A64 - What exactly IS the FSB???

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Harrkev

Limp Gawd
Joined
Dec 8, 2004
Messages
143
Quick question on the A64...

What exactly IS the FSB???

Back in my day we used to have to walk to school through 6 feet of snow in 110 degree heat -- uphill both ways! And back in my day, the FSB was the speed that the CPU communicated with the rest of the system -- memory, PCI, peripherals, everything!

With the A64, the memory has its own bus, which can be divided down from the FSB. But it does NOT actually have to run at the FSB speed. And the processor itself runs at some multiple of the FSB speed. And the link to the peripherals is at a multiple of the FSB. But does ANYTHING actually run at the FSB speed? It seems to me that you could have a FSB of 200MHz, and yet have absolutely nothing running at that speed.

Opinions? Comments? Insults?
 
there is no fsb in a traditional sense.

the front side bus was the bus that went between the cpu and northbridge, on which the memory controller resided.
now that the mem. controller is on die, the fsb is technically the speed of the cpu.
and 'HTT' is the base clock that sets the speed of everything else. the break to down simply:

clock speed = htt * cpu multi
ram speed = cpu speed / ram divider (check the link in my sig to figure out how the ram divider is set)
hypertransport link = htt * ht multi
and, if you have a board with an unlocked pci/agp bus, the bus is set from the htt speed.
 
i wish the ram speed was the cpu speed divided by the divider. That would mean i would have my ram running at well over ddr4000. Dude cpu speed is its effective clock rate. Ram speed is HTT divided by ram diver ram divider. For example. HTT = 200 divided by a 5/4 divider, and we all know when u r dividing a fraction to multiply its reciprocal. So 200 * 4/5 will give u the ram speed.
 
Take the RAM setting and divide it by 200. Then multiply it by the cpu internal clock. For example. My internal cpu clock is set to 245 but my ram divider is at 166. 166/200 = .83 Now multiply .83 by 245 to get your Ram speed.
 
And yet my point still stands. It is possible to have a FSB of 200MHz, and yet have absolutely nothing running at 200MHz. So, in a sense, there IS NO FSB!

I think that calling it the FSB is a misnomer. Maybe they should call it the "base clock" or "system clock". But calling it the "Front Side Bus" when there is no such bus is rather strange.
 
Harrkev said:
And yet my point still stands. It is possible to have a FSB of 200MHz, and yet have absolutely nothing running at 200MHz. So, in a sense, there IS NO FSB!

I think that calling it the FSB is a misnomer. Maybe they should call it the "base clock" or "system clock". But calling it the "Front Side Bus" when there is no such bus is rather strange.
Click to expand...

It's just a traditional carryover from the 'old days' ;) Just like a lot of people who use Tivo or other PVR's still say "I taped such and such last night..." even though there is no tape.
 
Harrkev said:
And yet my point still stands. It is possible to have a FSB of 200MHz, and yet have absolutely nothing running at 200MHz. So, in a sense, there IS NO FSB!

I think that calling it the FSB is a misnomer. Maybe they should call it the "base clock" or "system clock". But calling it the "Front Side Bus" when there is no such bus is rather strange.
Click to expand...

You're pretty close to the truth there. The 'FSB' is the master clock generator. There has to be something in the system that tells the system what 200MHz is, and that's the clock generator. By raising the frequency that this 'master' clock generator runs at, it forces everything else in the system to run at a higher speed as well, as they run at a given multiple of the that clock generator setting - regardless of its actual value. Your HT link and CPU will run at a multiple of it, RAM at a fraction of it. With the variety of ratios and multipliers available, it is possible to keep 2 of the 3 pretty close to stock while rasing the third, which is the entire foundation of A64 overclocking. You reduce the multipliers on HT and sometimes RAM so that you can raise the clock generator frequency, which forces the CPU to run faster, presuming that you leave its multiplier at stock. But that's correct, the FSB concept on K8 is something of a misnomer, although the HyperTransport link to the northbridge (MCP on NV chipsets) is pretty close to an 'FSB' Indeed, many mobo makers have exploited the high effective frequency of an HT link (1600-2000 MHz) as a marketing trick to say "This mobo supports 2000FSB, compared to a lowly 800FSB on this Intel board over here," this despite the fact that the 2000MHz HTT only has 25% more bandwidth than 800MHz NetBurst, and the fact that they serve very different purposes.
 
A fsb/htt is a physical set of wires that carry a frequency generated by the clock generator which each component uses to sync operations.

As for the memory speed, the ram sends data to the cpu at the htt freq, the memory controller however runs at cpu clock frequency.

