Memory

Memory

Memory–

• Memory is an essential part of a digital computer .
• It is a storage device that stores programs, data, information, results etc. 
• The data and instructions that are entered into the computer must be stored into the computer system before starting actual processing .
• Mainly memory are of two types:
  • primary memory
  • Secondary memory

1) Primary memory: Internal memory, main memory, temporary memory, Volatile memory.

2) Secondary memory: external memory, Auxiliary memory, Permanent memory, Non-volatile memory.

Units of memory: 

A computer stores and processes data or information in the form of binary numbers. The smallest unit of memory is a bit that can hold a single state either 0 or 1.

1 bit –→ 1 binary digit [0-1]

1Nibble –→ 4 bit

1Byte –→ 8 bit

1Kilobyte –→ 1024byte or 210 Byte

1Megabyte –→ 1024 KB or 220 Byte

1Gigabyte –→ 1024 MB or 230 Byte

1Terabyte –→ 1024 GB or 240 Byte

1Pentabyte –→ 1024 TB or 250 Byte

1Exabyte –→ 1024 PB or 260 Byte

1Zettabyte –→ 1024 EB or 270 Byte

1Yettabyte –→ 1024 ZB or 280 Byte

Concept of volatile and non volatile memory: 

Volatile memory:

Volatile memory is a type of computer memory that requires power to maintain the stored information. Once the power is turned off, all data is lost. 

Non-Volatile memory:

Non-volatile memory (NVM) is the type of computer memory that retains data even when the power is turned off. 

Note– In almost all computer systems the primary unit is volatile and the secondary unit is non -volatile Memory.

Primary memory: 

• Primary memory holds only those data and instructions on which computer is currently working. 
• It is generally made of semiconductors. 
• A computer can not run without primary memory. 
• Primary memories are of two types:

        1) RAM (Random access memory)

         2) ROM (Read only memory)

RAM(Random access memory):

• RAM is the read /write memory of a computer system.

• Information can be written into and read from a ram.

• It is a volatile memory that retains and stores information as it is supplied with power.

There are two types of RAM:

i. DRAM(Dynamic RAM)

ii. SRAM(Static RAM)

 DRAM: 

Dynamic RAM uses an internal circuit to refresh the storage every 2 milli second, It refreshes periodically.

Characteristics of Dynamic RAM:

• It has a very short data life.

• Need to be refreshed continuously.

• Slower as compared to SRAM.

• Large in size and Less expensive.

• Less power consumption.

SRAM:

A static RAM does not need any special regenerator circuit to retain the stored data.

The word static indicates that the memory retains its content as long as the power is being on.

Characteristics of static RAM:

• It has long life data.

• It is faster than DRAM.

• It needs not to refresh.

• Smaller in size and expensive.

• High power consumption.

ROM(Read only memory):

• ROM stands for read only memory.

• It is non-volatile memory which stores permanent programs that can not be altered.

• It stored a bulk of program where function is to start the computer, when power is turn on.

• Its contents are not lost when the power supply is switched off.

PROM :

• It stands for Programmable Read Only Memory.
• Its contents are designed by manufacturers. 
• It stores permanent program function data and instructions.
• once a data or program written in PROM, It can’t be erased.

2)EPROM(Erasable programmable ROM):

1. UVEPROM(Ultra Violet Erasable programmable ROM): 

In this ROM programs are erased by using ultra violet rays and then reprogrammed.

The chip has to be removed from the computer erased using an UV rays emitting device.

2. EEPROM( Electrically erasable PROM):
   1.  In this ROM the data can be erased by short circuit.
   2. It is easy to use because there is no need to remove the ROM from the motherboard.

Note: Booting: The process of starting up a computer system is called booting. 

1)Cold booting: when a computer starts for the first time it is called cold booting.

2)Warm booting: When the system restarts (shutting down and starting again or rebooting) is known as warm booting.

Secondary memory: 

• Secondary memory is also known as external memory. 
• This part of computer memory is non-volatile and has low cast per bit storage. 
• It is used to store large amount of data on permanent basis which can be partially transferred to primary memory and when required for processing.

