unit5 final..pptx - UNIT-5 MEMORY DEVICES Topics...

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UNIT-5 MEMORY DEVICES
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Topics : Classification of memories RAM organization – write operation – read operation – memory cycle timing wave forms memory decoding memory expansion static RAM cell Dynamic RAM cell ROM organization PROM – EPROM – EEPROM- EAPROM, Programmable logic devices, PLA and PAL.
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Topic-1 Introduction to memories: definitions
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Introduction: Memory : Memory is a means for storing data (or) information in the form of binary words. Data memory : memories used to store data is called Data memory Program memory : Used to store programs(set of instructions that a computer executes to achieve a desired result) Address : Memories are made up of storage locations in which data can be stored. Each location is identified by an address . Capacity: Each storage location can accommodate one or more bits. Generally ,the total no of bits that a memory can store is its capacity. Some times capacity is specified interms of bytes(8 bit group)
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Memories are made up of storage elements i.e. Flip-flops (or) Capacitors in semi conductor memories and magnetic domains in magnetic memories),Each of which stores one bit of data. A storage element is called a cell. Write :-store data in a memory device. Read :- Retrieve data from a memory
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Memory Main memory consists of a number of storage locations, each of which is identified by a unique address The ability of the CPU to identify each location is known as its addressability Each location stores a word i.e. the number of bits that can be processed by the CPU in a single operation. Word length may be typically 16, 24, 32 or as many as 64 bits. A large word length improves system performance , though may be less efficient on occasions when the full word length is not used
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Topic -2 Memory organization
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Capacity or size of a memory means total no of bits or bytes or words that it can store. For convenience , the size of the memory is expressed as a multiple of 2 10 =1024 , which is abbreviated as K Ex: Memory size of 2 11 =2048 is said to be 2K. Ex: 2 14 (16,384)=16K Ex: 2 16 (65,536)=64K
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Memory system functions : 1) Select the address in memory that is to be accessed for a read or write operation. 2) Select either a read or a write operation to be performed. 3) Supply the i/p data to be stored in memory during a write operation. 4) Hold the o/p data coming from during a read operation. 5) Enable(or disable) the memory, so that it will (or will not ) respond to the address inputs and read/write command.
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Fig shows the diagram of 32 X 4 memory that stores 32 , 4 bit words. since the word size is 4 bits, there are 4 data i/p lines I0 to I3 and 4 data output lines O0 to O3.during a write operation, the data to be stored in memory have to be applied to the data i/p lines, During read operation, the word being read from memory appears at the data o/p lines.
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