Good k8 (amd64) info...
http://www.cpuid.com/K8/index.php
 
dude, eclipse, u showed me one of ur posts in order to prove ur point? dude show me an outside source saying ur correct and i will be humble and admit it lol
 
slamgoku said:
dude, eclipse, u showed me one of ur posts in order to prove ur point? dude show me an outside source saying ur correct and i will be humble and admit it lol
Click to expand...

Eclipse is correct, so please give it a rest :)
 
(cf)Eclipse said:
http://www.hardforum.com/showpost.php?p=1026972298&postcount=10
Click to expand...
I think you have a small typo in your post, " the ram divider = cpu multi / ram ratio rounded up to the nearest whole number" should say that

the ram divider = (cpu multi / ram ratio) rounded down to the nearest whole number.


I think the nomenclature is very confusing (that comment isn't pointed at eclipse but at the subject in general), anyway here is a picture if it helps.
A64_speed_bus.jpg
 
*saves link to that picture*
i like that, though kinda complex at first, it's good for those visual learners among us :D
and the ram divider is rounded up to give a slower ram speed that one would expect, not down to give a faster ram speed ;)

thomas, thanks for sticking up for me... but to prove myself, i shall show something that shows that the method of calculation i have provided is correct.
 
So let me get this straight, im at 230*10.5 1:1, so my ram is really running at 10.5 / 1, which is 10.5 that gets rounded up to 11. then 2415/11 = ram freq, So my memory is really running at 219.5mhz not 230mhz like every app tells me?
 
(cf)Eclipse said:
and the ram divider is rounded up to give a slower ram speed that one would expect, not down to give a faster ram speed
Click to expand...
Yes, I think you're right. Me bad. :eek:


Lord of Shadows said:
So let me get this straight,...... So my memory is really running at 219.5mhz not 230mhz like every app tells me?
Click to expand...
When the memory ratio is 1:1 then things become simple so that
...memory divider = cpu multiplier
and at the end
...memory speed = clock speed

For some motherboards, the half multipliers can be weird and result in the answer you suggest. For others they seem to not be a problem and work as I suggest above. I would assume your apps are right. :)
 
Lord of Shadows said:
So let me get this straight, im at 230*10.5 1:1, so my ram is really running at 10.5 / 1, which is 10.5 that gets rounded up to 11. then 2415/11 = ram freq, So my memory is really running at 219.5mhz not 230mhz like every app tells me?
Click to expand...
yup, that exactly right.. all a64's round the half multi up, for the fact that there are no 1/2 ram dividers.
 
Just dled everest didnt get it right but was a neat little prog to add to my collection. I just updated to the latest cpu-z and sure enough its reporting according to what I found here. Now im going to have to find a nice int multipler, thanks for the info.
 
Wait I am a little confused here. I have an Fx-53 I have my divider set to 2:1 The option says dd400 right next to it so I assumed it was the right one. Corsair also told me to select this option. The chip is 200 * 12 = 2400 stock. So the divider indicates 12/(2/1) = 6, 2400/6 = 400? So my mem is running at 400mhz? Or is it just including the *2 since it is ddr?

I was playing around with the numbers and say I drop my multi to 11 and increase my HTT to 240 that gives me 2640. Of course change the divider from 2:1 to 5:4 (333) cause there is no way my ram is doing 240mhz. I would also drop the HT multi to 4 to keep it under a 1000 (240 * 4 = 960). So now my ram speed would be 11(5/4) = 8.8. 2640/9 = 293?
 
ddr400 = 200mhz = 1:1
ddr333 = 166mhz = 6:5
ddr266 = 133mhz = 3:2
ddr200 = 100mhz = 2:1

just an fyi :D
 
yup, if you guys really wanna know how ram speed is set with a64's, have a look at the link in my sig. :D
 
yeah, the problem is that the memclock to cpu ratio is higher than htt to cpu (meaning the ram divider is higher or equal to the cpu multi, never less..)

though, i wish for one thing: a board that lets you select the ram divider, not set a 'ratio' between the htt and ram speed. :eek:
 
(cf)Eclipse said:
yeah, the problem is that the memclock to cpu ratio is higher than htt to cpu (meaning the ram divider is higher or equal to the cpu multi, never less..)

though, i wish for one thing: a board that lets you select the ram divider, not set a 'ratio' between the htt and ram speed. :eek:
Click to expand...

My board doesn't even allow ratios. You just set the maximum allowed RAM speed (266, 333, 400, and up) and let it go. Granted that in the end it works out the same, but some people might not even have a clearly labeled RAM multiplier setting in their BIOS.
 
(cf)Eclipse said:
those 'max' speeds are ratios. as i said ddr400 = 200mhz = 1:1.. yeah
Click to expand...

I know, I was just pointing it out for any newbs that might be reading this thread.
 
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