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  • Memory is a storage device. 
  • It consists of cells; each cell is flip-flop/or transistor and can store one binary digit. 
  • Typically each line/or location is an 8 bits or one byte register that can store 8 binary bits. 
  • When several of these registers are arranged in a sequence it is called memory. 
  • The User writes the necessary instructions and data in memory through an input device, microprocessor perform the given task and answer/result is generally displayed an output device or stored memory. 
  • Memories are made up of registers. 
  • Each register in the memory is one storage location. 
  • Each location is identified by an address. 
  • The number of storage locations can vary from a few in some memories to hundreds of thousands in others. 
  • Each location can be accommodating one or more bits. 
  • Generally the total number bits that a memory can store are its capacity. 
  • Most of the types the capacity is specified in terms of bytes. 
  • Each register consists of storage elements (flip-flops or capacitors in semiconductor memories and magnetic domain in magnetic storage), each of which stores one bit of data. A storage element is called cell. 
  • The data stored in a memory by a process called writing and are retrieved from the memory by a process called reading. 
  • A memory stores the bulk of data, instruction and results. 
  • The basic unit of memory is called a word. 
  • A word can store one unit of data. 
  • The maximum number of bits of data that a word can store is called length. Each memory location is associated with an address, which is used to access that particular location. 
  • With every read or write operation on the memory that address where the operation is to be performed is also specified. 
For example if we were to write 02,03 and 04 at location with addresses 2050,2051 and 2052 respectively. Both data and address would need to be specified with each write instruction. The memory read/write operation are performed using two registers, namely read/write operation are performed using two registers, namely Memory Address Register (MAR) and Memory Buffer Register (MBR). MAR contains the address where the read/write operation is to be performed, whereas MBR contains data to be written or data after the read operation.

Memory Structure and Its Requirements
Read/Write memories consist of an array of registers, in which each register has unique address. The size of the memory is N*M where N is the number of registers and M is the word length,in number of bits.

Example: If memory having 12 address lines and 8 data lines then Number of registers/memory locations= N power of 2=12 power of 2=4096
Word Length=M bit= 8 Bit 
Example: If memory has 8192 memory locations,then it has 13 address lines.

Types of Memory
Memory unit is an integral part of any microcomputer system. Its primary purpose is to hold program and data. The main objective of the memory unit design to enable it to operate at a speed close to that of the processor.

A microcomputer memory can be logically divided into four groups 
  • Processor memory /register 
  • Cache memory 
  • Primary and main memory 
  • Secondary memory 
Processor memory refers to a set of CPU registers. Processor registers are the first set of storage devices available for the programmers to store any data, but they are generally few in number up to few tens or hundreds. As these registers are available within the processor, they are the fastest memory registers. The main disadvantage is the cost involved, which restricts the number of registers and their bytes.

Cache memory is the fastest external memory; it is placed to the processor. The instructions to be executed are placed in the cache memory for access by the processor. These are few kilobytes in size. Cache memory contains volatile semiconductor RAMs. The processor fetches instructions from the cache memory and if an instruction is not in cache, it refers to the primary memory.

Primary memory is the storage area from which all the programs are executed. All the programs and corresponding data for execution must be within the primary memory. The primary memory is much larger than the processor memory and the cache memory but its operating speed is slower. The primary memory in a system varies from few KB to a few MB.

Secondary memory refers to the storage medium for huge files such as program source codes, compilers, operating systems. These are not accessed directly or very frequently by the microprocessor in micro computer systems. Secondary memory consists of slow devices such as magnetic tapes and optical disks. Stored information in a magnetic tape or magnetic disk is not lost when power is turned off. Therefore these storage devices are called non-volatile memories.

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