism) have many greater options, for example system clock adjustable in 1Mhz increments from 100-600, adjustable PCIe clock, and additional memory multipliers like 5:4(667), 4:3(711), 5:3(888).

Remember this is DDR2 memory, so the I/O clock is half the label speed. So, 1:1 on a 266 Mhz bus = I/O of 266, double pumped to an effective rate of 533.

You wouldn't normally run your CPU at stock and go to 5:4 memory timings, since RAM typically will be 533, which wouldn't handle that speed, and 800, which would be faster running at stock 3:2. However, if you overclock and your CPU/FSB tops out at 330 Mhz, the 5:4 multiplier will work out to your memory running at very close to 800Mhz, so you don't need 1000Mhz RAM to reach this overclock.