Components for Building Computers From Scratch/CPU Multiplier


I think I am miss understanding the Multiplier setting on the Motherboard.

When you Buy a Motherboard you have to set the Multiplier I thought to match the CPU speed.

But I may be off or getting this mixed up with something diferant.

I know every Motherboard will have a diferant Multiplier some Multipliers move in 8x steps some move in 10x steps.

So if you have a CPU that runs at 300GHz and you ad a Multiplier of 10x you will make the CPU run 10 times 300MHz.

Now yes I know this is OverClocking and it is not good because the CPU will over Heat and Crash.

So I do not think this is what people are talking about when they say if you install a CPU you have to set the Motherboard to run at the CPU speed.

Now I know all Motherboards Auto Detect the speed of the CPU.

But I am talking about the Motherboards that you have to set the speed by a Jumper.

What was this called and how did it work?

I will use the Pentiume 2 CPU it ran at 133MHz.

So when you installed it into the Motherboard you would have to set the Motherboard to run at 133MHz with a Jumper.

But I never understod what this was or what you did?

Did they have a Jumper that made the Motherboard speed move in 100MHz steps and you would move it till you got to your CPU speed?

Thank you.

Alright, if I'm understanding your question correctly: "what is the CPU multiplier and how does it work?" then this should explain:

- The CPU multiplier is, along with the FSB (or reference frequency - on newer CPUs that use HyperTransport or QPI they will not have a conventional FSB, but will provide a "reference clock" that serves the same function for the purpose of generating clockspeed), how the clockspeed is generated; via the following formula:

CPU Multiplier * FSB = CPU Clockspeed

The FSB is a given constant for a certain model of CPU (overclockers may change the FSB to achieve a desired clockspeed, but every CPU has a certain FSB that it is designed to operate with), the multiplier is also generally fixed by the CPU to set it to a given clock frequency. Some very old CPUs, as well as most mobile CPUs and some modern high-end models, can operate with either "any" multiplier (so-called "unlocked CPUs" - usually they can't have any value under the sun plugged in, but can accept any valid value for their model series), or have a range of multipliers they can accept (for example between 6x and 8x, this situation is almost always for power savings - the lower multiplier is engaged when the chip is in an "idle" state to conserve how much power it consumes).

FSB also has to be considered in terms of whether it is SDR (single), DDR (double), or QDR (quad) - the "base" is what is multiplied to determine clockspeed, not the effective. So for a Pentium 4 (which uses a QDR FSB), divide the FSB by 4 to determine the base clock that will be multiplied. For example the Pentium 4 2.0GHZ with 400MHz FSB has a multiplier of 20.0x; the base FSB is 100MHz, multiplied by 20 to provide a CPU clockspeed of 2000MHz (or 2.0GHz), and the QDR bus used for the FSB provides an effective FSB of 400MHz.

By contrast, an AthlonXP (which uses a DDR FSB), divide the FSB by 2 to determine the same. So for example the AthlonXP 3200+ (which has a clockspeed of 2.2GHz) has a multiplier of 11.0x, and an effective 400MHz FSB - the base frequency is 200MHz, multiplied by 11 to achieve 2200MHz (2.2GHz), and the DDR FSB provides an effective FSB of 400MHz.

With older chips like the Pentium 1 (there is no 133MHz Pentium II - the Pentium II was available between 233 and 450 MHz) they have a lower overall FSB (and it is SDR), for example 66MHz, and lower overall multipliers (usually in the 1-4x range); so a Pentium 133MHz with its 66MHz FSB would have a multiplier of 2.0x, providing a clockspeed of 133MHz and effective FSB of 66MHz.

As far as setting things with jumpers - some motherboards use jumpers or DIP switches to set FSB and/or multiplier values, which have to be set to correctly configure (or overclock) the installed CPU. More modern boards have these settings adjusted directly from the BIOS, assuming the settings can be changed - in the example of the Pentium 4 above, the multiplier cannot be changed (it is "locked" at 20x), however the FSB can be changed (which will result in the new value being multiplied by 20x as well; so if the FSB were re-set to 110MHz it would produce an overall clockspeed of 2200MHz, assuming the CPU/motherboard/etc could run at that speed - not all CPUs and motherboards overclock to the same extent). As far as what you would properly set these switches to, it depends on the specific motherboard and CPU combination you're using - you need to know what the CPU's specified FSB and multiplier are, and then how that specific motherboard needs its jumpers re-arranged (very often this is printed right on the board, but otherwise would be available from its manual).

CPUs that adjust their clockspeed for power savings generally do this via lowering their multiplier and core voltage, for example the Intel Core 2 series with EIST will drop their multiplier a few levels and bring core voltage down a few points to allow them to idle on less power. The Core 2 Quad Q9550 for example has a "standard" operating specification of 8.5x333MHz (1333MHz QDR FSB and 8.5x multiplier), to yield an effective clockspeed of 2.83GHz. However with EIST it will lower its multiplier to 6.0x, bringing clockspeed down to 2GHz, along with a slight core voltage drop; it does this when it detects low activity (for example the computer is on, but not doing anything) in order to consume less power. Many laptop CPUs employ a feature like this, and in some cases it is much more aggressive (letting them drop to even lower relative clockspeeds for even greater power savings - the goal here being to conserve/extend battery life, as opposed to just saving power). The host operating system and motherboard must support the feature for full functionality as well. In general I would never suggest defeating/disabling this feature (as it serves no purpose but to waste power), however on a desktop computer it can often be safely disabled (on a laptop this feature may be used as part of the system's thermal management as well, and the system may be designed with the understanding that the CPU will not run at maximum heat output all the time).

From your example there are a few other points I'd like to touch on, that don't fit into the above answer neatly:

- A 10x multiplier on a 300MHz FSB would not produce a 300GHz CPU, it would produce a 3GHz CPU - 1000MHz is equal to 1GHz (it follows SI conventions for prefixes), and 10*300 produces a result of 3000 (3000MHz = 3GHz).

- In general you wouldn't need to "walk" the clock frequency up to the CPU's target - you would just set things to the correct level the first time upon installing the CPU (if the board/CPU fails to run at those settings something is likely defective/broken). For boards with jumpers, the system must be powered off to adjust the jumpers, for boards that provide this control via the BIOS configuration the system will have to be re-started to effect changes.

- Overclocking does not always mean overheating and failure - the heat output of the CPU is generally increased, however whether or not this is significant depends on how much the CPU is being overclocked (and whether or not its core voltage is being adjusted - if vCore is being raised, power consumption can go up considerably), and the capabilities of the CPU's cooler. As long as the CPU is kept below its maximum operating temperature, and can be run stably at the overclocked settings (not all chips will overclock to the same extent - in general it's safe to assume there's a reason the CPU you've bought has the specifications it does; that's often what the manufacturer's internal testing was able to validate it at) it should not have any problems. A lot of the instability associated with overclocking (and the "bad rap" that it gets) tend to be the result of devices overheating, being undervolted, or clock dividers not being set properly (so as the FSB is increased, the board drags the other bus frequencies up with it, which causes problems for most devices (e.g. if the PCI bus frequency is dragged up)).

- Automatic clock setting has been accomplished through a variety of means, but is most often done through jumpers being physically set on the CPU's package - in some cases users will modify these to allow different settings to be run, or enable new features. This feature should behave the same across equivalent boards - for example a Socket 478 CPU with a given FSB and multiplier should be treated the same no matter what compatible Socket 478 board it is installed within. While the exact settings for that board may not align with another model, the end result will be the same.

If you have further questions, feel free to ask.


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