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FSB and CPU confusion

ShujaaShujaa Registered User regular
I'm planning on getting a custom built pc fairly soon, from this site:

http://www.pcspecialist.co.uk/index.php?page=form&&select=intel2

Can someone explain to me the relationship between mobo FSB speed and CPU FSB speed? On that configuration I'm stuck with a 1066MHz FSB motherboard (I think), but the only CPUs I can choose are either 800 or 1333.

Now I know RAM clock speed should match that of the motherboard, but what about the CPU? If I get a CPU listed as "1333 FSB" with a 1066 mobo, will it

a) be compatible?
b) run slower than normal operating speed as a result?

Thanks.

Also, any CPU recommendations from that selection would be appreciated, all I've decided so far is to stick with a Core 2 Duo.

Shujaa on

Posts

  • waterloggedwaterlogged Registered User regular
    edited November 2007
    Shujaa wrote: »
    I'm planning on getting a custom built pc fairly soon, from this site:

    http://www.pcspecialist.co.uk/index.php?page=form&&select=intel2

    Can someone explain to me the relationship between mobo FSB speed and CPU FSB speed? On that configuration I'm stuck with a 1066MHz FSB motherboard (I think), but the only CPUs I can choose are either 800 or 1333.

    Now I know RAM clock speed should match that of the motherboard, but what about the CPU? If I get a CPU listed as "1333 FSB" with a 1066 mobo, will it

    a) be compatible?
    b) run slower than normal operating speed as a result?

    Thanks.

    Also, any CPU recommendations from that selection would be appreciated, all I've decided so far is to stick with a Core 2 Duo.

    Depending on the board it may or may not be compatible. However even if it is it will be forced to run at the slower FSB speed unless you overclock things.

    waterlogged on
    Democrat that will switch parties and turn red if Clinton is nominated.:P[SIGPIC][/SIGPIC]
  • ZxerolZxerol for the smaller pieces, my shovel wouldn't do so i took off my boot and used my shoeRegistered User regular
    edited November 2007
    Just some quick notes on the relationship between front-side bus and CPU clock speed:
    • The final clock speed of the CPU depends on the speed of the front-side bus and the CPU's multiplier. FSB * Multiplier = CPU clock speed. Thus, if the front-side bus is 266 MHz, and the CPU multiplier is 9, the final CPU speed would be ~2.4 GHz (which is the case of the CPU in my computer, a C2D E6600, at stock).
    • Why did I say 266 MHz instead of 1066 MHz? 266 MHz is the actual signaling rate of the FSB, but Core 2 chips are run on quad data rate. Thus, it is effectively four times the actual FSB (266 * 4 = 1066 MHz). My E6600 above is an older chip and runs at 1066 MHz (266 MHz FSB). Newer high-end Core 2s will be 1333 MHz (333 MHz FSB), and lower-end Core 2s -- including the new Pentium and Celeron branded chips -- run at 800 MHz (200 MHz FSB).
    • As stated before, the chip's multiplier along with the FSB will determine the final CPU speed. My E6600 has a multiplier of 9, so it runs at 2.4 GHz. Normally, the multiplier isn't something you can easily change, unless you're using the super-expensive "enthusiast" models (Core 2 Extreme and Athlon FX), which come with unlocked multipliers. Not an issue unless overclocking.
    • Running a 1066 MHz chip, like my E6600, on a board that can do 1333 MHz? No problem, the board will simply go to 266 MHz instead of 333 MHz (hopefully) -- same with 800 MHz chip on a 1066 MHz board. The caveat is that the board has to recognize the chip's model and that it supports it, this could require a BIOS update if the board came out before the chip.
    • Doing the opposite with a 1333 MHz chip on a 1066 board? Not good. At best, the board will run the CPU slower than it's rated -- baring some serious overclocking. At worse, the whole setup will fail and refuse to boot. There's a lot of other scenarios and variables here in play, too. Don't do it.
    • Memory is much more malleable. Generally, you can run a whole set of memory speeds that are different from the FSB and the system will take it fine (as long as the memory are all the same speed, of course), and a clock divider will make it play nice with your system. Again, with my E6600 running with a 266 MHz FSB, the memory that runs at that exact speed would PC4200 (DDR2-533) -- the system will use the memory without a clock divisor. People often run superfast memory that's faster than their FSB and it'll work dandy, but I hesitate to call that any real improvement. You run into nerd wars with shit like this.
    So, you have it opossite in regards to CPU and RAM speeds. CPU speed should match your motherboard's capabilities, but RAM doesn't necessarily have to (just so long as it isn't lower than the FSB you're running at, of course). I'm sure some leetsauce professional EE dude will come and correct and nitpick my explanations, but that's the gist of it, I hope.

    Zxerol on
  • ShujaaShujaa Registered User regular
    edited November 2007
    Thanks, that's a big help!

    Shujaa on
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