What determines your FSB setting?

  • Thread starter Thread starter Jim Caldwell
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Jim Caldwell

I have a Asus A7N8X with a XP 2600 Barton, 1 gb of PC 2700 DDR ram. I think
PC 2700 is equivalent to 166 Mhz., so is whatever your RAM is rated for what
determines what you set your front-side bus speed to (in this case 166
MHZ)?
 
You're on the right track, but there is a more precise, accurate way to
describe it. To keep it simple, let's assume no overclocking.

You typically configure the CPU (FSB) on the motherboard to match the clock
speed of the processor. Assuming this Barton is 333MHz DDR (166MHz actual,
or "clock"), that's what you use. Usually you buy *matching* memory so you
can run synchronously (1:1), which simply means it has the same "clock"
speed as the CPU FSB. PC2700 is 166MHz actual/clock (333MHz DDR), so it is
a perfect match in this case.

So in the end, you arrived at the same/right conclusion, but I didn't want
to leave a misimpression. You don't typically buy memory at some given
speed "X", then set your CPU FSB to match it. That's literally how you
described it. You never let the memory speed *drive* the CPU FSB, it's the
other way around. You buy the best CPU you can afford, they buy memory to
match it. The cart pulls the horse, not the other way around!

In the simplest case, you match CPU FSB "clock" speed to memory "clock"
speed. But nothing prevents you from running FASTER memory if you prefer,
just so long as your motherboard supports it. So it may also be possible to
run say, PC3200 (which is 200MHz "clock"). In this case, you'd have the CPU
FSB running asynchronously to the memory (166MHz vs. 200MHz). It would
require the motherboard to support a 5:6 CPU/DRAM ratio too (166 / 5 * 6 =
200), at least to run at spec. This isn't always more efficient,
asynchronous behavior sometimes undermines any benefits you expected by
increasing the memory speed, in this example, by 33MHz. But some people do
anyway, and claim more performance.

Some people also buy faster clocked memory because they intend to overclock
the CPU FSB (run above it's spec of 166MHz in this example). By doing so,
their attempts to get more clock speed out of the FSB and continue to run
synchronously (CPU clock = Memory clock) is not hindered by the memory, the
*extra* clock speed afforded by the memory at 200MHz provides *headroom* for
the overclocking attempt on the CPU FSB. IOW, on the march to reach 200MHz
w/ the CPU FSB, the memory is never overclocked itself, indeed, it always
remains underclocked.

HTH

Jim
 
PC2700 = 266MHz. The DDR (Double Data Rate) runs at Double the FSB of
133Mhz, thus at 266MHz.
 
PC2700 = 266MHz. The DDR (Double Data Rate) runs at Double the FSB of
133Mhz, thus at 266MHz.
Click to expand...

PC2700 = 2700MBps, not 266MHz, There's nothing in it running at 266MHz.
The Bus speed is 166MHz, DDR (333Mbps per line, NOT MHz). And the FSB and
memory bus are 2 seperate buses.
 
Hi Jim
you can have perfect synchronizing with your computer
your mother board support up to 400 Mhz FSB (provided that you have
BIOS version 2, see Asus website)
and your AMD XP2600 support 333Mhz FSB. At the same time your memory
speed is 2700/8=337.5
SO you can set the FSB in your BIOS to be 333Mhz. I think the mother
board decided the speed of the FSB.
HTH
 
Jim you said
<quote>... In this case, you'd have the CPU FSB running asynchronously
to the memory (166MHz vs. 200MHz).</quote>

But I thought AMD XP2600 has FSB = 333Mhz. So in this case even with
PC3200 he can get good synchronization! Am I right??
 
Wes
which bus speed you are talking about.. do you mean the memory bus??
in this case yes it is 166 but not the FSB which is 333Mhz
 
esara said:
Jim you said
<quote>... In this case, you'd have the CPU FSB running asynchronously
to the memory (166MHz vs. 200MHz).</quote>

But I thought AMD XP2600 has FSB = 333Mhz. So in this case even with
PC3200 he can get good synchronization! Am I right??
Click to expand...

This is where the confusion comes in. We have to differentiate between the
actual "clock" speed, vs. *effective* speed (for lack of a better term).
When you install that AMD XP2600 w/ FSB 333 in your mobo, the motherboard
will generate a 166MHz "clock" to drive the FSB. That AMD CPU supports an
actual clock of 166MHz (spec'd, assuming no OC'ing), but because it
implements DDR, it's doing twice (2x) the amount of work as a non-DDR
processor w/ the same 166MHz clock. It does this by sending data on BOTH
the up and down side of the clock cycle. IOW, it's more efficient. AMD
sells and markets the CPU as 333MHz FSB (2 x 166MHz clock) to make clear
this is the peformance level you can expect, but it's not the actual "clock"
speed that it runs at.

