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Module 3c2. About the 5th generations CPUs - continued

  • The Cyrix 6x86
  • The AMD K5
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  • Cyrix

    This is a low cost alternative to Pentium. The chip from the Cyrix company, which was introduced February 5, 1996, is a cheap Pentium copy.

    The chip was Pentium compatible, since it fitted into a Socket 7. When Cyrix suggested a 6th generation in their naming, it was because the 6X86 employed advanced techniques, which were not found in Intel's Pentium. Thereby Cyrix got improved performance from their chip with the same clock speed.

    The Cyrix 6X86 was marketed using a comparison to Intel's clock frequency.

    The 6x86 chips had lower internal speed than model name stated. Below, you can see the data for the different models:

    Cyrix model
    CPU speed
    Clock doubling
    System bus speed
    P120+ 100 MHz 2 50 MHz
    P133+ 110 MHz 2 55 MHz
    P166+ 133 MHz 2 66 MHz
    P200+ 150 MHz 2 75 MHz

    An interesting detail was, that the 6X86 P200+ was the first CPU to run a system bus speed above 66 MHz. However it was difficult to find motherboards with chip sets capable of this, so the chip never achieved an important position in the market.

    Cyrix 6X86s were known for poor performance regarding floating point operations. There also were problems with Cyrix and NT 4.0.

    In my experience, the 6x86 did quite a good job with common office programs in Windows 95. I was very satisfied with the P166+ I had. Of course I would prefer a genuine Pentium 166, but I was not willing to pay three times the price at that time.

    The 6X86 was later improved with Dual Voltage (like Pentium P55C). This reduced power consumption and heat generation.

    Also see the article on Cyrix M3. The company Cyrix was in 1999 taken over by Taiwanese chip producer VIA.

    AMD is another CPU brand, which has become very important. Their Pentium like chips offered Intel tight competition. AMD used their own technologies, and hence they are not clones. They had these series:

  • K5, corresponding to the classic Pentiums (with 16 KB L1 cache and no MMX)
  • K6, K6-2, and K6-3 which compete with Pentium MMX and Pentium II
  • K7 Athlon, from August 1999, which is not Socket 7 compatible.


    K5 was Pentium copy. The old K5 was for example sold as PR133. This means, that the chip should perform like a Pentium P133. However, it only runs 100 MHz internally. It still has to be installed in the motherboard like a P133.

    AMD's K5 also existed as PR166. As the name suggests, it was intended to compete with Intel's P166. It operated at just 116.6 MHz internally (1.75 X 66 MHz). According to the highly respected German magazine c't, issue 3.97 page 20, it actually ran at least as fast as the P166.

    This was due to an optimized cache and other new developments. The only feature on which it could not match the P166 was in floating point operations. These are typically necessary in 3D calculations in AutoCAD and similar applications.

    PR133 and PR166 cost far less than the similar Pentium models, and they were very popular in low budget machines.

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    Or continue with the 6th generation CPUs. Click for Module 3e.

    Read module 5a about expansion cards, where we evaluate the I/O buses from the port side.

    Read module 5b about AGP and module 5c about Firewire.

    Read module 7a about monitors, and 7b on graphics card.

    Read module 7c about sound cards, and 7d on digital sound and music.

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