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AMD K6-III+ and K6-2+ Shaptooth's Final Bite
(Review by Aaron Vienot, July 13, 2001)

In a companion article on the overall performance of the K6-III+ posted on ClickOn-CPU I have looked into what possible benchmark results can be achieved by upgrading a Socket7 system to a K6-III+. To put these numbers into perspctive, this article compares the K6-III+ with the K6-2 in a few selected benchmarks. The K6-x+ processors were released late last year as an interim solution for the mobile market, between the fading K6-2 and the mobile Athlon/Duron line recently launched. Discontinued by now, there are still many of these parts "in the wild" leading to the question what kind of gains can be achieved from using a K6-x+ processor.

Acknowledgement

Thanks to MS for providing the Shuttle HOT597 motherboard used in the test setup. A review of this very fast motherboard can be viewed here. Thanks also for the proof-reading of both articles and helpful editorial and technical suggestions.

The K6-2+/III+ processors

The K6-2+ and K6-III+ processors are an evolutionary step from the original K6-III processor based on a 0.25 µm die fab process. The same CXT core was released as the 380MHz and faster K6-2 processors with the added 256kB on-die Level 2 cache running at core speed.

On-die L2 cache was originally meant as AMD's response to Intel's move to the Slot1 format to eliminate the problems with external L2 caches on the mainboard. Those problems mostly comprised design variations between different manufacturers leading to differences in trace length and other minor nuisances in the days of 66-83 MHz bus speed. With increasing bus frequencies, however, the external L2 cache became effectively the limit for the bus frequency. The goal was to eliminate the external L2 cache but instead of using a back-plane cache like Intel, to integrate the L2 into the die itself.

Reality proved that the expectations of running the K6-III without external cache were somewhat off. In fact, AMD appeared rather surprised when the first independent benchmarks found that performance not only depended on bus speed but that the size of the former L2, now L3 cache was the major dominating factor regarding K6-III performance. A technical discussion of the K6-III's Tri-Level cache architecture can be found in this article on the AMD K6-III, and as RoadRunner demonstrated in AMD K6-2 vs. K6-III, the extra on-die cache permitted the K6-III to outperform the K6-2 by respectable margins in both business and gaming benchmarks.

The K6-2 denuded of its cover plate

Few things are free and the new K6-III was plagued by high heat dissipation and low overclocking potential. Even good cooling solutions often could not overcome the clocking issue. Most of the part would function at lower speed grades and AMD later released those as 300, 350, and 380MHz parts into the OEM channel whereas the retail versions were mostly 400 and 450 MHz. A handful of unannounced 500MHz parts turned up a while after AMD ceased manufacturing the CPU.

The speed barrier was broken with the move to a 0.18µm die fabrication process leading to the K6-x+ processor line. In January 2000, AMD resumed producing the K6-III core at 0.18µm as an interim solution for the mobile market in FAB25, Austin, Texas. At this time, the Athlon was making strides in desktop PCs, but could not be positioned in the mobile market due to power and heat issues. The company's K6-2 line, traditionally a mainstay in lower-end notebooks, was peaking at 550MHz, leaving the door wide open for Intel's mobile Celeron processor to become a stronger presence in the mobile market. Intel's regularly announced price cuts, along with added speed grades, quietly undercut AMD on both price and clock speed.

The K6-3 has a substantially larger core than the K6-2 since 256 kB L2 require an additional 12.6 million additional transistors plus the SRAM interface, making it a worthwhile 15 million transistors added to the core

A die manufactured at 0.18µ is smaller than a die manufactured at 0.25µm, and the reduced die size allows for reduced power consumption hence less heat dissipation. This finally qualified the K6-III core for notebook use, and both the K6-2+ and K6-III+ were actually K6-III+ cores. The K6-2+ with a 128kB L2 cache was positioned as a mid-range part and the K6-III+ with 256kB as the high-end version.

AMD officially positioned the 'Plus' CPUs in the mobile market, meaning that these CPUs never entered the retail channel. The use of the same packaging, that is, the standard socket 7 pinout, however, asked for transplanting the new CPU into existing mainboards. The primary limitations were that some boards would either would not POST or not boot properly after the BIOS failed to recognize the processor ID string; and a board theoretically had to support the K6-x+ 2.0V requirements.

Most of the K6-III+ parts that entered OEM channels were specked for 450MHz operation. However, overclocking to 550 MHz was almost a guarantee, and many of these CPUs would reach 600 or 616MHz; some even made it to 672 MHz.

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