Karbosguide.com - Module 3b1.

The CPU developments and improvements


The contents:

  • Clock frequency and -doubling
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  • Intro

    If you have to improve a CPU and that happens all the time it is not only a matter of technical development. There are many bottlenecks in and around the CPU, which are continually being bettered.

    To understand these technological improvements, one must remember that the CPU is a data processing gadget, mounted on a printed circuit board (the motherboard). Much of the data processing takes place inside the CPU. However, all data must be transported to and from the CPU via the system bus. But what determines the speed of the CPU?


    Clock frequency

    We know this from the ads: "A Celeron 466 MHz." The 466 MHz is the clock frequency. Actually, there is a small crystal on the motherboard. which continually ticks to the CPU at a steady number of clock ticks per second. At each clock tick something happens in the CPU. Thus, the more ticks per second the more data are processed per second.

    The first CPUs worked at a frequency of 4.77 MHz. Subsequently then, clock frequencies rates rose to 16, 25, 50, 66, 90, 133 and 200 MHz to the best today, which operate at almost 2000 MHz. Clock frequencies are still being increased. In a few years we will have CPUs operating at 3 GHz and more.

    To reach these very high clock frequencies, one has to employ a technique called clock doubling.


    Clock doubling in the CPU

    The problem with the high clock frequencies is to ensure that other electronic components keep up with the pace. It is rather simple to make data move very fast inside a chip where the print tracks are microscopic. But when we move outside the chip, other problems appear. The other components must be able to keep up with the pace. When the frequency gets too high, the circuit board print tracks start acting as antennae and various forms of "radio noise" appears. Briefly, it becomes expensive to make the rest of the hardware to keep up with these high frequencies.

    The solution to this problem was to split the clock frequency in two:

  • A high internal clock frequency, which governs the pace of the CPU.
  • A lower external clock frequency, which governs the pace on the system bus.

    Intel's 80486DX2 25/50 MHz was the first chip with clock doubling. It was introduced in 1992 with great potential. For a lower price you could acquire a chip, which provided 90% of the 486DX50 performance. The DX50 ran at 50 MHz both internally and externally. The DX2 ran at just 25 MHz on the system bus. This enabled lower cost motherboards. Also RAM speed demands were lower.

    Clock doubling occurs inside the CPU. If the motherboard crystal works at 25 MHz, the CPU will receive a signal every 40 nanosecond (ns). Internally in the CPU, this frequency is doubled to 50 MHz. Now the clock ticks every 20 ns inside the CPU. This frequency governs all internal transactions, including integer unit, floating point unit, and all memory management unit operations as well as others. The only area still working at 25 MHz are external data transfers. That is transfers to RAM, BIOS and the I/O ports.

    RAM speeds

    The speed of the CPU is also connected to the RAM. The ordinary FPM RAM and EDO RAM can functioned at a maximum of 66 MHz (possibly 75 MHz). Therefore, Pentium and similar CPUs were "clocked up" with factors from 2 to 5 internally.

    In 1998 the PC100 RAM was introduced together with new motherboards and chip set. This RAM works at 100 MHz, and using the clock factors 3.5, 4 and 4.5 we had CPUs running at 350, 400 and 450 MHz. The Intel CPUs Pentium II, Celeron, and Pentium III can operate with clock factors of up to 8.

    With chip set designs like i815 the internal clock frequency operates independently of the FSB (front side bus) connecting the CPU to the north bridge of the chip set. Hence we do not need to talk about clock doubling anymore, and the clock frequencies of the CPU reaches 1700 MHz and above.


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    Learn more

    Also see Module 3c about the 5th generations CPUs (Pentiums etc.)

    Click for Module 3d about the clock frequencies

    Click for Module 3e about 6th generations CPUs (Pentium IIs etc.)

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