# What speed RAM for a given mobo?

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How does one figure out the speed of RAM to get for a mobo? Is it supposed
to be some multiple that divides into the mobo front side bus speed or
something? Not sure how to figure out what type and speed of RAM chips to
get for a mobo. There must be some logic to it.

## Re: What speed RAM for a given mobo?

Nope, you've got to read the manual.  The motherboard manual.  The reason
there is no easy way to figure it out is that most chipsets allow RAM and
CPU to run asynchronously (at different base clock speed).  In some cases,
you might want to match your RAM to your CPU.  Let's say you have a
processor based on 200MHz (times the multiplier), then you might want DDR400
(200MHz, doubled) RAM, if your motherboard will allow that.  But in many
cases, DDR333 ram will work, also, and you won't notice the difference!

## Re: What speed RAM for a given mobo?

On Fri, 10 Mar 2006 12:39:42 -0500, Mike T. inscribed to the world:

So what exactly does the frontside bus speed relate to, if anything?

## Re: What speed RAM for a given mobo?

Good question.  It used to be that all components of a PC were timed by a
clock, and ran at the same speed.  Think of a drill sergeant calling cadence
while all the troops are marching.  But modern CPUs run much faster than the
clock, so they need multipliers, also.  (that is, they run at some MULTIPLE
of the clock speed).  Also, the clock in some cases is too fast, so some
components run at a DIVISION of the clock speed.  For convenience of
customers, many mainboard manufacturers are now using chipsets that allow
different components to run at different clock speeds.  That's how you get a
200MHz CPU running just fine with 166MHz (DDR333) RAM, for example.  This is
possible as the chipset will act as kind of a buffer between them,
communicating at different speeds with different components.

Typically, most components have their timing related IN SOME MANNER to the
front side bus speed, which is what the CPU is using to communicate with the
chipset, and with everything else.  Think of this bus as a hotline tied to
everything, though not necessarily directly tied to everything.

Where overclockers get in trouble (if they don't know what they are doing)
is that they don't realize that changing the FSB will change everything.  A
common problem is, cranking up FSB too far can overclock your PCI bus (even
though it's slower, it's still related to the FSB speed), and cause massive
system instability.  That's just one example of a problem with overclocking.
This is where BIOS settings that are overclocker friendly come in.  If you
really want to tweak the FSB, you've got to be able to control speeds that
other components that get timing related to FSB are operating at.  -Dave

## Re: What speed RAM for a given mobo?

Go to www.crucial.com
and input your computer\motherboard, you`ll be told what ram you need.

--
bw..OJ

## Re: What speed RAM for a given mobo?

The method used to determine the correct speed of RAM for a given
motherboard depends on which design of RAM the motherboard requires.  Most
motherboards of the past few years use DDR RAM (Double
Data Rate).  Determine what the rated maximum standard FSB speed of your
motherboard is; let's say that it's 200MHz for this example.  For DDR RAM
you then double the speed of the motherboard's FSB
(200 MHz x 2 = 400MHz.)  Thus the motherboard requires 400MHz DDR RAM, which
is known in the industry language as PC3200 DDR RAM.

--
DaveW

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