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AMD K6

After AMD's plans to conquer Intel with the K5 turned into a debacle, there was a great deal of skepticism when AMD again said that they had the solution to make them more than just "discount alternatives" to Intel. Still, the promise was enough to make most PC users salivate: a chip with the power of a Pentium Pro, with MMX instruction support and intended to fit into a standard Pentium socket 7! AMD delivered its much anticipated K6 chip in April of 1997, beating Intel's Pentium II to the market by a month and assuring themselves of at least a shot at the brass ring.

AMD showed their commitment to establishing themselves in the market when they purchased NexGen in 1996 and with it, the design for the NX686 processor. With AMD's K5 over a year late to market and basically a disappointment, AMD needed a solution now and got it with the NX686. NexGen had been intending to market this chip in its own socket, but AMD changed the design to fit the standard socket 7, added MMX support, and renamed it the K6. Despite the name implying a design evolving from the K5, it is in fact a totally different design that was created by the NexGen team and adapted after the AMD purchase.

The K6 is a very advanced processor internally. It uses x86 translation/emulation, decoding x86 instructions into what it calls RISC86 operations. It compares internally very similarly to the Pentium Pro and the Pentium II processors, which work in a similar way. The K6 has several key improvements and a few minor ones. The most important performance-enhancing architectural features are:

• Large Level 1 Cache: The K6 uses a full 64 KB of level 1 cache, a number that a few years ago was large for level 2 cache. AMD pretty much had to do this in order to ensure that the K6 would be able to compete with the Pentium Pro and Pentium II, because the K6 still uses the conventional, 66 MHz, motherboard-based second-level cache.
• Extra Decoders: The K6 has four x86 instructions decoders, compared with only three for the Pentium Pro and Pentium II.
• Large Branch Prediction Table: The K6's branch history table contains 8,192 entries, while most other processors top out at 1/8th this size. Due to this large size the branch prediction accuracy is estimated at 95% by AMD.
• Six Integer Execution Units: The K6 has more internal execution units than any other x86 processor right now, allowing for more parallelism and efficiency.

One trend that AMD has continued quite well with the K6 is its reputation for compatibility. The K6 is not a direct clone of the Pentium family at all, because it is its own design. However, K6 chips work with what are at least to my knowledge very few if any compatibility issues. Since the K6 does run on socket 7 motherboards, it is an excellent choice for upgrading provided that the voltage issues are dealt with. A BIOS upgrade is often also required to enable the K6 to be recognized by the BIOS.

A nice thing about the K6 is that unlike the K5, there are no silly "P" ratings to be concerned with. The K6 competes close enough to the Pentium Pro and Pentium II that AMD decided clock-for-clock was close enough. So a K6-200 really does run at 200 MHz, unlike most of the K5 series. (Not to mention the Cyrix chips.)

The voltages on the K6 can be a bit confusing. The K6 is a split-rail voltage chip, much like the Pentium with MMX. It runs at 3.3 volts externally; internally, the 166 and 200 run at 2.9V and the 233 at 3.2V. The newest K6, the 266, drops the internal voltage all the way down to 2.2V, as a result of its using the state of the art 0.25 micron process. Like the Pentium with MMX 233, the K6-233 runs at a 3.5x clock multiplier, which is selected when you set the motherboard to a 1.5x clock multiplier (since 3.5x isn't a clock option for socket 7 motherboards). As an aside, when you consider that the core voltage of the K6-233 is only 0.1V away from the external voltage, the K6-233 might in theory work without split rail, since 0.1V is within the margin of tolerance for the input voltage, according to AMD's own specifications (although it is right on the edge). Combined with the 1.5x setting, this may mean that the K6-233 could function in a regular Pentium 100 motherboard? Might be interesting to try that some day if I am feeling particularly adventurous... :^)

In terms of performance, the K6 is quite similar to the Pentium Pro at the same clock speed, and tends to be slightly faster than the Pentium with MMX. Despite the fact that AMD would like to think the K6 competes directly with the Pentium II, in fact it really does not. If you consider relative performance, and the fact that the K6 and the Pentium with MMX both go into socket 7 motherboards with split rail voltage, it's obvious what people are trying to decide between. The Pentium II is still a step above the K6, but in terms of price/performance the K6 is pretty hard to beat. When looking at upgrading, the K6 has the Pentium II beat hands down, competing only with the Pentium with MMX (unless money really is no object for you, and you don't mind the idea of a motherboard upgrade).

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