AMD's 850MHz Athlon is the third processor released based on the K75 core. Built on a .18-micron process, the K75 is just over half the size of the older K7 core (102 vs. 188 mm˛), meaning less heat, less voltage, and faster frequencies. Because more processors per wafer can be cut, we should also look for lower prices and better availability.
This last note is of special interest. Since Intel can't keep up with the demand for their Coppermines, consumers looking for a high-end system may be glancing over to look at a speedy new Athlon system.
Originally, AMD had planned to move the L2 cache onto the die concurrently with the transition to the .18-micron process (i.e. with the 750MHz Athlon). Unfortunately, the 850MHz Athlon is the third processor we've seen after that point without the added benefit of an on-die L2 cache.
What is the significance of having the Level 2 cache off of the CPU's actual die? SRAM (Static RAM, which makes up the L2 cache) in and of itself is a fairly expensive memory. Faster SRAM increases manufacturing costs, amplifies heat, and at some point hits a technological limit at which the RAM is produced. Normally, a CPU's cache would run at half the speed of the core, however, at 750MHz and beyond, AMD is forced to set the cache at 2/5 of the core frequency. Dubbed Pluto 2.5, we can only guess the AMD engineers secretly wished to call this cache divider Goofy. All jokes aside, our 850MHz Athlon endured a similar fate as the Athlon 750 and 800 before it and has an L2 cache set at 2/5 of the core frequency, or 340MHz.
Despite this limitation, the Athlon is still able to aggressively compete against Intel's Coppermine. Of course AMD knows this, meaning there is no dire rush to deliver an on-die cache and copper interconnects just yet. For now, we can only imagine the performance increases we would see with one of these decked out Athlon processors.