The Pentium III 866MHz is essentially identical to other Coppermine based Pentium IIIs, only faster. Like the Pentium III 1GHz, the only differences between the 866MHz and any lower speed version lay in the bin the CPU qualified for and the multiplier lock Intel used to tie it down. In this case, the CPU is locked to a 6.5x multiplier and designed to run on a 133MHz bus.

Because we've gone through it so many times before, we'll spare you the in-depth discussion of the Coppermine architecture and just give you the highlights of this SC242 connector CPU (SC 242 is Intel's new name for Slot 1).

Like its Coppermine siblings, the Pentium III 866MHz owes much of its performance to 256K of on-die "Advanced Transfer Cache." Running at full processor speed and connected via a 256-bit wide data path, the cache keeps the P6 CPU core of the Pentium III well fed, even at speeds of 866MHz and beyond. The older version Pentium III design carried 512K of "Discrete" cache running at half the processor speed with less efficient transfer methods. The current AMD Athlon also carries 512K of cache running at half the processor speed. This full speed cache is the source of the Pentium III's primary performance advantage over Athlon CPUs and becomes more of an advantage as clock rates increase. As speeds go higher, the Athlon's external cache speeds remain around 300MHz because high-speed SRAM is extremely expensive. You can read more details of the ATC system in our Intel Pentium III 500E and 550E FC-PGA CPU Review.