Introduction to Computers Lecture # 9

Introduction to Computers Lecture # 9

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  Introduction to Computer Lecture# 9 Course Instructor : Sehrish Rafiq Govt. Girls Degree College Maneri Swabi Higher Education Department KP
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  Lecture Overview  Cachememory  Level-1  Level-2  Level-3  RAM  SRAM  DRAM  ROM  PROM  EPROM  EEPROM
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  Cache Memory  Cachememory is a very high speed memory that is placed between the CPU and main memory, to operate at the speed of the CPU.  CPU is the fastest component of computer but the speed of fetching instruction from memory is normally slower than the speed of CPU.  As a result, the speed of CPU is also affected because for doing its work first it has to fetch instructions from memory.  To bridge this speed gap, Cache memory is placed between CPU and main memory.  The speed of cache memory is faster than main memory but is slower than registers.
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  Cache Memory
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  Cache Memory  Cacheis cheaper than registers which justifies their use.  Cache is used to store that data and instructions which are frequently demanded by the running program, so that the number of main memory accesses can be reduced.  There are three types of cache memory.  Level-1 cache  Level-2 cache  Level-3 cache
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  Level-1 Cache  Level-1cache is built in to the actual processor core.  It is a piece of high speed memory which operates at the same speed as CPU.  It is directly accessible by the processor.  It holds data that the processor needs to execute instructions.  It is very expensive and small in size.  Its typical sizes are 8,16,32,64 or 128 KB.
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  Level-2 Cache  Level-2cache pulls information from RAM which is then accessed by the Level-1 Cache.  Level-2 Cache memory is faster and smaller in size than RAM but is slower and greater in size than Level-1 cache.  It is less expensive than Level-1 Cache.  Its size varies between 64 KB to 16 MB.  The purpose of Level-2 cache is to constantly read in slightly larger quantities of data from RAM to be available to Level-1 cache.  Level-2 cache is mostly built in to the processor chip but in some cases it lies on the motherboard.
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  Level-3 Cache  Level-3cache is specialized memory that works hand in hand with Level-1 and Level-2 cache to improve system performance.  Level-3 cache is the biggest cache memory.  It anticipates data requests so that processing instructions are provided without delay.  Level-3 cache is faster than RAM but slower than Level-1 and Level-2 cache.  CPU first looks for data and instructions in Level-1 cache then it checks Level-2 and Level-3 cache respectively.  Level-3 cache lies on motherboard.
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  RAM  RAM standsfor Random Access Memory is the most common type of memory found in computers.  It is also known as main memory or primary memory.  Instructions and data cannot be used by the processor until they are brought in to RAM.  It is a R/W memory and allow the computer to store and access data and instructions randomly.  RAM is volatile in nature which means that if the computer is turned off or a power failure occurs all the data contained in RAM is lost.
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  Continued…  If thecomputer is turned off, all data contained in RAM is lost.  There are two types of RAM.  Static RAM(SRAM)  Dynamic RAM(DRAM)
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  Static RAM(SRAM)  SRAMis a type of semi-conductor memory.  The word static indicates that it does not need to be periodically refreshed to retain its data.  SRAM is more expensive but faster and less power hungry than DRAM.  SRAM is mainly used to make CPU cache.  Because of its complex structure, it is expensive and cannot be used for high capacity storage.
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  Dynamic RAM  DRAMis also a semi-conductor memory.  DRAM needs to be periodically refreshed to retain its contents.  It stores each bit of data in a separate capacitor within an integrated circuit.  The capacitor can be either charged or discharged.  In personal computers DRAM is used as main memory.  Because of their simple structure they allow high storage and are low cost.
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  ROM  ROM standsfor Read Only Memory.  ROM is a non-volatile memory which means that when power goes off it does not lose its information.  ROM is used for permanent storage of information.  It possesses random access property like RAM.  The contents of ROM are manufacturer supplied and cannot be changed by users/programmers.  There are three types of ROM.  PROM  EPROM  EEPROM
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  PROM  PROM standsfor Programmable Read Only Memory.  It is a type of ROM which can be programmed once and then can never be changed.  PROM is manufactured blank and then it is programmed just once.  The process is irreversible.  The data is fed in to it using PROM programs.
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  EPROM  EPROM standsfor Erasable Programmable Read Only Memory.  EPROM are non-volatile, they do not lose their content when power supply is cut off.  The content of EPROM chips can be erased using ultra-violet light.  They can be reprogrammed and reused many times.
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  EEPROM  EEPROM standsfor Electrically Erasable Programmable Read Only Memory.  EEPROM chips are also non-volatile.  The content of these chips can also be erased but using electricity instead of ultra violet light.  EEPROM chips can also be reprogrammed many times.