Clock and bus speeds

Hello! A quick question -- how are the clock and bus speeds
related?  I.e. looking at the motherboard manual, it seems
that the bus speed needs to be set differently depending on
the speed the chip is running at.  What is the reason
behind this?  Also, what chip speeds for the Pentium2 would
I need if I want to take advantage of the 100MHz bus speed?


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The clock and bus speeds need not be related at all. Any Pentium 2 can run with a 100MHZ bus speed. If you are to use the max bus speed the card you plug into the bus must be able to run at 100MHZ.
You will also need SDRAM selcted for appropriate speed.

ap9Author Commented:
Hmmm, ok, but what about those jumpers you some times have to set
when you move to a faster processor?  I guess they are called
clock and bus multipliers?  What is the deal with that?  I guess
that is what I meant to ask.

Also, how do you check to see if a card can run at 100MHz bus
speed?  Does this mean that all the cards I have now in my old
system cannot be transfered to the new system if I want to
run at that speed?


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If the proposed answer does not solve your problem, please re-open the question so that other experts may provide a solution for you.
Clock frequency and bus speed multiplier jumper settings are very important when configuring the motherboard for use.
If you tell me the manufacturer & model number of the motherboard, I can give you specific information concerning your system.

ap9Author Commented:
Ok, I've reopened the question -- sorry aburr, I guess it was a
bit of miscommunication.

To restate the question (more clearly):

How does the clock frequency and bus speed multiplier work -- I can read and follow the manual, but I'd like to know the
reasons for it.


The CPU clock frequency is set as fast as the cpu will take.
The bus clock runs at 8 MHZ. (ISA, EISA and PCI). The 8 MHZ was a fast speed when it was set. It has not yet been raised because that would make most plug in cards obsolete and unworkable. (different machines will do DMA transfers at different speeds on the EISA and other buses. The video also runs at different speeds).
Most of your old clock dependant cards will not work on the 100 MHZ bus. Ask the card maker.
Various caches have been set up to interface between the bus and the cpu, hard drives, and CD ROM to speed things up.
The clock frequency is the frequency at which a voltage changes on the clock output pin. That voltage change signals a cpu or card to do something.When it chages slowly the signal is easy to handle. When it changes very rapidly some of the energy is broadcast as a radio signal and interfers with other computer parts (and lots of other things). You can listen to your computer on an FM radio.
If a card is driven at a speed faster than what it is designed for, the voltages will not have time to settle down toi the required value before the voltage is required to change again. The faster the circuit the more difficult and expensive to design.


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Clock frequency is the rate at which a crystal vibrates in an oscillator circuit. It is then multiplied and/or divided by digital counter circuits to obtain the desired frequencies at which to operate components in the computer.
There are several pathways in the computer which are known as buses.  A bus can be thought of as a multi-lane highway on which data travels between the CPU, memory, and peripheral devices inside the computer.  Data travels on this highway at the rate of the bus speed which is on the order of many millions of cycles per second.  Components for the computer are designed to operate at certain speeds.  This may limit their usefulness as speed increases beyond that for which they were designed
I hope this helps.  There is much more detailed info available on the net if you want to go deeper.


  Hey AP9! I see you now have two questions here :

(1)  "How does the clock frequency and bus speed multiplier work?"


(2)  "Does this mean that all the cards I have now in my old system cannot be transfered to the new system if I want to run at that (100MHz) speed?"

  I saw your original question last night, but figured someone would answer it immediately. I think I know what you're asking in the first question, so I'll try to answer that one, and maybe that will also answer your second one.

  If I understand you correctly, you're asking about the "external clock frequency" and the "internal CPU to BUS frequency ratio."  A lot of this computer technical jargon is used interchangeably, (clock speed, clock frequency, clock rate, etc.,) especially in certain manuals that have been translated from one language to another by a computer program. Although good for a few laughs, the vernacular sometimes does nothing but confuse the issue. Even a manual utilizing perfect grammar, regardless of the language, can be somewhat ambiguous!

  The "external CPU clock (BUS) frequency" and "internal CPU to BUS frequency ratio" are used together to configure the speed for a specific CPU- typically with jumpers- or perhaps automatically on a "jumperless" mainboard.

  Depending on the speed at which you want to run the CPU, you would choose an External (BUS) Clock Speed, together with a Frequency Ratio, to determine the ratio of the internal frequency of the CPU to the bus clock.

  Now that we're both thoroughly confused, I'll append some examples illustrating how it works. Hopefully your manual or board shows the jumper settings used to achieve the required numbers. These apply to a few Intel Pentium CPUs :

 CPU Speed           External           Internal

 200 MHz               66 MHz            x 3

 166MHz                66 MHz            x 2.5

 150 MHz               60 MHz            x 2.5

 133 MHz               66 MHz            x 2

 120 MHz               60 MHz            x 2

 100 MHz               66 MHz            x 1.5

 90 MHz                 60 MHz            x 1.5

 75 MHz                 50 MHz            x 1.5

  The formatting of these examples may end up crooked, but it should give you an idea of how the frequencies are related. As you can see, there can be more than one configuration for a specific CPU. Also note that for some, the math doesn't always equal the CPU's rated speed... i.e., as is the case of the 200 shown above, you actually end up with 199 MHz. (Other factors, such as mainboard throughput, will affect your actual net. There are some good utilities around that will give you a real-time reading of your eventual speed.)

  Notice that the examples assume a mainboard clock supporting 50, 60 and 66 MHz, but the faster clocks and core frequencies- like your PII and board- would work in the same manner. Also note that you can push a CPU beyond its rated speed. (In the above example, a 133 set at 2.5 would run at 165 MHz.) For info on the do & don't of that subject, as well as more extensive info on other mainboard and CPU issues, take a look at "What is Overclocking?" at this address :

  Hope this clears some of it up for you... Let us know!

ap9Author Commented:
Thanks to all that replied!  That really helps!

Aburr:  Thank you!
rin1010:  *Great* supplement!  Thanks!


  You're welcome, AP9... I was notified that you had appended a   comment re my response, so I came here to read it...

  Unfortunately, I was charged 15 points to get in!

   ...whatever ¿

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