If you are editing page one of a text you are probablygoing to request page two. Some existingblocks or pages must be removed for a new one to be brought in.ĭisk caching is different from memory caching, in that it uses a formulabased on probabilities. Otherwise, a so-called missoccurs, and the processor has to look in the next, lower level of hierarchy.When a miss occurs, the whole block of memory containing the requested missinginformation is brought in from a lower, slower hierarchical level. When the processor needs information, it looks at the top of the hierarchy(the fastest memory). The memory hierarchy is a way to handle differences in speed."Hierarchy" is a fancy way of saying "the order of things fromtop to bottom, fast to slow, or most important to least important." Goingfrom fastest to slowest, the memory hierarchy is made up of registers, caches,main memory, and disks. By keeping as much of thisinformation as possible in SRAM, the computer avoids having to access the slowerDRAM. Memory caching is effective because mostprograms access the same instructions over and over. Amemory cache (sometimes called a cache store, a memory buffer, or a RAM cache)is a portion of memory made up of high-speed static RAM (SRAM) instead of theslower and cheaper dynamic RAM (DRAM). Two types of caching are commonlyused in personal computers: memory caching and disk caching. Memory CachesĬache (pronounced "cash") is derived from the French word cacher, meaning to hide. This allows the caches tocapture a bit every time (clock tick) the processor sends a bit, or thereverse. L-1 and L-2 caches run at the speed of theprocessor bus (also known as the front side bus). It also uses memory caches to store data on the way tomemory, where the memory controller is too slow to capture each bit in relationto the CPU's timing speed. Remember that the CPU uses memory caches to store data from registers that itwill be using again soon. It takes a bit longer for thesecretary to print a job and carry it back to the boss's office, butit's still much faster than having to run the job through the entirebuilding. Level 2 (secondary) cache memory is like thesecretary's printer in the next room. Level 1 (primary) cache memory is like the boss's own personal printer,right there by his desk. The CPU uses Level 1 and Level 2 caching ina similar fashion. Because theydidn't have to go all the way to the press room (main memory), thesetemporary jobs were extremely quick. Whenever the boss was working up a price quote for a customer, he could setup various calculations and have his secretary print them off. Finally, he bought a laser printer for himself (L-1cache), and one for his secretary (L-2 cache) so they could do these quicklittle jobs themselves. At first, they sent these small jobsto the press roomafter all, they were a printing companybut that wascosting too much money. Between print jobs, they had to send copiesof financial statements and records off to the accounting department and thegovernment. A+ Certification Exam Cram 2 (Exam Cram 220-301, Exam Cram 220-302), 2nd Editionĭo you remember the printing business? Well, the company expanded, meaningthere was more and more paperwork.