Primary Memory: Need, Classification & Nature

Primary memory is the segment of the computer memory that can be used to store current data, programmes, and instructions. Primary memory is often referred to as main memory or RAM (Random Access Memory). It plays a crucial role in computer systems, providing fast and temporary storage for data and programs actively being used by the CPU.

• It is the main memory of the computer and is an integral part of the Motherboard. 
• It basically houses the primary data, which can be easily accessed and stored by the Primary memory.
• Understanding primary memory is crucial for GATE exam preparation, as it forms the foundation of computer architecture and system design.

Key Terms: Computer System, Memory Hierarchy, Primary Memory, Secondary Memory, Cache Memory, Primary data, Processor, RAM, ROM, 

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Primary Memory- Definition 

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Primary memory is the in-built volatile storage mechanism of a Computer system.

The Primary memory is that part of the computer memory which is directly accessed by the data processor with the help of Data bus.

• It essentially stores the running programs and currently processed data (that are stored in a specific memory location). 
• In the hierarchy of memory when compared to cache memory it has greater storage capacity and access time. 
• The Primary memory has lesser access time and storage capacity than Secondary memory. 
• It is therefore faster than Secondary memory. 

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The Need for Primary Memory

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For the sake of good efficiency of the system, the memory of a computer is organised in a manner which reduces the access time to a minimum value. It works especially to reduce the access time for an immediately required process. This is brought about through the following method:

1. All the files, data and the programs are usually stored in the Secondary Memory. The Secondary Memory has a greater access time.
2. The CPU or the processor of the system has no direct access to this Secondary Memory.
3. As a solution, the operating system loads the immediately required process into the Primary Memory.
4. Now, this process can be easily accessed by the CPU or Processor. 
5. One thing to note is that the operating system stores only those processes which are ready to run on the system. 
6. This optimises the overall performance of the computer along with improving its efficiency. 
7. The organisation of Computer System Memory in the above mentioned manner is known as the Memory Hierarchy. 

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Classifications of Primary memory

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The Primary Memory can be classified into two broad categories:-

RAMs: Random Access Memory

Random Access Memory is named so because it can randomly store and access any data and from any location in the system. Here are the features of RAM:

• It is a sort of temporal storage memory.
• It is the fastest memory.
• It is volatile- it retains the data as long as the computer is powered.

The RAM works to load any immediately required process which is then interpreted and accessed by the CPU as per the Instructions in the Program. For instance, if one wants to open 'My Computer', the operating system will first load the code for 'My Computer' in the RAM. The CPU can access this code to open the concerned Application.

On the basis of behaviour RAMs are of two types:-

SRAM (Static RAM)

The SRAM holds the information as long as the power supply to the system is on.

• It stores information in the form of sequential Circuits, similar to that of flip-flops, which does not require to be refreshed from time to time.
• SRAM is expensive and hence used only in situations where speed is the biggest need.

DRAM (Dynamic RAM)

DRAM is made up of a series of capacitors and transistors that store memory for a very short duration (few milliseconds), even if the power supply is constant. 

• DRAM stores information in the capacitors and transistors in the form of electric charges that tend to leak out of the capacitors (every few milliseconds). 
• Hence, it needs to be periodically refreshed to retain data.
• As compared to SRAM, DRAM is cheaper and can be conveniently used in Computers and Servers.

ROMs: Read- Only Memory

ROM primarily stores all the data and information that does not require any modifications, it is because the data once stored in ROMs cannot be altered. 

• It is for this reason that this type of memory is known as 'Read Only' memory, as the stored Data can only be read and not modified. 
• Most commonly, ROM stores the data that is loaded at the time of booting (also known as the bootstrap program that starts the Operating system)

The different types of Read- Only Memory are:

1. MROM: MROM stands for Masked ROM. This type of ROM is pre- programmed and hardwire, making it impossible to make any changes in its data.
2. PROM: PROM is the Programmable ROM. It can only be programmed once. A consumer buys a blank PROM, sets in it a desired content which cannot be altered afterwards.
3. EPROM: EPROM is for Erasable and Programmable ROM. As the name suggests, its contents can be erased, modified and reprogrammed any number of times. 

• One can easily erase existing content of an EPROM by exposing it to UV radiation which dissipates the charge on ROM
• After which a new content can be set up easily.

4. EEPROM: EEPROM, it's full form is Electrically Erasable and Programmable ROM is similar to EPROM in the sense that its content can be changed as per the user's desire and for any number of times. 

• The only difference is that its contents can be erased electrically. 
• In EEPROM, the content is usually deleted one byte at a time.
• Hence, reprogramming in this ROM takes a lot of time.

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Volatile Nature of Primary Memory

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Primary Memory can be either volatile or non- volatile. The Volatility of information and data stored in the Primary memory largely depends upon the type of Primary Memory utilised for such situation.

• The data stored in RAM is volatile as the contents are lost as soon the power supply is stopped.
• The data stored in RAM is non- volatile, it retains the data which persists even after switching off the system.
• Thus, Primary Memory can be both volatile or non- volatile depending upon the use of ROM or RAM.

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The Development of Cache Memory

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Data can be accessed in the Primary memory faster than in the case of Secondary memory, owing to the faster access time of Primary Memory. 

• The standing problem here was that the access time of Primary Memory is in milliseconds while a CPU can perform operations within a few nanoseconds.
• Only using Primary memory in the Computer System caused a time lag between the reading of the process and acting upon it. 
• Moreover, the CPU cannot be utilized to its fullest potential, as it stays idle while the data is being read. 
• This led to the development of a new segment of memory known as the Cache Memory. 

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Things to Remember 

• The memory Hierarchy is the organization of the different segments of the memory in a Computer Operating system 
• The Primary Memory is the main memory of a computer that helps in processing and opening files, data and programs. 
• Primary Memory has a lesser access time as compared to Secondary Memory, but it has limited storage capacity.
• The Primary memory is of two types- RAM (Random Access Memory)and ROM (Read- Only Memory).
• There are two types of RAM on the basis of behaviour- SRAM (Static RAM) and DRAM (Dynamic RAM)
• Some of the types of ROM are- MROM, PROM, EPROM and EEPROM.
• RAM is volatile while ROM is non-volatile.
• The Cache memory helps in the complete use of the CPU, thus minimising the time lag that comes from using Primary Memory.