Cache Memory: Definition & Concept

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  • 0:01 Cache Memory & Related…
  • 0:58 CPU Cache
  • 1:54 How the CPU Cache Works
  • 3:52 Multi-Level Caches
  • 5:10 Lesson Summary
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Lesson Transcript
Instructor: Paul Zandbergen

Paul is a GIS professor at Vancouver Island U, has a PhD from U of British Columbia, and has taught stats and programming for 15 years.

Cache memory is a type of memory used to hold frequently used data. Cache memory is relatively small, but very fast. Most web browsers use a cache to load regularly viewed webpages fast. The most important type of cache memory is the CPU cache.

Cache Memory & Related Definitions

Computer engineers are always looking for ways to make a computer run faster. A multi-core chip, more memory, and a faster video card can all be used to improve the performance of a computer. One very effective method to improve speed is the use of cache memory. Cache is a type of memory that is relatively small, but can be accessed very quickly. It essentially stores information that is likely to be used again.

For example, web browsers typically use a cache to make webpages load faster by storing a copy of the webpage files locally, such as on your local computer. This is referred to as a web cache. Clever protocols are used to make sure that the copy is still current - if not, the web browser will get the new files from the original source. The general purpose of the web cache is to make the web pages load faster. Cache memory is used in other parts of a computer system. The most important type is the CPU cache, which will be the focus of the rest of the lesson.

CPU Cache

The central processing unit (CPU) is the brain of the computer. All of the instructions have to run through the CPU for the various parts of a computer to work together. CPU chips have been getting smaller and faster as chip technology has advanced. One of the slower aspects of computer processing is the interaction between the CPU chip and the main memory in the form of random-access memory (RAM). Installing more memory is not always a solution - the bottleneck is often the time it takes to access the memory.

So, what have chip designers come up with? A small form of memory located directly on the chip itself. This is the CPU cache. It is much smaller, but can be accessed much faster than the main memory. The CPU cache stores the most frequently used pieces of information so they can be retrieved more quickly. This information is a duplicate of information stored elsewhere, but it is more readily available.

How the CPU Cache Works

To carry out a particular instruction, the CPU needs a specific piece of information. The CPU will first check to see if this information is available in the CPU cache. If the information is found, this is called a cache hit. If the information is not found, this is called a cache miss, and the CPU goes on looking for the information elsewhere. In the case of a cache miss, the piece of information will be found in the main memory, but it will simply take longer.

A lot of research has gone into how to optimize the design of cache memory. The result has been somewhat counter-intuitive: smaller is faster. What this means is that a relatively small CPU cache improves speed, but as the cache gets really large, it no longer helps as much, and the CPU might as well look for the information in the main memory.

Think of it this way: let's say the main memory is like a 500-page textbook packed with information. It contains everything about the subject, but finding information means you have to look through the table of contents or search the index. You know the information is there, but it takes a bit of time to find the right page. Now, think of the CPU cache as a small notebook with only the most important information.

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