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Computer Maintenance Unit Subtitle: Cache Concepts Excerpted from Copyright © Texas Education Agency, All rights reserved.1
Summary Introduction to cache Cache Computer Caches Caching Subsystems Cache Technology 2Copyright © Texas Education Agency, All rights reserved.
Introduction to Cache One method of increasing the memory- access speed of a computer is called caching. This memory management method assumes that most memory accesses are made within a limited block of addresses. There are many different types of caches. Some caches include memory caches, hardware and software disk caches, page caches and more. Even virtual memory is a form of caching. 3Copyright © Texas Education Agency, All rights reserved.
Cache Caching is a technology based on the memory subsystem of your computer. The main purpose of a cache is to accelerate your computer. Caching allows you to perform computer tasks more rapidly. 4Copyright © Texas Education Agency, All rights reserved.
Cache Cache technology is the use of a faster but smaller memory type to accelerate a slower but larger memory type. A cache has some maximum size that is much smaller than the larger storage area. It is possible to have multiple layers of cache. The diagram on the next slide illustrates the concepts of multiple layers of cache. 5Copyright © Texas Education Agency, All rights reserved.
Cache Layers 6 Diagram showing how each level's information is stored in a layer below it in a traditional memory hierarchy (www.howstuffworks.com). Copyright © Texas Education Agency, All rights reserved.
Computer Cache Microprocessors can have cycle times as short as 2 nanoseconds. If we were to build a small but very fast special memory bank, (around 30 nanoseconds), it would be two times faster than the main memory access. That's called a level 2 cache or an L2 cache. What if we build an even smaller but faster memory system directly into the microprocessor's chip? That way, this memory will be accessed at the speed of the microprocessor and not the speed of the memory bus. That's an L1 cache. 7Copyright © Texas Education Agency, All rights reserved.
Computer Cache Conventional processors use level 2 cache on the motherboard and connect to it using the standard memory bus arrangement. A special backside bus manages this high-speed data link between the processor and the level 2 cache having separate caches and buses to run them is far superior for multiprocessing. Each cache has an independent, non- interfering bus to service it. 8Copyright © Texas Education Agency, All rights reserved.
Cache Subsystems The Internet connection is the slowest link in your computer. So your browser uses the hard disk to store HTML pages. The first time you ask for an HTML page, your browser renders it and a copy of it is also stored on your disk, it is cached. 9Copyright © Texas Education Agency, All rights reserved.
Cache Subsystems (cont.) The next time you request access to this page, your browser checks if the date of the file on the Internet is newer than the one cached. If the date is the same, your browser uses the one on your hard disk instead of downloading it from Internet. In this case, the smaller but faster memory system is your hard disk and the larger and slower one is the Internet. 10Copyright © Texas Education Agency, All rights reserved.
Cache Subsystems Cache can also be built directly on peripherals. Modern hard disks come with fast memory. The computer doesn't directly use this memory -- the hard-disk controller does. When the computer asks for data from the hard disk, the hard-disk controller checks into this memory before moving the mechanical parts of the hard disk. 11Copyright © Texas Education Agency, All rights reserved.
Cache Subsystems (cont.) If the hard-disk controller finds the data that the computer asked for in the cache, it will return the data stored in the cache without actually accessing data on the disk itself, saving a lot of time. Copyright © Texas Education Agency, All rights reserved.12
Cache Subsystem Summary The normal caching system (www.howstuffworks.com): L1 cache - Memory accesses at full microprocessor speed (10 nanoseconds, 4 kilobytes to 16 kilobytes in size) L2 cache - Memory access of type SRAM in size) Main memory - Memory access of type RAM (around 60 nanoseconds, 32 megabytes to 128 megabytes in size) Hard disk - Mechanical, slow (around 12 milliseconds, 1 gigabyte to 10 gigabytes in size) Internet - Incredibly slow (between 1 second and 3 days, unlimited size) As you can see, the L1 cache caches the L2 cache, which caches the main memory, which can be used to cache the disk subsystems, and so on. 13Copyright © Texas Education Agency, All rights reserved.
Cache Technology The idea behind caching is to use a small amount of expensive memory to speed up a large amount of slower, less-expensive memory. Without L1 and L2 caches, an access to the main memory takes 60 nanoseconds, or about 30 wasted cycles accessing memory. 14Copyright © Texas Education Agency, All rights reserved.
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