Types of secondary memory:

1) Sequential access memory:

Sequential Access Memory (SAM) is a type of memory or storage where data is accessed in a fixed, sequential order

magnetic tape: 

• It is one of the oldest forms of secondary storage devices .
• It is used as a backup storage. 
• The magnetic tape is a plastic ribbon that is coated on one side with a magnetic material such as iron oxide.
• Range: 1/4 inch 1/2 inch 50-240 feet long

2) Direct access memory: 

• Direct Access Memory (DAM) refers to a type of memory or storage where data can be retrieved directly from any location, without the need to read through data sequentially. 
• This allows for faster data retrieval compared to sequential access memory.
• It has two types:

1) Magnetic disk:

• A magnetic disk is a type of storage device that uses magnetic media to read and write data.
• Magnetic disks are commonly used for storing large amounts of data and provide direct access to stored information.
• It has two types:

1. Floppy Disks:

• A floppy disk is a round flat piece of flexible plastic coated with magnetic material .
• It is encased in a square plastic jacket that gives holding protection to the disk surface .
• Direct access but very slow compared to modern storage.
• Floppy disk are available in two size:

2. Hard disk: 
   1. A hard disk is a magnetic disk made up of metal and covered with a magnetic recording surface.
   2. It is the most popular medium for direct data access to secondary memory.
   3. Unlike floppy disks, hard disks are made of rigid metal (aluminium). 
   4. A single hard disk usually consists of several platters. 
   5. It is used to store huge amounts of data on a permanent basis. 
   6. There are two types of hard disk:

1) Internal hard disk

2) External hard disk

Components of hard disk:

1)Platter: the circular disk on which magnetic data is stored in a  hard drive is called a platter.

There can be several platters within the hard drive.

These platters are stacked and spin at high speeds.
2)Tracks: The surface of a disk is divided into a number of circles that is called track.

3)Sector : Each track is further subdivided into sectors.

4)Head: Each side of the platter is used to read/write data on disk.

5)Cluster: It is a group of sectors within a disk.

Access time: The data access time for a disk depends on three parameters:

1) Seek time : The time required to position the read/write head over the desired track is called seek time.

2)latency: The time required to spin (move) the desired sector under the head is called latency.

3)data transfer rate: The data transfer rate refers to the rate at which data is read from the disk.

2) Optical disk: 

• The optical disk is a round disk made of Poly carbonate plastic
•  on the surface of the optical disk is made of shiny with a thin layer of Aluminum to reflect light. 
• An optical disk is a type of data storage medium that uses laser light to read and write data hence it is called a laser disk.
• Optical disks are commonly used for storing large amounts of data, such as for music, movies, software, and backups.
• type s of optical dick:
  • Compact disk 
  • Digital video disk 

1. Compact disk:

• It stands for compact disk.
• CD’s  are circular disks that use optical rays, usually lasers, to read / write data.
• A CD is a  round disk of polycarbonate plastic coated with a very thin reflective layer of Aluminium.
• They are very cheap.
• CD can explore up to 700mb of data.
• CD’s are inserted in a CD drive built into the CPU cabinet. 
• They are coatable as you can eject the drive, remove the CD and carry it with you.
• Types of CD:
  • CD-ROM
  • CD-R
  • CD-RW

1. CD-ROM:

• CD-ROM stands for compact disk read only memory.
• The dala in the CD-Rom is stored by the manufacturer in the factory itself which can not be changed later.
• data can be read repeatedly from CD-Rom but new data can't be Stored (write)

2. CD-R:

• CD-R stands for compact disk Recodable.
• It is a type of optical disk like a normal compact disk in which data can be stored by a computer with the help of a CD-R drive.
• This is called a worm (write once, read many) Disk repeatedly.
  Data can't be changed once written.

3. CD-RW:

• CD-RW stands for compact disk Rewritable.
• Data can be written and deleted on these optical disks again and again.

2. Digital video disk: 

• It stands for digital video disk or Digital versatile disk.
• It looks just like a CD and uses technology as that of CD’S
• DVD Can Store 15 times the date held by CD's.  
• A DVD holds 4.7 Mb to 17 GB of data.
• DVDs achieve high storage capacity by using both sides of the disk, by using data comparison technology.

Mass storage device–

Mass storage device is referred to as the storage  device which can hold more data than normal storage device. 

USB Thumb Drive (Flash Drive)–

USB thumb drive normally known as flash drive or pen drive is used to store bulk data as per capacity. They are small in size with the USB face being portable to be used with any computer. 

Storage size: Small to medium (8 GB – 1 TB)
Speed: Faster than HDD, slower than SSD
Use today: File transfer, backups, portability
Solid State Drive (SSD)-- 

High-speed flash memory storage
Storage size: Medium to very large (128 GB - several TB)
Speed: Very fast (much faster than HDD)

Use today: Modern laptops, desktops, gaming, professional work.