Memory

MEMORY

Memory is the part of the computer system that is used to store data or instruction temporarily and permanently. The memory can be assumed as the set of boxes and the boxes are numbered from zero upwards so that each box can be identified.

Once data is stored in the memory of memory box, it holds there till it is replaced by other data in the computer system. The computer cannot function without the memory. So, memory is the key factor of the computer to run smoothly and nicely and it is used to store data.

CPU (Central Processing Unit ) contains necessary data processing and controlling other components of the computer. It does not have any other place to store programs and data during data calculation and data processing. We also know that the CPU obtain several registers for storing data and instructions.

On the basis of technological point of view, memory can be classified as below:

1. Semiconductor memory
   It is the memory that is built by using transistor and IC. It is faster, lighter, has smaller storage capacity and consumes less power. It is used as the main memory of the computer. It has an access time of about 50ns. Examples: ROM, RAM, Cache memory, etc.
   
   
2. Magnetic memory
   It is the memory that consists of magnetic property for storing data. It stores data in the form of magnetized dots. It is slower for read / write and cheaper than semiconductor. It is used as the secondary or backup memory of the computer. Examples: Hard disk, Floppy disk, Tape drive, etc.
   
   
3. Optical memory
   It is the memory that uses optical property (light) to the read-write operation of the data. It is slower and cheaper than semiconductor memory and used for mass storage and backup. Example: CD, DVD, BD, etc.

The time to fetch and load data from the storage space into CPU must be very small to that from disk storage to reduce the speed mismatch problem with CPU speed. Every computer has a storage space called main memory or primary memory. It is temporary area built into the computer hardware.

Memory consist of IC (Integrated circuit), chips either on the motherboard or on a small circuit board attached to the motherboard of a computer system. Hence, the rate of data fetching from the main memory is faster than that from high-speed secondary storage like a disk.

Primary Memory

Primary memory is also known as system memory or main memory or primary storage or internal memory. Primary memory is the working area for the computer's processor.

It holds data for processing. It holds instructions for processing data. It holds data after it is processed waiting to be sent to an output or storage device.

Features of Primary memory

1. It is used for storing data and programs for temporary.
2. It is normally used for smaller storages.
3. It is expensive and faster for read / write operations than secondary memory.

Here are some of them that comes under the primary memory

• RAM (Random Access Memory)

RAM is the read/write memory of the computer. It is used for both purpose read and write. It possesses random access property that means CPU can access any memory location in a random sequence form.

It is a volatile memory. It stores data or information as long as power supply is on. But ,after power supply is off,data will be gone. There are two types of RAM.

Types of RAM

• Static RAM (SRAM)

SRAM stores data and programs as long as the computer is in ON state. It is expensive but faster for read/write than DRAM.it is typically used for CPU cache while DRAM is used for a computer's main memory.

Fig: Static RAM

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• Dynamic RAM (DRAM)

The data stored in DRAM will be lost after a few milliseconds even if the computer is on. It is cheap but slow for read/write than SRAM.

Fig: Dynamic RAM

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• Read Only Memory (ROM)

ROM stands for Read only memory. It can be used only to perform the read operations. It does not have write access. It is non-volatile memory i.e, the information stored in it is permanent and is not lost even if the power supply is cut down.

Fig:Read Only Memory

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It is used for permanent storage of programs. It is cheaper than RAM on per unit basis but while compared to storage ROM is expensive than RAM.

Types of ROM

• PROM (Programmable Read Only Memory)

It is a programmable memory that users can write data at once only. The user can store permanent programs, data or any kind of information in PROM.

Fig: Programmable Read only Memory

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• EEPROM ( Electrical erasable programmable read only memory)

It is a memory that can also be erasable via electric. It can be erased and reprogrammed on the byte by byte basis single data or the entire chip can be erased.

Fig: Electrical erasable programmable read only memory

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• EPROM ( Erasable Programmable Read Only Memory)

It is an erasable memory. Data stored on it can be erasable via passing the Ultra-Violet light so that entire data will be erased.

Fig: ErasableProgrammable Read Only Memory

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Secondary Memory

Secondary memory is non-volatile and has a lower cost per bit. It generally has an operating speed far slower than that of primary memory. Secondary memory is also called as "Auxiliary Memory". It is primarily used to store a large volume of data on the permanent basis that can be partially transferred to primary storage.

Secondary storage is the storage device that is designed to store data and instructions (programs) in a computer relatively in permanent form. It is non-volatile storage.ie. stored data and instructions can be retained even if the power supply is cut off. It is also known as backup memory.

Features of secondary memory

1. It is not directly accessible to CPU; it requires primary memory for its operation.
2. It is usually non-volatile memory.
3. It is usually cheaper and slower for read /write operation than primary memory.

Cache Memory

Cache memory pronounced as cash, is a special high speed and expensive memory used in CPU, so that CPU doesn`t have to wait for data to be delivered. The most frequently used instructions are kept in the cache memory.

Cache memory computer runs fast and smoothly. It doesn`t have to swap instruction in and out of RAM. Hence, the cache memory is placed in between CPU and RAM so that the speed of operation of main memory and cache memory together can meet speed requirements.

There are usually two levels of cache memory. They are:

• Level 1 (L1)

L1 is smaller and the faster. Multi-level caches generally operate by checking the L1

cache first If it hits the processor proceeds at high speed.

• Level 2 (L2)

L2 is larger and the slower. If the smaller cache misses the next larger cache L2 is checked and so on.

Buffer

It is a temporary holding area for data; unlike register it stores more than one piece of data. It is normally used between the CPU and the input/output devices. The purpose of a buffer is to hold data prior being used by CPU.

(Shrestha, Manandhar, and Roshan)

Bibliography

Shrestha, Prachanda Ram, et al. Computer Essentials. Kathmandu: Asmita's Publication, 2014

Gurung,Juddha Bahadur et.al.,computer science-XI,Bhundipuran Pakashan.