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Computer Primary Memory

Memory is the most essential part of the computer. We need memory to store the data. This storage enables us to use the stored data to in future. Computer memory is mainly divided as primary and secondary memory. The memory and storage devices are measured in terms of bits and bytes. 
Here we will discuss about the primary memory and in next section we will see the Secondary memory or Storage devices.

1.    Primary memory:
The term memory refers to the main memory of the computer, whereas the word storage is used for memory that exists on disk, CDs etc.. The main memory usually called a physical memory which refers to the ‘chip’ capable of holding data and instruction.
Functions of the primary memory
i. Primary memory holds copy of the operating system, when the computer is turn on.
ii. Temporarily store a copy of the application program that is currently being executed.
iii. Temporarily store the data input from the keyboard, which is required for processing.
iv. Temporarily store the result, which is generated from processing unit.
There are 2 types: ROM  and RAM

Read Only Memory(ROM):
In ROM the information is burnt (pre-recorded) into the ROM chip at manufacturing time. Once data has been written into a ROM chip, it cannot be erased but you can read it. When we switch OFF the computer, the contents of the ROM are not erased, but remain stored permanently. ROM is non volatile memory. ROM stores programs that boot the computer.

Programmable Read Only Memory (PROM):
PROM is a memory on which data can be written only once. A variation of the PROM chip is that it is not burnt at the manufacturing time, but can be programmed using PROM programmer. It is also a non volatile memory

Erasable programmable read only memory (EPROM):
In EPROM information can be erased and reprogrammed using a special
PROM –programmer using an ultraviolet light.

Electrically Erasable programmable read only memory(EEPROM):
It is a recently developed type of memory. In EEPROM information can be erased by exposing it to an electrical charge. A flash memory is a type of EEPROM 
Random Access Memory (RAM):
RAM is the most common type of memory found in the modern computers. This is really the main store and is the place where the program gets stored temporary. When we switch OFF a computer, whatever is stored in RAM gets erased. It is a volatile form of memory.
The memory has a number of locations in it store. Each location in a memory has a unique number called its memory address. This serves to identify it for storage and retrieval. Operations on memories are called reads and write, a write instruction transfers information from other device to memory and a read instruction transfers information from the memory to other devices.
Types of RAM:
There are 2 types:
v Static RAM (SRAM): The word static refers to constant or the data remains stable until the power is ON. Static RAM is costlier and consumes more power. They do not need refreshing circuitry. They have higher speed than dynamic RAM.
v Dynamic RAM (DRAM): It is named so because it is very unstable. The data continues to move in and out of the memory as long as power is available. Therefore, dynamic ram have to be refreshed periodically, generally every 2 milliseconds. It is cheaper and they consume less power.
Difference between SRAM and DRAM:
                 SRAM    
           DRAM
1) It need not be refreshed
1)Need to be refreshed periodically
2) These are not as dense as DRAM
2)They are dense
3)It is fast
3)Little slow
4)Low latency
4)Longer latency
5)Access time is 25  nanoseconds
6)Best for internal memory
7) Expensive, consumes more power
8) Stores information as long as the power supply is on.
5)Access time is 60 nanoseconds
6)Best for system’s memory
7) Inexpensive, consumes less power
8) Dynamic RAM loses data in a very short time.

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