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We encouted an odd question during a Cisco IT Esstenials course which didn't make sense.

Which memory module has a front side bus speed of 200 MHz

Possible choices were

  • DDR-333
  • DDR-400
  • DDR3-667
  • PC100 SDRAM

The answer was DDR-400. The question we're asking is what is the relationship between FSB speed and RAM, and why wasn't DDR-333 a valid answer?

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up vote 3 down vote accepted

In older systems, the front-side bus (FSB) was synchronously tied to the northbridge and memory controller. This meant that, without the use of clock dividers (introducing complicated and expensive PLL circuitry to keep control of the different clock rates), your memory bus would operate at the FSB speed. In your case, DDR-400 was the answer, since DDR-400 memory modules have a clock rate of 200 MHz.

Now, as history progressed, systems that still used an FSB now had a clock divider between itself and the memory controller. This allowed for the use of different memory speeds independent of the FSB speed (so if we set the FSB to 400 MHz, and had a clock ratio of 1:2, the memory would run at 400 * 1 / 2 = 200 MHz).

I assume that since this isn't a computer architectures course, and since there was only one answer, it was implicitly implied that the system did not have a clock divider. If it did (and indeed, nearly all computers since the late 90's did), we could simply solve the ratio to make any of the above listed memory modules work with the computer.

To make DDR-333 work for example, we need a memory clock of 166 MHz, or a clock divider of 5:6. For DDR3-667, we need a memory I/O clock (not memory speed, DDR3 is different) of 333 MHz, or 5:3. Finally, PC100 would work with a divider of 1:2 for a memory clock of 100 MHz.

TL,DR: Without a memory clock divider, the FSB has to match the memory clock speed. With a clock divider, so long as you can create an integer ratio X:Y to match the memory:FSB speeds, then you can use that memory module (and that ratio can be satisfied for all of the memory modules listed in your question).

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Reading your explanation, I still don't see why other answers are wrong... In fact all could be right if proper dividers are set up. Am I missing something? Isn't i t more related with the "DDR" meaning of tranferring data on clock rise and fall (which would actually have a real 200 MHz frequency but work as 400 MHz? – m0skit0 Oct 5 '11 at 12:26
@m0skit0 again, your only hint was that there was only 1 answer - if you were allowed multiple selections, the true correct answer would be all of them! – Breakthrough Oct 5 '11 at 15:57

If you're required to choose one answer, you have to make assumptions such that only one answer is correct. The only sensible assumptions that will do that is assuming that the FSB speed equals the RAM clock speed. In that case, DDR-400 is correct since DDR-400 RAM has a RAM clock speed of 200MHz.

If you were permitted more than one answer, then the question is broken.

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