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[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System Cache ]

Role of Cache in the PC

In early PCs, the various components had one thing in common: they were all really slow :^). The processor was running at 8 MHz or less, and taking many clock cycles to get anything done. It wasn't very often that the processor would be held up waiting for the system memory, because even though the memory was slow, the processor wasn't a speed demon either. In fact, on some machines the memory was faster than the processor.

In the 15 or so years since the invention of the PC, every component has increased in speed a great deal. However, some have increased far faster than others. Memory, and memory subsystems, are now much faster than they were, by a factor of 10 or more. However a current top of the line processor has performance over 1,000 times that of the original IBM PC!

This disparity in speed growth has left us with processors that run much faster than everything else in the computer. This means that one of the key goals in modern system design is to ensure that to whatever extent possible, the processor is not slowed down by the storage devices it works with. Slowdowns mean wasted processor cycles, where the CPU can't do anything because it is sitting and waiting for information it needs. We want it so that when the processor needs something from memory, it gets it as soon as possible.

The best way to keep the processor from having to wait is to make everything that it uses as fast as it is. Wouldn't it be best just to have memory, system buses, hard disks and CD-ROM drives that just went as fast as the processor? Of course it would, but there's this little problem called "technology" that gets in the way. :^)

Actually, it's technology and cost; a modern 2 GB hard disk costs less than $200 and has a latency (access time) of about 10 milliseconds. You could implement a 2 GB hard disk in such a way that it would access information many times faster; but it would cost thousands, if not tens of thousands of dollars. Similarly, the highest speed SRAM available is much closer to the speed of the processor than the DRAM we use for system memory, but it is cost prohibitive in most cases to put 32 or 64 MB of it in a PC.

There is a good compromise to this however. Instead of trying to make the whole 64 MB out of this faster, expensive memory, you make a smaller piece, say 256 KB. Then you find a smart algorithm (process) that allows you to use this 256 KB in such a way that you get almost as much benefit from it as you would if the whole 64 MB was made from the faster memory. How do you do this? The short answer is by using this small cache of 256 KB to hold the information most recently used by the processor. Computer science shows that in general, a processor is much more likely to need again information it has recently used, compared to a random piece of information in memory. This is the principle behind caching.

Next: "Layers" of Cache

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