In mid-2000 AMD introduced an enhanced version of the Athlon processor, codenamed Thunderbird. Fabricated using AMD’s 0.18-micron process technology, the new core replaced the K75 chip’s 512KB of off-die Level 2 cache by 256KB of cache integrated onto the die itself and running at the full clock speed of the processor. This is in contrast to the original Athlons that operated their L2 cache at a certain fraction of the core clock speed; for example, in the case of the Athlon 1GHz, its external L2 cache ran at a maximum of 330MHz.
As well as boosting performance, moving the cache on-die also allowed AMD to follow Intel’s lead in moving from slot-based processors in favour of a socket form factor – in AMD’s case, a 462-pin format, christened Socket A. Supporting PC133 memory, the enhanced Athlon processor was initially available in six speeds, from 750MHz to 1GHz, in both Slot A (albeit available to OEMs only) and the new Socket A packaging.
The integration of the 256KB of L2 cache on-die increased the die size of the Thunderbird core by about 20% compared with its predecessor – 120 mm2 compared to the 102 mm2 of the K75 core. It is, however, still smaller than the original (0.25-micron) K7 Athlon, which weighed in at a hefty 184 mm2. Adding the 256KB of L2 cache to the die also dramatically increased the transistor count of the CPU. The new Thunderbird featured a 37 million transistor core, meaning that the integrated L2 cache accounted for an additional 15 million transistors.
In the autumn of 2000 the AMD760 chipset was released, featuring support for PC1600 (200MHz FSB) and PC2100 (266MHz FSB) DDR SDRAM. Other features of the chipset included AGP4X, 4 USB ports, 8GB addressing with 4 DIMMs and ATA-100 support. By this time Athlon processors were available in the Socket A form factor only.
The last Athlon processors based on the Thunderbird core were released in the summer of 2001, by which time speeds had reached 1.4GHz. Hereafter, the Athlon was to be replaced by the Athlon XP family – XP standing for extra performance – based on the new Palomino core.