In conception, the design was simple: it was essentially a K6-2 with an additional level of cache memory (Level 3). The original K6-2 had a 64k primary cache and a much larger amount of motherboard-mounted cache (usually 512k or 1MB but varying depending on the choice of main board). In contrast the competing Intel parts used 32k of primary cache and either 128k of full-speed secondary cache integrated into the CPU itself (Celeron) or 512k of half-speed cache mounted on a processor daughter board (Pentium III). The K6-III, however, used both methods: it had 64k primary cahe, a massive 256k on-chip, full-speed secondary cache (similar to the Celeron's but twice the size), and the variable size motherboard mounted cache on the Socket 7 main board became tertiary cache.
In execution, the design was not simple: with 21.4 million transistors, it was a very large chip to manufacture with early 1999 technology, and the K6 core design did not scale well past 500MHz. Nevertheless, the K6-III/400 sold well, and the K6-III/450 was clearly the fastest X86 chip on the market on introduction, comfortably outperforming AMD's K6-2s and Intel's Pentium IIs.
Intel then introduced a slightly revised version of the Pentium II and named it the "Pentium III". Although the base design was unchanged (except mainly from the addition of SSE instructions and a hardware serial number), Intel's new production process allowed slightly higher clockspeeds, and it became difficult to determine which part was, all things considered, the faster one. Most industry observers regarded the latest Pentium III as superior for floating-point intensive tasks, but the K6-III as the best for integer-based work.
Both firms were keen to establish a clear lead, and both experienced manufacturing problems with the production of higher-frequency parts. AMD chose not to release a 500MHz or faster K6-III, preferring to concentrate on their soon-to-be-released Athlon instead. Intel produced a 550MHz Pentium-III with some success but their 600MHz version had reliability issues and had to be recalled with customers refunded.
With the release of the Athlon, the K6-III became something of an orphan. No longer a top-of-the-line part, it nevertheless required substantial manufacturing resources to produce: at 21.4 million transistors, it was almost as expensive to make a K6-III as a 22-million-transistor Athlon, and the same area of silicon could make more than two of the 9.3 million-transistor K6-2 parts. For a time, the K6-III was a low priority part for AMD—something to be made only when all orders for high-priced Athlons and cheap-to-produce K6-IIs had been filled—and it became difficult to source in quantity.
The original K6-III went out of production when Intel released their "Coppermine" Pentium III (a much improved part that used internal, on-die cache like a Celeron or a K6-III) and, at the same time, switched to a new production process. The changeover was fraught with difficulties and Intel CPUs were in global short supply for 12 months or more. This, coupled with the outstanding performance of the Athlon, resulted in even many former Intel-only manufacturers ordering Athlon parts, and stretched AMD's manufacturing facilities to the limit. In consequence, AMD stopped making the K6-III in order to leave more room to manufacture Athlons (and K6-2s).
By the time the global CPU shortage was over, AMD had developed revised versions of the K6 family: the K6-2+ and the K6-III+. Essentially, both parts were K6-IIIs (the 2+ with a 128k cache, the III+ with the full 256k) made on a new production process. Although mainly targeted at notebook computers, both parts were also available for desktop systems. AMD continued to devote their marketing resources to the Athlon and neither part became well-known outside the industry, but both had modest success and became firm favourites with the overclocking community. K6-III/450+ parts routinely overclocked to almost 600MHz before hitting motherboard frontside bus limits (indicating considerable room for further clockspeed ramping) and had it not been for the outstanding success of the Athlon and the gradual dissapearance of suitable Socket 7 mainboards from the market, the K6 family may well have stayed in production longer. Many K6-IIIs remain in service today (as of 2003).
see also: List of AMD microprocessors