September '98 BX Motherboard Comparison

One new feature that made its way into AnandTech's BX Motherboard Comparison is the SEL100/66# signal setting.  One of the biggest disappointments with the Pentium II 350/400 processors was their partial clock locked nature, when using the 100MHz FSB the Pentium II 350 would only recognize 3.5x clock multipliers and the Pentium II 400 would only recognize 4.0x clock multipliers.  This is caused because the SEL100/66# Signal present on those two chips is set to "High".   Now, when operating either of the chips at a FSB other than 100MHz (i.e. 66MHz or 112MHz for that matter) the SEL100/66# Signal is set to "Low," effectively unlocking the chip and allowing it to recognize clock multipliers in the 3.5x - 5.0x range.  So in theory, if one could set the SEL100/66# Signal to "Low" while operating at the 100MHz FSB you could clock the chips at 450MHz (4.5 x 100) without any problems other than those directly related to overclocking.  Setting any electrical signal to "High" or "Low" is much like to flipping a light switch, in this case when the light switch is on (or in the "high" position) the processor is clock locked when used with the 100MHz FSB, but when the light switch is off (or in the "low" position) the processor is now available for operation in the 3.5x - 5.0x range.  The setting of this signal can be controlled within the BIOS setup of one motherboard out of the 18 reviewed by AnandTech and it introduces a few new overclocking possibilities for 350/400MHz Pentium II users.

Which leads us to the next question, how far can current Pentium II's be taken on an overclockable BX board?  Let's discuss the options:

Pentium II "Klamath" 233/266/300

The very first Pentium II - 233/266/300 chips weren't too incredibly overclockable, however they still left the competition far behind.   The best Pentium II - 233's for overclocking are the ones that were purchased around the time Intel clock-locked the 233/266's.  The reason for this is because Intel realized that their 233 and 266's were being overclocked to/remarked as 300MHz parts without any signs of difference between them and the real thing (courtesy of Intel's excellent yield on the processors).  Therefore a counteraction was necessary, in this case, Intel clock-locked the chips at 4.0x.  While that kept users from running their processors at 66 x 4.5, owners of those processors will be happy with the strong possibility that they will be able to run their 233/266's at 100/103 x 3.0 without too many problems, provided proper cooling measures are taken. 

As far as the 300 is concerned, this processor can be taken up to 100/103 x 3.5 with a bit of elbow grease helping the process.   Unfortunately anything above 350MHz is virtually unachievable using conventional overclocking methods.

Recently there have been reports of original Klamath processors (the 0.35 micron Pentium II's) being manufactured using the newer 0.25 micron Deschutes core, as well as being manufactured with the newer 4.5ns L2 cache that is present on the Pentium II 450 processor.  What does this mean?  Well it means that if you buy a 266MHz P2 and it happens to be a 0.25 micron core, you are essentially getting a Pentium II 450 that is clock locked at 4.0x since 4.0 x 66MHz = 266MHz.   What Intel isn't counting on is you running the chip at 4.0 x 100MHz, giving you a fairly stable 400MHz chip for the price of a 266MHz chip.  At the same time there have been reports of 300MHz chips making it all the way up to 450MHz, however at the current state of things in the Pentium II 266/300 market you can never be sure as to whether or not you're getting a 0.25 micron CPU unless your vendor is nice enough to tell you.  It is a gamble, but considering that the 233/266/300 processors are on their way out and their prices are dropping considerably it may just be worth it.

Pentium II "Deschutes" 333/350

Right now the Pentium II 333/350MHz processors are nothing more than clock crippled Pentium II 400 processors, an unfortunate yet extremely helpful truth since both of these processors can usually make it up to 400MHz and beyond with proper cooling.  On a motherboard that supports the manipulation of the SEL100/66# signal, the Pentium II 350 becomes a Pentium II 400 and even gives you the possibility of a 450MHz clock speed provided you were lucky and picked up a high quality chip.  The 333 is slowly being phased out as it can be easily replaced by a Celeron 333 which gives you virtually equivalent performance at a much lesser cost.  

