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.
AMD Overclocking - Memory Dividers???