Same holds true for PC3200. This memory "clocks" at 200MHz (spec'd,
assuming no OC'ing). The PC3200 label is simply a reflection of its
performance, which is 200MHz (clock) * 2 (for DDR) * 8 (bits wide) = 3200!
IOW, the speed of the memory is 3.2Gb/sec (theoretically).

Finally, for synchronization purposes, it's the clock that matters. Since
that AMD is 166MHz clock, and the PC3200 memory is 200MHz clock, these are
NOT synnchronized. Using that AMD processor (w/ 166MHz clock), the *ideal*,
synchronized memory would be PC2700 (2700 / 2 (for DDR) / 8 (bit wide) =
166MHz clock).

Of course, some people try to *force* synchronization by OC'ing one or the
other, but that's another story for another time.

HTH

Jim
 
Wes
which bus speed you are talking about.. do you mean the memory bus?? in
this case yes it is 166 but not the FSB which is 333Mhz
Click to expand...
Fixed top posted message.
AMD (or Intel for that matter) doesn't make a cpu with a FSB speed of more
than 200MHz. Actually, they don't make cpu's with any FSB speed. FSB speed
is determined by the MB and can be set to any value the MB supports. Bus
speeds are measured by the clock speed in Hz. This clock speed along with
the multiplier define the cpu internal clock. Why am I typing this
again... See The Real Front Side Bus in link below.
 
Wes
which bus speed you are talking about.. do you mean the memory bus??
in this case yes it is 166 but not the FSB which is 333Mhz
Click to expand...

333MHz is the *effective FSB speed* on an AMD FSB at a *FSB clock
speed* of 166MHz.

Just "bus speed" is a good way to confuse things. ;-)

The frequency of data transfers occurring on the bus is 333MHz. The
width of the bus is 8 Bytes, so we get a max data rate of 8B X 333MHz
= 2667x10^6 Bps

(And here's a question for Wes, if he reads this:
Is 2667x10^6 Bps equal to 2667 MBps or 2543 MBps?)

The transfers are synched on both rising and falling flanks of the bus
clock, so the *FSB clock speed* is only 166MHz.

There is one clock. This clock controls both cpu clock speed and the
FSB. And this clock is in fact identical to 'FSB clock speed'.

There's some risk of confusing 'effective FSB speed' - introduced by
the cpu makers - and 'FSB clock speed'. So for completeness sake, a
little table:

'effective FSB' 'FSB clock speed'

Intel P4 at 800FSB 200MHz
Intel P4 at 533FSB 133MHz
Athlon at 400FSB 200MHz
Athlon at 333FSB 166MHz
Athlon at 266FSB 133MHz

And this doesn't involve 'memory bus' in any way. But on the memory
bus, you have the corresponding confusion with *DDR* rating, Like
DDR333 and the memory bus frequency,166MHz for DDR333. This is analog
to *effective FSB speed* and *FSB clock speed*, but concerns another
bus.

ancra
 
The frequency of data transfers occurring on the bus is 333MHz.
Click to expand...

For clearity, that's 333Mbps per line.
The width of the bus is 8 Bytes, so we get a max data rate of 8B X
333MHz = 2667x10^6 Bps
Click to expand...
Don't know how you come up with these numbers. I get 64x333M = 21312Mbps
or 2664MBps.
(And here's a question for Wes, if he reads this: Is 2667x10^6 Bps equal
to 2667 MBps or 2543 MBps?)
Click to expand...
2667x((10^6)x8)=21336Mbps or 2667MBps. :-)
Calculators work wonders. I still don't know where you came up with 2667.
2664 maybe?
 
For clearity, that's 333Mbps per line.
Click to expand...

The nomenclature in wide use is "333MHz FSB effective speed", so it's
not objective that Mbpspb is clearer. But my question was rather if it
really is the same thing. Superficially, one might be tempted by:

333 Mbps per bitline = 333 Mb ps pb = 333 M * b * s^-1 * b^-1 =
= 333 M * s^-1 = 333MHz

Here's the problem: MHz is 10^6 * Hz.
But what does the M stand for in Mb?
Don't know how you come up with these numbers. I get 64x333M = 21312Mbps
or 2664MBps.
Click to expand...

Why is 2* 166 = 333 instead of 332? Because it's not exactly 166.
....And 333 is not exactly 333 either. And my 2667 is the same thing
again, 2666.66666.... rounded up.
But never mind, this is a shit detail, and god knows what our PC is
exactly running...
2667x((10^6)x8)=21336Mbps or 2667MBps. :-)
Click to expand...

Ok, so MBps is 'M Bps'? rather than 'MB ps'?

And 2667MBps is the same as 2543MB per second?

