# 133MHz FSB will work on a 100MHz FSB, won't it? **URL:** \ **Category:** Factual Questions\ **Created:** [January 13, 2001, 3:32pm UTC](https://boards.straightdope.com/t/133mhz-fsb-will-work-on-a-100mhz-fsb-wont-it/49860 "2001-01-13T15:32:52Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![vandal](https://avatars.discourse-cdn.com/v4/letter/v/858c86/32.png) [@vandal](https://boards.straightdope.com/u/vandal)\ **Post date:** [January 13, 2001, 3:32pm UTC](https://boards.straightdope.com/t/133mhz-fsb-will-work-on-a-100mhz-fsb-wont-it/49860/1 "2001-01-13T15:32:52Z")
I have this Pentium III 800MHz with a 133MHz front side bus. However, my motherboard (an Abit BE6 1.0) can only support a 100MHz FSB. I haven’t installed the CPU yet, but I’m wondering, is the CPU backwards compatible with the 100MHz FSB? ---
**Author:** ![Ringo](https://avatars.discourse-cdn.com/v4/letter/r/779978/32.png) [@Ringo](https://boards.straightdope.com/u/Ringo)\ **Post date:** [January 13, 2001, 4:02pm UTC](https://boards.straightdope.com/t/133mhz-fsb-will-work-on-a-100mhz-fsb-wont-it/49860/2 "2001-01-13T16:02:13Z")
I think you’re going to have a problem. But get the word from someone who knows on the discussion board at [Hardware Central](http://www.hardwarecentral.com). ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [January 13, 2001, 4:50pm UTC](https://boards.straightdope.com/t/133mhz-fsb-will-work-on-a-100mhz-fsb-wont-it/49860/3 "2001-01-13T16:50:23Z")
**beatle** , you’re not emplying that someone here on _this_ message board doesn’t know. Are you? ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [January 13, 2001, 5:51pm UTC](https://boards.straightdope.com/t/133mhz-fsb-will-work-on-a-100mhz-fsb-wont-it/49860/4 "2001-01-13T17:51:23Z")
It will work fine as a stand alone. The only problem you may may have is mixing and matching with existing 100 mhz modules in the the same system. Some boards are tolerant of this and some are not. If the PC’s BIOS setup section for memory speed settings has adjustable parameters set it to auto or you can experiment with different spped settings, just don’t get too aggressive with timing issues. The real world performance increase is neglible. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [January 13, 2001, 6:03pm UTC](https://boards.straightdope.com/t/133mhz-fsb-will-work-on-a-100mhz-fsb-wont-it/49860/5 "2001-01-13T18:03:05Z")
> [@](#): > > \*Originally posted by astro \* > \*\*It will work fine as a stand alone. The only problem you may may have is mixing and matching with existing 100 mhz modules in the the same system. Some boards are tolerant of this and some are not. > > If the PC’s BIOS setup section for memory speed settings has adjustable parameters set it to auto or you can experiment with different spped settings, just don’t get too aggressive with timing issues. The real world performance increase is neglible. > > \*\* DOH! I’m going to have stop posting before I’ve had my afternoon tea. I misread the question (How, I don’t know, as it was perfectly straightforward) . I have no clue as to whether this will work but it doesn’t seem like a particularly good idea. I’ll see if I have any references. As an aside on most ABIT motherboards you can juice the bus speed past 100 mhz up a couple notches using the BIOS setup facility. You might get it close enough to 133 that the CPU is happy if the BIOS will accept the CPU and your memory modules will cooperate. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [January 13, 2001, 6:24pm UTC](https://boards.straightdope.com/t/133mhz-fsb-will-work-on-a-100mhz-fsb-wont-it/49860/6 "2001-01-13T18:24:30Z")
Look at what 100mhz/PIII speed top end the latest BIOS update gives you? If it maxes out at 500 mhz or so just let it go. If it will punch out to 800 mhz (but at 100 mhz) it might be worth a try. [http://www4.anandtech.com/showdoc.html?i=1022](http://www4.anandtech.com/showdoc.html?i=1022) ---
**Author:** ![Jman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jman/32/4416_2.png) [@Jman](https://boards.straightdope.com/u/Jman)\ **Post date:** [January 13, 2001, 7:52pm UTC](https://boards.straightdope.com/t/133mhz-fsb-will-work-on-a-100mhz-fsb-wont-it/49860/7 "2001-01-13T19:52:47Z")
> [@](#): > > \*Originally posted by vandal \* > \*\*I have this Pentium III 800MHz with a 133MHz front side bus. However, my motherboard (an Abit BE6 1.0) can only support a 100MHz FSB. I haven’t installed the CPU yet, but I’m wondering, is the CPU backwards compatible with the 100MHz FSB? \*\* Yes, you can do it…but only if you don’t mind having a Pentium III 600MHz. See, the clock speed is derived by multiplying the FSB by the multiplyer, in your 800MHz chip case, it’s 6. All chips past the P2-333 are multiplyer locked, so you can’t just up the multiplyer to 8…the only way to adjust clock speed is to change the FSB. Usually, people do this to overclock their chips and get a nice faster chip for less money (they’re all the same chip anyway), but you’d be effectively underclocking the chip to 600MHz. Thems the breaks. Jman