The merits/weak points of the cache system

Cache memory, also called CACHE, a supplementary memory system that temporarily stores frequently used instructions and data for quicker processing by the central processor of a computer. The cache augments, and is an extension of, a computer's main memory. Both main memory and cache are internal, random-access memories (RAMs) that use semiconductor-based transistor circuits. Cache holds a copy of only the most frequently used information or program codes stored in the main memory; the smaller capacity of the cache reduces the time required to locate data within it and provide it to the computer for processing.

When a computer's central processor accesses its internal memory, it first checks to see if the information it needs is stored in the cache. If it is, the cache returns the data to the processor. If the information is not in the cache, the processor retrieves it from the main memory. Disk cache memory operates similarly, but the cache is used to hold data that has been recently written on, or retrieved from, a magnetic disk or other external storage device.

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.

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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: