The AMD K6-2 was probably AMD's most successful and popular Socket 7 processor. It sported the extended 3D-Now! instruction set and was very, very cheap. Although it could not compete speed-wise with the Pentium III series, it ran rings around the Pentium II and Celeron offerings at the same price. Unlike the Celeron, the K6-2 ran very hot and could hardly be overclocked at all without the application of large pelzier coolers and other such over-the-top methods. (Although the way things are going, I would expect all entry level PCs will have refridgeration units built in in a few years time).

The K6-2 had the additional bonus of AMD's close working relationship with 3dfx (back when they were good), leading to very nice accelerated versions of Quake 2 designed for this chip and the Voodoo2 graphics card. For an outlay of practically nothing I was able to maintain a decent games machine for well over two years based around these chips. Although the FPU was a great improvement on the doddering K6 and K5, it couldn't really compete with Intel's ever-increasing performance. It was eventually succeeded by the Athlon (K7).

The K6-2 is a great upgrade for ageing socket 7 boxes that would otherwise be useless. While not documented (or indeed recommended) by AMD, k6-2s will happily run at FSB speeds below 100Mhz, and interpret a clock-multiplier setting of 2x as one of 6x. The only requirement for using one is that the motherboard can support the core voltage of 2.2v1.

K6-2 500s can now be had for around a tenner, and will happily run at 66 x 6 = 400Mhz, or 66 x 4.5 = 300Mhz on a plain socket 7 motherboard. If you get a good one, you can reduce the core voltage to 2.1v or even 2.0v and use a slower, quieter fan.

For extreme overclocking (that is to say, extreme measures are taken to overclock it, not that the results are extreme), the heat spreader can be taken off the top of the chip using a stanley knife, and a socket A heatsink stuck directly on the core. At a low enough core voltage and clockspeed, the processor can be cooled passively.

A word of warning - socket 7 processors don't provide the motherboard with information on their core voltage in the way later processors do. Any 'auto voltage' setting a motherboard has is simply a lookup table based on what the clockspeed/multiplier are set to. This table will not have accurate entries for processors released after the motherboard was. Always set the core voltage manually if your motherboard allows it.2

1 - Some motherboards won't identify the processor correctly in the bios, and therefor won't set up any of its advanced features. http://www.geocities.com/uart0/k6tweak.html contains programs that can help in this case.

2 - My motherboard (a Gigabyte GA586TX3) selected a core voltage of 2.8v, and the poor k6-2 400 baked to death over the course of a year. When I took it out, the top of the heat spreader was scorched where the die had been.

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