(since the M is not decimal for Bytes, M = 2^(10+10) instead of 10^6,
just like K = 2^10 instead of 10^3
So, in reverse, 2667 * 10^6 B is 2543 MB.)

And the deal is the same for Mbps (and your weird 'Mbps per bitline'
unit for frequency ;)) ?
Calculators work wonders.
Click to expand...
:-D Yes, but they can't think! ;-)

ancra
 
The nomenclature in wide use is "333MHz FSB effective speed", so it's
not objective that Mbpspb is clearer. But my question was rather if it
really is the same thing. Superficially, one might be tempted by:

333 Mbps per bitline = 333 Mb ps pb = 333 M * b * s^-1 * b^-1 =
= 333 M * s^-1 = 333MHz

Here's the problem: MHz is 10^6 * Hz.
But what does the M stand for in Mb?
Click to expand...
Since it originally equaled 1 million (in 333MHz or 333Mbps however you
want to put it), then it carries that value through the equation, and not
1024K. It started as a decimal value, not hex, so it can't be changed
during the equation. And throughput is always defined in decimal afaik.
Why is 2* 166 = 333 instead of 332? Because it's not exactly 166.
Click to expand...

Because the nominal speed of a 166MHz FSB is 166.67MHz. Times 2 is is 333
truncated. Close enough.
...And 333 is not exactly 333 either. And my 2667 is the same thing
again, 2666.66666.... rounded up.
But never mind, this is a shit detail, and god knows what our PC is
exactly running...
Click to expand...
Exactly.
 
ancra
How did you know the "Effective FSB" and "FSB Clock Speed" for Intel
and AMD?? Does the table below that you wrote come with the spec of
each CPU? If not how can I know the "effective FSB" and the "FSB Clock
speed or the actual FSB speed"
Thanks a lot.

Intel P4 at 800FSB 200MHz
Intel P4 at 533FSB 133MHz
Athlon at 400FSB 200MHz
Athlon at 333FSB 166MHz
Athlon at 266FSB 133MHz
 
ancra
Sorry I also forgot to ask about the DDR memory .. can we say that the
"actual speed or clock speed of DDR memory is 1/2 the effective
speed". I mean if the DDR is 333Mhz then the actual speed is 333/2 and
if DDR is 266 (which is the effective speed) then the actual speed is
266/2. Can I say so??
Thanks..
 
ancra
How did you know the "Effective FSB" and "FSB Clock Speed" for Intel
and AMD?? Does the table below that you wrote come with the spec of
each CPU? If not how can I know the "effective FSB" and the "FSB Clock
speed or the actual FSB speed"
Thanks a lot.

Intel P4 at 800FSB 200MHz
Intel P4 at 533FSB 133MHz
Athlon at 400FSB 200MHz
Athlon at 333FSB 166MHz
Athlon at 266FSB 133MHz
Click to expand...

The cpus are marketed with their 'effective FSB speed'. That's how you
buy them. The 'FSB clock speed' is what you need to set the mobo FSB
clock to.
(There's other older P4s and Athlons too, other than those I listed.)

"actual FSB speed" I don't understand. Please keep to "effective" or
*clock* respectively. Both are actual and real.
FSB Clockspeed is the frequency of the clock that syncs transfers.
Effective FSB speed is the frequency of transfers.

The Intel P4 varieties (P4B, P4C, P4E and P4EE) with 533 and 800 MHz
FSB, have what Intel call a "quad pumped" bus. This means the
transfers can occur 4 times per clock cycle. So 4 * 200 = 800 and 4 *
133 = 533.

AMD is using DEC's old Alpha bus. This syncs on both rise and fall of
clock. So effective speed is simply twice that of the clock.

ancra
 
ancra
Sorry I also forgot to ask about the DDR memory .. can we say that the
"actual speed or clock speed of DDR memory is 1/2 the effective
speed". I mean if the DDR is 333Mhz then the actual speed is 333/2 and
if DDR is 266 (which is the effective speed) then the actual speed is
266/2. Can I say so??
Thanks..
Click to expand...

This (333/2, 266/2 etc) is what the memory bus should be set to, for
DDR ram. So, for instance, DDR333 should run on a 166MHz memory bus.

ancra
 
Since it originally equaled 1 million (in 333MHz or 333Mbps however you
want to put it), then it carries that value through the equation, and not
1024K. It started as a decimal value, not hex, so it can't be changed
during the equation. And throughput is always defined in decimal afaik.
Click to expand...

Thank you. I thought I remembered having seen that (decimal) stated
before somewhere, but I wasn't sure.

That means 'MHz' and 'Mbps per bit' are exactly the same unit for
frequency. (I prefer MHz ;) And Mbps is something different,
troughput, rather than frequency)

But it also means things like:

"MB per second" is not the same as "MBps". Interesting observation, I
think.

ancra
 
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