Cache Memory

1. Cache Memory How caching works

2. The cache is a very high speed, expensive piece of memory, which is used to speed up the memory retrieval process. Due to it’s higher cost, the CPU comes with a relatively small amount of cache compared with the main memory. Without cache memory, every time the CPU requests for data, it would send the request to the main memory which would then be sent back across the system bus to the CPU. This is a slow process. The idea of introducing cache is that this extremely fast memory would store data that is frequently accessed and if possible, the data that is around it. This is to achieve the quickest possible response time to the CPU.

3. 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. In fact, on some machines the memory was faster than the processor. With the advancement of technology, the speed of every component has increased drastically. Now processors 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. The best way to keep the processor from having to wait is to make everything that it uses as fast as it is. But that would be very expensive. 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 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

4. Memory Cache: A memory cache, sometimes called a cache store or RAM cache, is a portion of memory made of high-speed static RAM (SRAM) instead of the slower and cheaper dynamic RAM (DRAM) used for main memory. Memory caching is effective because most programs access the same data or instructions over and over. By keeping as much of this information as possible in SRAM, the computer avoids accessing the slower DRAM. • Disk Cache: Disk caching works under the same principle as memory caching, but instead of using high-speed SRAM, a disk cache uses conventional main memory. The most recently accessed data from the disk (as well as adjacent sectors) is stored in a memory buffer. When a program needs to access data from the disk, it first checks the disk cache to see if the data is there. Disk caching can dramatically improve the performance of applications, because accessing a byte of data in RAM can be thousands of times faster than accessing a byte on a hard disk.

5. Levels of Cache: Cache memory is categorized in levels based on it’s closeness and accessibility to the microprocessor. There are three levels of a cache. • Level 1(L1) Cache: This cache is inbuilt in the processor and is made of SRAM(Static RAM) Each time the processor requests information from memory, the cache controller on the chip uses special circuitry to first check if the memory data is already in the cache. If it is present, then the system is spared from time consuming access to the main memory. In a typical CPU, primary cache ranges in size from 8 to 64 KB, with larger amounts on the newer processors. This type of Cache Memory is very fast because it runs at the speed of the processor since it is integrated into it. • Level 2(L2) Cache: The L2 cache is larger but slower in speed than L1 cache. It is used to see recent accesses that is not picked by L1 cache and is usually 64 to 2 MB in size. A L2 cache is also found on the CPU. If L1 and L2 cache are used together, then the missing information that is not present in L1 cache can be retrieved quickly from the L2 cache. Like L1 caches, L2 caches are composed of SRAM but they are much larger. L2 is usually a separate static RAM (SRAM) chip and it is placed between the CPU & DRAM(Main Memory) • Level 3(L3) Cache: L3 Cache memory is an enhanced form of memory present on the motherboard of the computer. It is an extra cache built into the motherboard between the processor and main memory to speed up the processing operations. It reduces the time gap between request and retrieving of the data and instructions much more quickly than a main memory. L3 cache are being used with processors nowadays, having more than 3 MB of storage in it.

6. Diagram showing different types of cache and their position in the computer system

7. Cache is a really amazing technology. A 512 KB level 2 cache, caching 64 MB of system memory, can supply the information that the processor requests 90-95% of the time. The level 2 cache is less than 1% of the size of the memory it is caching, but it is able to register a hit on over 90% of requests. That's pretty efficient, and is the reason why caching is so important. The reason that this happens is due to a computer science principle called locality of reference. It states basically that even within very large programs with several megabytes of instructions, only small portions of this code generally get used at once. Programs tend to spend large periods of time working in one small area of the code, often performing the same work many times over and over with slightly different data, and then move to another area. This occurs because of "loops", which are what programs use to do work many times in rapid succession.

8. Locality of Reference Let's take a look at the following pseudo-code to see how locality of reference works Output to screen « Enter a number between 1 and 100 » Read input from user Put value from user in variable X Put value 100 in variable Y Put value 1 in variable Z Loop Y number of time Divide Z by X If the remainder of the division = 0 then output « Z is a multiple of X » Add 1 to Z Return to loop End This small program asks the user to enter a number between 1 and 100. It reads the value entered by the user. Then, the program divides every number between 1 and 100 by the number entered by the user. It checks if the remainder is 0. If so, the program outputs "Z is a multiple of X", for every number between 1 and 100. Then the program ends. Now it is easy to understand that in the 11 lines of this program, the loop part (lines 7 to 9) are executed 100 times. All of the other lines are executed only once. Lines 7 to 9 will run significantly faster because of caching. This program is very small and can easily fit entirely in the smallest of L1 caches, but let's say this program is huge. The result remains the same. When you program, a lot of action takes place inside loops. This 95%-to-5% ratio (approximately) is what we call the locality of reference, and it's why a cache works so efficiently. This is also why such a small cache can efficiently cache such a large memory system. You can see why it's not worth it to construct a computer with the fastest memory everywhere. We can deliver 95 percent of this effectiveness for a fraction of the cost

9. Cache is responsible for a great deal of the system performance improvement of today's PCs. The cache is a buffer of sorts between the very fast processor and the relatively slow memory that serves it. The presence of the cache allows the processor to do its work while waiting for memory far less often than it otherwise would. Without cache the computer will be very slow and all our works get delay. So cache is a very important part of our computer system.

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