So, you want to overclock a Celeron?
You've read a few post, maybe visited a few web sites. Everyone is reporting their success and claiming fantastic speeds from a lowly 266 or 300 MHz CPU. You're excited at the prospect of a high performance CPU for, essentially, small change and you want to get in on the action. The speed of a P2-400 or -450 for $90 or $150 sounds too good to be true. But wait, they're talking about S-codes, multiplier locking, Pin B21, CAS-2, and other esoteric terms. Names like Deschutes, Klamath and Mendocino are bandied about while you wonder what these words have to do with computers. Now you're confused. How hard is this going to be? Is it worth it? Do you need to be an Electrical Engineer to overclock a Celeron? In a word, no. With the right hardware and a little luck, it should be a snap.
Why is the Celeron so overclockable?
As you may know, a given chip design is used for CPU's of many different speeds. The P2 and Celeron designs are named after Western US rivers: Deschutes, Klamath and Mendocino. More on this later.
In theory, a CPU is tested first at it's maximum speed. The ones that pass the testing process at this speed are marked as such and sold as top-of-the-line CPU's. Those that fail at the fastest speed are tested at successively lower and lower speeds until they run reliably. These slower cores are then marked with the speed at which they passed the testing process and sold as slower processors. At least, that's the theory. No one really knows how Intel decides which cores get marked for a given speed. Several other factors, such as customer demand and production quality, affect how many processors of each speed are produced.
A CPU of any given speed can usually be made to run somewhat faster if one is willing to play around with the motherboard settings. This is the overclocker's bread and butter. Now, through a convenient turn of events, Intel has produced a CPU with an unusually high capacity for overclocking.
Intel has long controlled the high-end CPU market while its competitors, Cyrix and AMD were gaining market share in the low- and mid-price range because of the popularity of lower priced PC's. Intel finally realized what was happening and wanted to recover the low ground while also keeping the high end market (can you say "total market domination"?). When Intel designed the CPU core for their newest line of processors, the P2, they changed the way the CPU was mounted. All P2's are mounted on a circuit board, called an SECC (Single Edge Contact Cartridge), that plugs into a special, patented CPU slot (Slot 1) similar to a PCI slot. [Intel calls the Celeron packaging a SEPP (Single Edge Processor Package) but it's still compatible with the Slot 1 connector, go figure.] AMD and Cyrix do not have a Slot 1 CPU, so if you want high-end speed, you need to buy an Intel processor. Thus the high-end market is preserved for Intel. Now, Intel needed a cheap Slot 1 CPU to corner the low-cost PC market.
Enter the Celeron line. To reduce production costs, Intel left out the expensive Level 2 cache. Also, to eliminate design costs, the original Celerons (C266 and C300) used the same CPU core as the new 350-450 MHz P2's (code name Deschutes). [Remember, design costs account for a huge percentage of the total cost of a CPU. Once in production, it costs exactly the same to manufacture a core destined for use as a 266 MHz processor as it does to use that same core in a 450 MHz processor.] Many media pundits immediately dubbed the Celeron a backward-stepping piece of crap because of the lack of the L2 cache. Later, perhaps due to the poor reviews from hardware critics, Intel released the Celeron 300A and 333 with 128 Kb of built-in cache. Again, they used basically the same core design with some modifications to incorporate the on-die cache. The C300A and the C333 modified Deschutes core carries the code name Mendocino. Since Celerons use a Slot 1 motherboard, you can't upgrade to one of Cyrix's or AMD's fast new CPU's later, when prices come down. They don't have Slot 1 CPU's and Intel has the patent. Now Intel has again regained a foothold in the below-$1000 PC market and insured that the upgrade dollars also come home to Papa Intel too.
Here's where it gets interesting. The fastest P2 CPU's (350 to 450) require a relatively new type of Slot 1 motherboard with the BX chipset. The BX motherboard runs at a bus speed of 100 MHz. They can also run at 66 MHz bus which allows them to accept slower P2 CPU's (233, 266, 300 and 333) and Celerons. The Celerons are supposed to be used on the earlier EX and LX generation of Slot 1 motherboards which run at 66 MHz only. Since the Celerons have the exact same core as the new architecture P2 CPU's, there's nothing to stop you from setting the bus to 100 MHz and running a Celeron at 400 or 450 MHz.
People started buying BX motherboards and Celerons and overclocking the hell out of them by setting the bus speed to 100 MHz. A chip meant to run at 266 running at 400 MHz and more was unheard of previously. It's all because Intel is trying to capture the low-cost CPU market without the R & D costs of a new chip. It's really a marketing stroke of genius when you think about it. Produce one type of CPU. Take the best ones, add 512 kb of fast, expensive cache and sell it as the top-of-the-line CPU for $700+. Take the rejects, leave off the expensive L2 cache and sell them as cheap Celerons. Except they're too smart for their own britches. The production yield of 450 MHz cores is too good and the "rejects" are too few and far between. Because they want to flood the market with $100 CPU's, they have to mark them as 266 to 333 MHz Celerons and sell them cheap anyway. It doesn't cost them any more since both chips came off the same production line. Because the P2-450 market is relatively small compared to the low- and mid priced market, the demand is greater for Celerons.