Computer Memory

Computer Memory                           - Dhan Bahadur Thapa

Computer Memory is the most essential part of the computer system. It is used as mass storage device as well as the work place processor. Generally memory is used to store data programs result of operation and many more. In computing, memory refers to the physical devices used to store programs (sequences of instructions) or data (e.g. program state information) on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which function at high-speed (i.e. RAM), as a distinction from secondary memory, which are physical devices for program and data storage which are slow to access but offer higher memory capacity. Primary memory stored on secondary memory is called "virtual memory". An archaic synonym for memory is store.

Computer memory can be classified into two types.

1)      Primary Memory (Main Memory)

2)      Secondary Memory ( Auxiliary Memory)

1)      Main Memory

-          Main memory is the semiconductor and is very faster than secondary memory. Also main memory consumes less power than secondary memory. But primary memory is expensive than secondary memory.

                The main memory of the computer is also known as RAM, standing for Random Access Memory. It is constructed from integrated circuits and needs to have electrical power in order to maintain its information. When power is lost, the information is lost too! It can be directly accessed by the CPU. The access time to read or write any particular byte are independent of whereabouts in the memory that byte is, and currently is approximately 50 nanoseconds (a thousand millionth of a second). This is broadly comparable with the speed at which the CPU will need to access data. Main memory is expensive compared to external memory so it has limited capacity. The capacity available for a given price is increasing all the time. 

             

             For example many home Personal Computers now have a capacity of 16 megabytes (million bytes), while 64 megabytes is commonplace on commercial workstations. The CPU will normally transfer data to and from the main memory in groups of two, four or eight bytes, even if the operation it is undertaking only requires a single byte.

2)      Auxiliary Memory]

-          It is the storage device can store large amount of data and information permanently. It may of three types.

i)                    Read Only Memory

ii)                   Read Mostly Memory

iii)                 Read/Write Memory

1)      Main Memory

-          Main memory consists of random access memory (RAM) and Read only memory. The access method for both these memories is random that is any memory location can be accessed wherever required. We cannot write data on TOM but we can write to RAM easily. RAM is a read / writes memory. RAM is called volatile memory because it continuous data and information till the power is ON. Similar ROM is call non-volatile memory because it contains data and information even if power is off.

-          Three types of main memory are explained below.

i)                    RAM (Read/Write)

-          It stands for Random Access memory which is used by processor or all the time. So it is the main work place of the computer. RAM can load any program that is to be executed data that is to be operated and the result of the operation. This storage device because program and data will remain in RAM as long as the computer has electrical power. So it called volatile memory. It is of tow types.

a)      Dynamic RAM (DRAM)

b)      Static RAM (SRAM)

a)      Dynamic RAM: dynamic RAM is the type of memory in modern computer; the main advantage of DRAM is that it is dense meaning that we can pack a lot of bits into a very small chip. It is also cheaper than SRAM. The main disadvantage of DRAM is that it must be constantly refreshed otherwise data will be lost. E.g. DDR1, DDR2, DDR3 etc.

b)      Static RAM (SRAM): SRAM does not need periodic refresh like DRAM. It is much faster than DRAM. Transistors are used instead of capacitor in SRAM transistor do not lose their change so refreshing is not necessary.

ii)                  Read only memory (ROM)

-          ROM stands for read only memory. It only performs read operation but not write operation. It stores data and information permanently. It is also called nonvolatile memory because any data stored in ROM will remain even if power off.

a)      Masked ROM

b)      PROM (Program Read only memory)

iii)                Read mostly Memory

-          Read mostly memory are actually ROM that can be reprogrammed later to modify the program that already stored in it. That are used where reading is frequently done and writing is very rare. It is of following types.

a)      EPROM

b)      EEPROM

c)       flash memory

·         Cache memory

-          Cache memory: it is a type of memory that is placed between CPU and main memory. It is a semiconductor memory and speed memory used to increase the speed of the processor cache holds the important and frequently used parts of instruction and data and it can provide to CPU at a faster rate.

2)      Auxiliary Memory

-          It is also known as secondary memory. It stores data and information for future use. Secondary memory is called permanent because it stores data had instruction till the data and instruction in it are not deleted. The memory that supplement to main memory is called secondary memory. The common auxiliary device used in computer are CD-ROM, CD-RW, DVD, magmatic Tape, Hard Disk etc. secondary memory are slower and cheaper than main memory . They have higher storage capacity than main memory.