More on Cache

Original Slot 1 and Slot A Style Cache

With the introduction of the Pentium II, Intel gave us the Slot architecture. Why the slot? Intel judged the bus-speed cache available on their previous Pentium offerings too slow to keep up with the Pentium II, but the full-speed, on-chip (not on-die) cache of the Pentium Pro was too expensive. Thus, a compromise was made and Intel created a package that contained a chip and separate SRAM cache chips on a single card, which was then plugged into the motherboard. By moving the L2 cache off the motherboard and closer to the chip, Intel was able to increase the speed of the cache significantly. The L2 cache on the Pentium II runs at half the speed of the processor. The older Pentium III processors based on the Katmai core also have the same type of L2 cache running at half of the processor speed.

Of course, there is a drawback to this approach. Previously, the SRAMs used for cache had to be capable of only 66MHz, since that's what they would always run at no matter how fast the processor was going. Often, they could run at lower speeds - 60 or even 50MHz. Suddenly, there was this demand for 116, 133 and 150MHz SRAMs to be used as cache for the original 233, 266 and 300MHz Pentium II processors based on the Klamath core. This need for faster memory, along with the additional materials necessary for the slot card itself increased the production costs of the CPU. Worse, however - for the overclocker, they added a roadblock on his path to more speed!

Cap'n, The Cache, She Canna Take Anymore!

So what's the mighty, dreadful issue with cache that hurts overclocking so badly? Well, in addition to worrying about how far you can push your chip, you're now limited by the speed of the cache. What good is it if your brand-spankin' new Athlon 550, made on the .18 micron process, has cache that only goes to 300MHz? Your Athlon would go to 600 and no more. Yes, most motherboards allow you to disable the L2 cache, but the performance loss from not having any L2 cache will greatly outweigh any speed gains.

Overclockers faced the same dilemma when they were presented with the Pentium II 333. Sure the processor was multiplier locked, but it was still only using the 66MHz bus, and it was the first P2 made on .25 microns. Just crank that bus speed up, and you have a nice little overclock, right? Nope, the cache wasn't fast enough to handle the extra speed. The scarcity of good cache kept the chip from becoming an overclocking wonder.

Athlons, too, have cache issues. Starting with the 700, AMD switched from a 1/2 to a 2/5 multiplier for cache, and the 900, 950 and 1GHz Athlon are running their cache even slower at only 1/3rd processor speed. Lower cache multipliers can have drastically negative effects on performance. While a 600MHz Athlon with half-speed cache can stand toe to toe with a Coppermine (which has full-speed cache), a 1GHz Athlon with its one-third multiplier becomes slower than a Coppermine at the same or slightly slower speed. Cache speed limitations have a huge impact in Katmai P3 and Athlon overclocking. What does moving the cache on-die do for the overclocker? Read on and find out!