David Farber wrote:
> Athlon XP3000+ M CPU in an ASRock K7VT4A+. 512Mb PC2700 ram.
>
> The default settings according to AMD is fsb=133MHz and a multiplier of 16
> (2128MHz).
>
> Would there be an advantage gained if I change the fsb to 166MHz and the
> multiplier to 13 (2158MHz)? I believe this motherboard allows you to change
> the multiplier settings. Could the increased fsb cause problems with the
> other devices on the motherboard or does the motherboard automatically set
> the correct dividers for the other components?
>
> Thanks for your reply.
Executive summary:
Yes. There is an advantage. More FSB speed allows better memory transfer speeds.
There is no "lock" setting or discrete frequency setting ( AGP/PCI 66.66/33.33MHZ )
in your BIOS.
There are four frequencies of interest.
1) Clock feeding CPU.
2) Clock feeding memory (usually a ratio of simple integers, times CPU clock signal)
3) Clock feeding PCI cards
4) Clock feeding AGP card.
3 & 4 tend to be related to one another (AGP = 2 * PCI), so I won't treat them
separately, and assume they are related by a factor of 2. If PCI is 37.5 MHz
(the max useful value with no danger), then AGP would be 75MHz. The standards
define AGP and PCI as 66.66MHz and 33.33MHz as nominal values. Values above 75MHz
for AGP, cause problems for cards more modern than ATI 9800Pro. Older cards,
can take clocks higher than that.
In the clock generator chip, for older technologies (where the BIOS doesn't show
a separate setting for AGP and PCI clocks), the PCI clock is derived from the
CPU clock. If CPU is 133Mhz, they use a divider of 4 inside the clock generator
chip. If CPU is 166MHz, they use a divider of 5. The "special" frequencies
of 133MHz and 166MHz, also happen to be the values selected by the processor maker.
Intermediate frequencies may be selected manually in the BIOS. For example, bumping
up the 133Mhz clock to 150MHz, gives 150/4 = 37.5MHz for PCI. Therefore, for maximum
safety on an older motherboard, you avoid frequencies between 150 and 165MHz, and the
next useful value is 166MHz (if it works). It means that while the BIOS may offer
a fine tuning option, there are still "zones" of frequencies to avoid. Corruption
of your disk drive may result if you violate the rule. (Which is why, if testing
for initial stability, I like to boot with a Knoppix CD in the CDROM drive. You
can't hurt a CD. Knoppix is a 700MB .ISO download from knopper.net and is a Linux
operating system.)
The memory clock settings in the BIOS, are not absolute frequencies as such. If you
select 133MHz for memory, and the CPU is 133MHz as well, the design probably has a
1:1 ratio between the two clock signals. If you then bump the CPU clock manually
to 150MHz, that should result in the memory overclocking by the same amount. A
memory at 150MHz, is running at DDR300. When the CPU clock is using one of the magic
values of 133MHz or 166MHz, it is also possible that the memory values will
again read correctly. (So, in a sense, "sane" ratios exist at the magic clock
values. Oddball values, which you can still adjust around, exist for intermediate
settings. You can observe how this works, by using a copy of CPUZ from cpuid.com
and make tiny changes, then observe the reported results in CPUZ.)
So, by all means, try the CPU jumper of 166MHz. Your RAM is PC2700 or DDR333, and it
is a perfect fit for a DDR333 setting in the BIOS. The best transfer efficiency, is
often when the CPU and memory on these systems, is at the 1:1 ratio. So if using
FSB266, then DDR266 might be good. (To do better on the memory, you might have to
go to DDR400 setting, to beat the latency disadvantage, with the clock rate advantage.
Add latency happens, when the memory bus samples need to be resynchonized to the
FSB interface.)
If using FSB333, then DDR333 might be good. Check that the multiplier setting you need,
works properly while you are still at FSB266 or lower. If you are sure the multiplier
works properly, then switch up to FSB333 and try it. If the setting doesn't work,
try FSB266 jumper and DDR266 BIOS settings, then use the manual BIOS control to go
from a CPU input clock of 133MHz, to 150MHz. 150MHz should be the next lowest safe
clock choice.
To see some of how the clock generator chip works, this is a sample datasheet.
This clock generator is a couple generations older than the one on your motherboard.
You can see how the PCI clock is affected by the main clock choice here. The jumpers
on your motherboard, would go to the equivalent of the FS3,FS2,FS1,FS0 signals,
and helps to select a "magic" value for system startup.
http://web.archive.org/web/20041014100151/www.icst.com/pdf/ics9250-08.pdf
There are many clock generators made by that company. To see how many, have
a look at this list.
http://web.archive.org/web/20041014100151/www.icst.com/pdf
And yes, these are all aspects of overclocking.
HTH,
Paul
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