Pentium II "Deschutes" 400

The 400MHz was the leader of the overclocker's pack until the Pentium II 450 made its debut alongside the new Celeron 300A.  When used with a board that supports the manipulation of the SEL100/66# signal, it has the potential to become a Pentium II 450.  While it can be pushed even further with the use of the 124/133MHz FSB settings you need to keep in mind that most AGP cards fail at any speeds greater than 75MHz, meaning that the 83MHz AGP frequency resulting from a 124MHz FSB overclock won't make your AGP video card too happy.  If you happen to have a PCI video card, then this changes things quite a bit, however most users out there will want to opt for one of the newer AGP 2X solutions that seem to be popping up everywhere.   Rapidly dropping in price, the Pentium II 400 doesn't seem to be the waste of money that it once was, however it still is if you compare it to the bang for your buck you get with the new Celeron's.

Pentium II "Deschutes" 450

The new overclocking king is here, but it isn't the Pentium II 450.  While the 450's are almost guaranteed to work at 504MHz, the jump from 450MHz to 504MHz (112 x 4.5) isn't great enough to justify the price of Intel's only official 450MHz chip.  The 450MHz chips are clock locked at 4.5x, meaning they only recognize 4.5x clock multipliers.  Keep this in mind while planning to overclock the chip, however the only reasonable setting outside of 450MHz seems to be 504MHz as no faster settings are reliable enough to be considered viable alternatives.

Pentium II Celeron 266/300

Intel's release of the cacheless Pentium II, otherwise known as the Celeron, gave overclockers and low-budget, hard core, gamers another chance at a high end system.  Both the Celeron 266 and 300 are clock locked at 4.0x and 4.5x respectively, hindering the overclocking capabilities of both processors.   The 266's will generally make it up to 400MHz with a Cooler Master Celeron fan, however there have been cases in which an adjustment of the core voltage of the processor was necessary to achieve optimal stability at 400MHz which can only be done on two motherboards reviewed by AnandTech.  The 300's aren't as great of overclockers as the 266's, some have been able to make it up to 450MHz however the success rate isn't as great as the 266MHz to 400MHz rate has been. 

Pentium II Celeron 300A

The cacheless Pentium II finally got cache and received an annoyingly simple name-change, this is of course, the Celeron 300A.  Like the Celeron 300, the 300A is clock locked at 4.5x, and like the original Celeron it is a beautiful overclocker.  Most 300A's can make it up to 450MHz without a problem, although some do require a modification of the core voltage setting which, once again, is only possible on two motherboards out currently.  There haven't been too many failure reports of 300A's at 375MHz (83 x 4.5) so that is another setting to aim for if 450MHz won't cut it.  Wishful thinking with the 300A seems to be the 504MHz setting that is possible with the Pentium II 450, considering that the 300A is rated for 300MHz operation, you can't really complain if it doesn't work at 504MHz.

Pentium II Celeron 333

Clock locked at 5.0x, the Celeron 333 is basically a higher yield 300A, and currently isn't the world's best overclocker.  In the future this may change, however most reports show that the 333's don't make it up to 500MHz reliably enough to justify the price difference between these and the 300A's which appear to be much better suited for overclocking due to their incredible success rate at 450MHz.

With that said, there remains one question: What is "proper cooling" for a Pentium II?  If you plan on overclocking your CPU, there is virtually no other option (as far as cooling is concerned) than Net-N-Dude's top of the line Glacier^TM 4500 Slot-1 Heatsink/Cooling Fan Combo.  It features three, count 'em, three fans on a massive heatsink virtually identical to the enormous OEM Pentium II Heatsink.  

It was using this that AnandTech managed to take the Pentium II - 400 up to a whopping 466MHz.   For an extra $35 tacked on to the overall price of your system purchase, this heatsink/fan combo is definitely worth it, if you can ignore the noise of the fan that is (which is mostly muffled by your case, although it is noticeably louder than the retail Pentium II Fan).