The formula for a cpu's operating "speed" is FSB x multiplier. Ex 200 x 11 = 2200 (3200+ speeds).
So, the multi is useful:
1) you want to acheive the highest FSB you can. So, lets say your cpu wont run above 2200mhz- you can reduce the multi to 10 and then run a FSB of 220. 220 x 10 = 2200
2) Your cpu is capable of speeds faster than you ram or mobo. Ex. say your ram or mobo can't go above 200, well then you can keep the FSB @ 200, but raise your multi to speed up your cpu without encountering instability due to ram or mobo chipset.
Basically, if the multi is locked, your only option for OCing is to raise the FSB. Just ONE variable to change.
If the multi is UNlocked, you now have two variables to play with in finding your optimal OC. It can simply make OCing a bit easier (and perhaps get you further).
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