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What is Computer Memory? | कंप्यूटर मेमोरी क्या हैं ?

computer memory units

 

What is Computer Memory?

What is the computer memory? Computer Memory is essential part of a computer. Without memory, computer cannot work. Memory is used for storing both instruction and data. Memories that can be read from and write into are called read-write memories. Some memories have programmed or data permanently memories have program or data permanently store and are called read only memories. Programs are data should be i main memory for execution and reference. Programs or data not needed immediately may be kept in off line secondary storage. This can be brought into main memory, for execution or reference when needed. Memory can be measure by the unit term as BIT. BIT is the acronym for BINARY DIGIT. It is represented by 1 or 0 (1 is referred as High Bit and 0 is referred as Low Bit). It corresponds to the hardware functioning of the presence or absence of a pulse. The presence of a pulse is represented by 1 and absence by 0. 8 Bits make a byte and 1024 bytes make 1 KB (Kilobyte) and so on. Three main criteria’s are relevant for a storage Unit. Three main criteria are relevant for a storage Unit. These are;

Access Time:

Stored data is located and retrieved from the storage unit under the direction of a program instruction. The time required to do it is called access time. A faster access means quicker process.

Storage Capacity

This is measured by the amount of data that the storage unit can contain. A large capacity gives better performance.

Cost per unit storage

Unit of storage is bit (Binary Digit). A lower value of cost per bit of storage is preferred.

INDEX

1. What is Main memory or Primary Memory?

1.1. RAM (Random Access Memory)

1.1.1. DRAM (Dynamic RAM

1.1.2. SRAM (Static RAM)

1.2. ROM (Read only memory

1.2.1. PROM (Programmable Read Only Memory)

1.2.2. EPROM (Erasable Programmable Read Only Memory)

1.2.3. EEPROM (Electrically Erasable Programmable Read Only Memory)

2. What is Secondary Memory?

2.1. Magnetic Tapes

2.2. Magnetic Disk

2.2.1. Winchester Disk

2.2.2. Floppy Disk

2.2.3. Hard Disk

2.2.4. Optical Disk

2.2.5. CD-ROM (Compact Disk Read Only Memory)

2.2.6. WORM (Write Once Read Many)

2.2.7. EOD (Erasable Optical Disk)

2.2.8. Some Others Type Memory

2.2.8.1. Magnetic Drum

2.2.8.2. Magnetic Bubble Memory

2.2.8.3. Cache Memory.

 

1. What is Main memory or Primary Memory?

In computer, memory can be classified primarily into two types. They are Main memory or Primary memory and Secondary memory. Primary memory or Main memories are basically two types like;

  • RAM (Random Access Memory)

  • ROM (Read Only Memory)

 1.1. RAM (Random Access Memory) or What is computer ram

The Random Access Memory (RAM), the memory cell can be accessed for information. Any location of primary storage can be accessed at random to directly retrieve or store data and instructions. So it is called Random Access Memory (RAM). Information stored in RAM is lost as soon as the power is switched off; this is main drawback of RAM memory. It is therefore called a VOLATILE MEMORY. Two types of RAM chips are found, they are;

1.1.1. DRAM (Dynamic RAM

DRAM (Dynamic RAM) are volatile in nature because capacity have tendency to leak off and a provision is made periodically to regenerate or refresh the storage charge ,as the stored data are lost in event of power failure.

1.1.2. SRAM (Static RAM)

SRAM (Static RAM) is volatile in nature, yet as long as that are supplied with power they need no special regenerator circuit to retain the stored data. These SRAMs are used in specialized storage system due to its complication and occupying more space than DRAM.4

1.2. ROM (Read only memory

A read only memory (ROM) performs the read operation only, it cannot be written into. The binary information stored in a ROM is made preeminent during the hardware production of the unit. It cannot be altered by addition or deletion. It is called NON-VOLATILE MEMORY. A ROM is restricted to reading worlds that are permanently stored within the unit. ROMs come with special internal electronic fuse that can be "programmed" for a special configure. There are various types of ROMs as follows;

1.2.1. PROM (Programmable Read Only Memory)

PROM is a ROM that can be programmed to record information using a facility known as PROM - Programmer. Once the chip has been programmed, the recorded cannot be changed, i.e. PROM becomes same ROM.

1.2.2. EPROM (Erasable Programmable Read Only Memory)

EPROM is a type of ROM that can be erased and chip can be reprogrammed to record different information using a special PROM program facility. The ROM can be erasing by EXPOSING the chip to ultraviolet light. When an EPROM is a use, information can only be "read" until it is eased

1.2.3. EEPROM (Electrically Erasable Programmable Read Only Memory)

EEPROM is a type of ROM that cannot program and eased by electrical signals.  It provides an easy means to load and store temporary or permanent information in a form of ROM memory.

2. What is Secondary Memory or सेकेंडरी मेमोरी क्या हैं?

As the storage capacity of primary storage is limited, to store large amounts of data permanently secondary storage devices are required. This is also termed as backup storage. Data are stored in secondary storage, in the same binary codes as in primary storage and are transferred to primary storage as needed. While a wide range of secondary storage devices is available, typical devices are magnetic tape and Magnetic Disk.

2.1. Magnetic Tapes

Magnetic Tape Drive is a machine that can either read data from a tape into the CPU or it can write the information produced by the computer on to a tape. It is a popular sequential device that can store large volume of data. The tape is plastic ribbon usually 1/2 inch wide that is coated on one side with an iron-oxide material which can be magnetized. The tape ribbon in reels of 20 to 2400-ft. size or small cartridge or cassette. It can be erased and reused. Old data on a tape are automatically erased as new data are recorded thereon.

