The Second Level Cache
Let's move from outside to inside. The second level cache of the Athlon models we've got in our lab is 512 kB, running at 1/2 the core clock speed, just the same as Pentium III. However, Athlon was designed with a very flexible second level cache in mind. The L2-cache size can range from 0.5 to 8 MB, the L2-speed can range from a ratio of 1/3, 1/2, 1/1.5 to 1/1 L2-cache clock / processor core clock. This opens up a very wide field of possibilities for segmentation of Athlon. The low cost versions could get small and slow caches, workstation and especially server version could be equipped with up to 8 MB of L2-cache running at core clock.
In my eyes the L2-cache is also one of Athlon's weaknesses. Due to the fact that Athlon's L2-cache is 'external', meaning 'not on the die', limits it to the SlotA-SEC-package. Intel is on the way to leave Slot1 and the single edge cartridge, simply because it's more expensive and less practical than a socket. Intel is only able to move away from Slot1 because the upcoming and delayed 'Coppermine' will have its 256 kB L2-cache integrated on the processor-die. AMD will have to go in this direction (and thus to .18 µ-process) very soon too, if it wants to stay competitive.
Anyway, the L2-cache of Athlon is a very flexible thing and as long as Pentium III is also using external L2-cache, there's no reason to complain.