KarbosGuide.com. Module 3b2.

The CPU – developments and improvements

The contents:

  • Cache RAM
  • Cache overview
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  • About CPU cache RAM

    The CPU must deliver its data at a very high speed. The regular RAM cannot keep up with that speed. Therefore, a special RAM type called cache is used as a buffer - temporary storage. To get top performance from the CPU, the number of outgoing transactions must be minimized. The more data transmissions, which can be contained inside the CPU, the better the performance. Therefore, the Intel 80486 was equipped with a built in mathematical co-processor, floating point unit and 8 KB L1-cache RAM. These two features help minimize the data flow in and out of the CPU.

    Cache RAM becomes especially important in clock doubled CPUs, where internal clock frequency is much higher than external. Then the cache RAM enhances the "horsepower" of the CPU, by allowing faster receipt or delivery of data. Beginning with 486 processors, two layers of cache are employed. The fastest cache RAM is inside the CPU. It is called L1 cache. The next layer is the L2 cache, which are small SRAM chips on the motherboard. See the illustration below of a traditional Pentium PC:

    How much RAM

    The L2 cache can cache a certain amount of RAM. How much is determined by the chip set and the so-called TAG-RAM, the circuit controlling the cache.

    One of the most popular chip sets for the original Pentium was Intelīs 82430TX. it worked very well - except for detail. it could not cache more than 64 MB RAM. If you added more RAM to the PC, it was not cached by the L2 cache. Hence, using more than 64 MB of RAM on a TX-based motherboard decreased the performance.

    This situation has caused a lot of rumors about Windows not being able to use more than 64 MB RAM. However: Windows 98 can use up to 2 GB RAM! The only problems with the amount of RAM has come from poorly designed chip sets as the TX.

    Cache overview

    L1-cache first appeared in Intel's 80486DX chip. Today, bigger and better CPU cache is a natural step in the development of new CPUs. Here we only see the internal caches, i.e. cache integrated to the CPU and working at the full clock speed.

    CPU Cache size in the CPU
    80486DX and DX2 8 KB L1
    80486DX4 16 KB L1
    Pentium 16 KB L1
    Pentium Pro 16 KB L1 + 256 KB L2
    (some 512 KB L2)
    Pentium MMX 32 KB L1
    AMD K6 and K6-2 64 KB L1
    Pentium II and III 32 KB L1
    32 KB L1 + 128 KB L2
    Pentium III Cumine 32 KB L1 + 256 KB L2
    AMD K6-3
    64 KB L1 + 256 KB L2
    AMD K7 Athlon
    128 KB L1
    AMD Duron
    128 KB L1 + 64 KB L2
    AMD Athlon Thunderbird
    128 KB L1 + 256 KB L2

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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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