The orginal Athlon was based on the 250 nanometer process and was only found in the slot form, due to the 2 256K SRAM chips that were used as its L2 cache. These two chips, equaling 512K, ran at a fraction of the cpu speed, usually 2/3 or 1/2. There was a second revision of the orginal athlon that used the 180 nanometer process and thus was capable of being clocked higher, but it still had the 2 SRAM chips and was in slot form. Towards the high clock rates, lower fractions had to be used in order to keep the SRAM chips functioning stably, such as 1/3. The Thunderbird Eschews the SRAM chips altogether, and implements them onto the core itself. This allows the Chip to be used in socket form, which is a cheaper to make. Since the SRAM in the L2 is on the chip it is run at the same clock speed as the rest of core, thus creating a marked performance increase. The thunderbird is also made using the 180 nanometer process. Some Thunderbirds were released in the slot form, but these are relatively rare, and aren't supported under the popular VIA chipset for the original athlon, the KX133. Thus if you buy the socket form, you are guranteed to be purchasing the thunderbird as no original athlon was packaged that way. The thunderbirds are also released in 1.1 GHz and 1.2 GHz, whereas none of the original athlons were. This is because some of them are made at AMD's new fab in Dresden, Germany that utilizes copper interconnects, resulting in a green tint to the core, and better clockability.
<P ID="edit"><FONT SIZE=-1><EM>Edited by redgoat3 on 01/24/01 12:55 PM.</EM></FONT></P>