Memory Dividers???

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Can somebody explain what "memory dividers" are for overclocking? I've
seen a couple of articles with tables that mention their use. But I
can't figure out what they mean when they say choose the appropriate
divider in the BIOS. I have an Asus K8N (socket 754) can't find anything
in the bios that refers to memory dividers.

Re: Memory Dividers???

On Tue, 29 Nov 2005 01:04:58 +0000, mort wrote:

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Not all boards offer it as a bios option. Basically, they are something
like 1:1, meaning run the ram bus 1 to 1 with the system clock. Or maybe
4:6, meaning run the ram bus at 4/6 (2/3) the system clock. These settings
acheive the same thing as manually setting the base ram bus lower than
200MHz when overclocking the system clock to overclock the default cpu
speed. May be a little easier for some. Certainly not needed as long as
you have some way to control the ram bus speed.

KT133 MB, CPU @2400MHz (24x100): SIS755 MB CPU @2330MHz (10x233)
Need good help? Provide all system info with question.
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Re: Memory Dividers???

A divider is what allows you to run the CPU FSB and DRAM (memory) at
different clock speeds.  Ideally, your CPU FSB and memory should run at the
same clock speed (i.e., synchronous).  However, it is *possible* to run the
CPU FSB and memory at DIFFERENT clock speeds (i.e., asynchronous), but only
within certain limits of the mobo chipset.  The available CPU FSB/DRAM
settings are defined as a "ratio", or sometimes called dividers.

For example, suppose you have an Intel CPU w/ FSB of 133MHz (clock).  You
want to use a memory of PC3200, which is 200MHz clock.  By default, your
BIOS will typically configure your system for "synchronous" operations by
downgrading the memory clock speed to 133MHz to match the CPU.  This is
called a CPU/DRAM ratio (or divider) of 1:1 (synchronous).  Not very
appealing when you consider that you've just lost 66MHz of memory clock
speed.  That PC3200 is effectively now PC2100.

Now suppose you'd like to fully exploit that PC3200 and run it
"asynchronously" to the CPU, iow, at its fully rated clock speed of 200MHz.
To do that, the BIOS must provide an appropriate "divider", or CPU/DRAM
ratio.  Chipsets will typical provide support for a very limited # of
dividers (1:1, 3:4, 4:5, 5:6, etc), and it varies chipset to chipset.  In
order to support, in this case, a CPU FSB of 133MHz (clock) and memory speed
of 200MHz (clock), we need an appropriate divider.  In this case, if the
motherboard supports a 4:6 divider, the configuration is supported (and
here's the calculation):

133MHz / 4 = 33MHz
33MHz * 6 = 200MHz

IOW, the motherboard uses the divider to tell you what relationships are
configurable for the CPU FSB and memory, and you simply do the math as above
to see what's possible.  If you don't have, in this case, the 4:6 divider,
you can't run a CPU FSB of 133MHz (clock) and memory speed of 200MHz
(clock), end of issue.

But sometimes you can workaround the problem.  Let's suppose we overclocked
the CPU, oh let's say, to 166MHz (not unprecented for an Intel P4
Northwood).  By doing so, we can now take advantage of the 5:6 divider
(let's assume the mobo in this case supports it), then we have:

166MHz / 5 = 33MHz
33MHz * 6 = 200MHz

Whalla!  By overclocking the CPU FSB, we've manage to not only run the
memory "at spec", that is, PC3200 or 200MHz (clock), we've even increased
the CPU FSB for better performance.  Naturally, this doesn't address the
issue of CPU stability.  But it illustrates how overclockers use the
CPU/DRAM ratio (or dividers) to configure the mobo to their liking.

Let's say the CPU is not stable @ 166MHz clock.  We still have the 5:6
divider available, so we'll simply increase the CPU FSB a few increments at
a time.  As we do, we'll be also increasing the DRAM clock speed thanks to
the divider.  Perhaps we find that the CPU is stable until, say, 144MHz
(clock).  We apply the formula again and get:

144MHz / 5 = 28MHz
28Mhz * 6 = 172MHz (or, 172MHz * 2 (DDR) * 8 (bits) = PC2752)

Not as much DRAM performance as we wanted. but certainly closer to PC3200
than before!  Just for fun, let's consider using the 4:6 divider instead:

144MHz / 4 = 36MHz
36MHz * 6 = 216MHz !!!

Hmm..., interesting we've now overclocked the memory as well.  This is a
good example of why overclockers often buy performance RAM, with speeds of
PC3500, PC4000, etc.  The extra headroom in that memory allows the
overclock, with the assurance of stability.

Why don't you not always see dividers in the BIOS?  Some mobo's don't
configure based on dividers, not directly.  What I mean is, they are still
restricted to a given set of CPU/DRAM ratios (dividers), but the user
interface may present those options in a different way.  For example, if you
specify a CPU FSB of 133MHz, then the BIOS will only allow you to select
DRAM settings that are compatible w/ the dividers it supports.  It doesn't
directly expose dividers, per se.  They still exist as
limitations/restrictions, it's just that the interface the manufacturer has
chosen conveys the information differently.  Many will use the term CPU/DRAM
ratio instead.  In the end, the mobo chipset ALWAYS has some limit on these
CPU/DRAM relationships.  IOW, you can't just plug in any ol' values you
want, there's a relationship between the CPU and DRAM that must always be
maintained.  And one chipset may be more flexible than another (e.g., NVIDIA
nForce4 vs. VIA).  The CPU/DRAM ratio possibilities for a given mobo chipset
are always defined in terms of these dividers.



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Re: Memory Dividers???

Excellent post. Now,how come more motherboards don't allow the user to
fiddle with the mem divider?

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Re: Memory Dividers???

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You have to realize that there are two different types of needs in the

There's the OEM market (Dell, Gateway, etc.) and the vendors who typically
provide their components (ECS, Intel, etc.).  These OEM's relish STABILITY,
they want as little mucking w/ the BIOS as possible, in order to reduce
support calls and naive (perhaps bumbling) customers making a mess of
things.  If you don't know what you are doing, futzing w/ the CPU FSB,
memory clock, voltages, and everything else required to support this level
of control, you may damage your system.  So these OEMs (and their supporting
vendors) often purposely limit access to these features.  Instead, they rely
on features like SPD (Serial Presence Detect) which allows the BIOS to query
memory for proper clock speed, CAS settings, etc.  IOW, the CPU FSB and RAM
speed in this case would be determined/negotiated dynamically, at start up.

The other end of the spectrum is the performance market.  These guys want
ALL the bells and whistles.  They want to tweak their systems and insist the
tools be provided in the BIOS (e.g., dividers).  Of course, they pay for
that privilege w/ higher costs.  These mobo's are also DESIGNED to support
OC'ing, which means access to voltage changes, for example.  There has to be
greater tolerances as well so the mobo is not easily damaged.  Overall
component quality will typically be higher.  Again, the performance market
PAYS for it, and as long as they do, vendors like Abit, ASUS, and others
will oblige.

Perhaps some might consider there to be a third market (or third way), which
lies somewhere between OEM and performance markets.  Such mobo's will
typically allow *some* BIOS changes not found on a run-of-the-mill OEM
board, but it might be limited to CPU and/or memory clock changes, but
usually no voltages changes (a little too dangerous for some ppl, but a
critical feature for serious OC'ers).

But your right, you're not going to find the ability to tweak the dividers
on ALL mobo's.  It's not a technical obstacle, but a politically/market
driven obstacle as I've described above.


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