2.2. Magnetic Disk

Magnetic disks storage provides large storage capabilities but speed is moderate. Magnetic disks store information on one or more circular platters/plates or disks. A disk pack generally contained six platters, each platter/plate having two sides. These disks which are coated with a magnetic material, are continuously spinning. Information is recorded on the rotating disks by magnetic heads on both the surfaces of the bottom plate. Each disk consists of a number of invisible concentric circles called tracks. A set of corresponding tracks in the entire surface is called a cylinder. Each track is further subdivided into sectors. Information is recorded on the track of a disk surface in the form of invisible tiny magnetic stops. The presence of magnetized sport represents a 1 bit and its absence represents 0 bit. Data are recorded on the tracks of a spinning disk surface and read from the surface, by one or more read/write heads. These are two basic types of disk system like; Moving Head System & Fixed Head System.

Magnetic disks come in different sizes. They can be portable or permanently mounted in their storage device. Magnetic Disks are as follows;

2.2.1 Winchester Disk

This disk system is housed in permanent sealed container. The disks are coated with a special lubricant, have increased number of tracks and higher storage density per track. Read/write heads and access mechanism are also sealed along with the disks. These are fast, reliable and used for minicomputer and microcomputers. Typical storage capacities are 10,20,40,80 MB

2.2.2. Floppy Disk

Floppy disks are direct access secondary storage medium for micro and mini computer system introduced by IBM in 1972. It is made of flexible plastic coated with magnetic oxide.

Enclosed in a square plastic jacket, it has a special liner that wipes out dust particles from that disk surface. Information is written to or read from that disk surface in the jacket. The read/write heads make direct contact with disk surface during read/write process. Typical size of floppy disks is 5.25, 3 ½ inches in diameter. They can be single sided or double sided and single density or double density. Capacity of floppy disks are depended on their size and recording mode and may vary from 100 KB to 2MB. These disks are very cheap and provide inexpensive storage for small systems.

2.2.3. Hard Disk

It comprise of a metal disk coated with ferrous oxide. It is fitted inside the computer but still considered to be an external storage device. In a hard disk system, the disk is sealed along with the read/write heads. Hard disk can store much more data than the conventional floppy and data access to a form is much faster. Unlike floppies, which can be taken in and out of their disk drives? Hard disks are usually permanently installed in the machine, which is called fixed disks. They have the capacity that range from 10 MB TO 40 GB and higher.

2.2.4. Optical Disk

Optical storage involves the uses of higher power laser to burn microscopic spots in an aluminium disk's surface coating. The 1 or 0 of holes in the location represent data. A much lower power laser beam is used to retrieve the data. The patterns of spots detected by the laser (during read operation) are converted into electrical signals used by the computer. The cost of optical media is very low. Their access time and data transfer rates are fast or better. The data recorded on optical media is not damaged by stray magnetic fields are surface problems like finger prints, dust, scratches and so on. Optical disks can also be used to back up a fixed hard disk.

2.2.5. CD-ROM (Compact Disk Read Only Memory)

CD-ROM (Compact Disk Read Only Memory) are typically single sides with a diameter of 12 centimetres, made of a polycarbonate plastic 1.2 millimetres thick. The spiral tracks are packed to written 1.6 million of a meter of one another and in all are about three miles long. CD-ROM plays back like an audio disk in the mode called Constant Liner Velocity (CLR). Compared to a standard fixed magnetic disk driven such as a Winchester or similar, CD-ROM search times are of the order of ten times longer. CD-ROM can contain audio information, but typically stores textual data that can be accessed and interpreted by a computer. A single disk can hold about 600 megabytes the equivalent of about a quarter of a million printed pages of text. It is helpful to understand what a laser can do. Laser is a kind of light "gun". It is load by being pumped up with electrical and magnetic energy. When the trigger is pulled, the laser fires a pencil-thin beam of very pure light. The user can only read the data, user cannot erase it. Change it or write on the disk.

2.2.6. WORM (Write Once Read Many)

Write Once Read Many these disks are also imprinted by the manufactures, but they can write on the disks. Once the disks have been written on, change cannot be made on them. They can only be read form.

2.2.7. EOD (Erasable Optical Disk)

Erasable Optical Disk these disks can be changed and erased. The removable cartridges provide convenience and security along with huge storage capacity.

2.2.8. Some Others Type Memory

Some others type memories are like; Magnetic Drum, Magnetic Bubble memory & Catch Memory.

2.2.8.1. Magnetic Drum

Magnetic Drum used in early days, for storing mathematical tables, programs etc., these are permanently mounted and cannot be removed. Basically a cylinder, whose outer surface is coated with magnetic material, it rotates on its axis. Tracks exist the surface of the drums. Data are stored as magnetized spots on these tracks. These are read/written by a set of stationary read/write head. These drums are mostly not used now.

2.2.8.2. Magnetic Bubble Memory

Magnetic bubble memories, which are non-volatile, can use as a low cost secondary storage. Bubble chips are used on portable terminals, telephone systems, machine tools, robots and military applications.

2.2.8.3. Cache Memory.

CPU speeds are very high compare to the time required accessing main memory. Consequently the speed of processing is lowered. To remedy this, an extremely fast, small memory, whose access time is close to the processing speed of the CPU is inserted between the CPU and the main memory. This type of memory is called a high-speed buffer or cache memory.

 

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