Virtually unlimited capacity because the cost per bit of storage is very low.Has larger capacity than main memory.Store large volumes of data: Used to stores system software, assembles, compiler, useful packages, large-data files etc.Used to store information on permanent manner which is not possible in main memory.Due to Non-volatile in nature, data cannot be lost when power goes off.
It is permanent type memory. It is non-volatile. It is used as secondary and backup memory.Digital data 1 or 0 is stored on the magnetic coating surface applying electric pulses of suitable polarity to the magnetizing coil of a write head.In the longitudinal recording the magnetic regions are oriented parallel to the disk surface along the track. In the vertical recording the magnetized regions are vertical to the disk surface. The vertical recording stores many times more data as compared to longitudinal recording.In low-capacity disks and older hard disks a single write/read head is employed. In other words the same head acts as a write head during write operations, and as a read head during read operation.
Concentric Rings or tracks Each track has the width same as of disk head.Tracks separated by gaps, minimizes errors in case of interference or head misalignment.tracks divided into sectors- pie shaped segments, data transferred to and from the disk sectors.Minimum block size of one sector; 512 bytes in most systems.
Constant angular velocity: All tracks has same number of sectors. Inner sectors decides the storage capacity for all other sectors on the disk. Due to this reason some storage space remains unused on the outer tracks.Multiple zones recording: To utilize unused space, the disk surface is divided into zones. In a zone the number of sectors in each track is same, but is different than the number of sectors in a track of the outer zone. In outer zones the number of sectors per track is more than in inner zones.
The term on-line means that the device is permanently connected to the computer system. When computer is on, the device is available to store information or to give information.Size 3.5 inch and 2.5 inch in diameter.
Actuator has head arm assembly with read/write heads which moves inward or outward to select track on a particular platter surface. Wait for particular sector reaching under it.Access Time = Seek Time + Latency Time.Seek Time: Time taken by head to reach the desired track.Latency Time: Time in which desired sector placed under the head.
Zip disks used a single hard disk platter encased in a plastic cartridge. Disk drives may be portable or fixed type. Fixed type is part of the computer system, permanently connected to it. Portable type can be carried to a computer system, connected to it for the duration of use, and then can be disconnected and taken away when the work is done.Disk Packs uses multiple hard disk platters mounted on a single central shaft. Disk drives have a separate read/write head for each usable disk surface (the upper surface of the top-most disk and the lower surface of the bottom most disk is not used). Disks are of removable/interchangeable type in the sense that they have to be mounted on the disk drive before they can be used, and can be removed and kept off-line when not in use.Winchester Disks uses multiple (two or more) hard disk platters mounted on a single central shaft. Hard disk platters and the disk drive are sealed together in a contamination-free container and cannot be separated from each other. In this all the surfaces of all disks are used for data storage so, the Winchester disks have larger storage capacity than disk packs.
5 1/4 inch diskette, whose diameter is 5 1/4 inch. It is encased in a square, flexible vinyl jacket.3 1/2 inch diskette, whose diameter is 3 1/2 inch. It is encased in a square, hard plastic jacket. Storing capacity is 1.44 MB. It uses 80 tracks per surface, 135 Track Per Inch (TPI), 18 sectors per track, 360 rpm and stores 512 bytes per sector. Both sides of the disk store information.
Magnetic tapes are used for backup memory. They are sequential access device whereas a disk drive is a direct access device. In disk drive system the head moves to the position of the desired record. But in the case of tape drive system, the head moves sequentially.Comes in real form and in cartridge form or cassette form.
Data recording density is the amount of data that can be stored on a given length of tape. It is measured in byte per inch (bpi).
A half-inch magnetic tape is divided into horizontal rows called tracks or channel and vertical columns called frames. For parallel recording a 9-track tape drive system is provided with 9 read/write heads. 9 bit are written/read in a vertical column simultaneously. 8-bits (1 byte) are for the binary code of a character and the 9th bit is the parity bit for error checking.
Optical disk is consists of a circular disk, which is coated with a thin metal or some other material that is highly reflective (polycarbonate). Laser beam is used to read/write data from/to highly reflective surface.
Has one long spiral track, which starts at the outer edge and spirals inward to the centre.Tracks is divided into equal size sectors.
To write 1, the laser beam is turned on, which burns a pit upto the reflecting layer. The laser beam is focused from polycarbonate surface side. The laser beam source and the photo detector are positioned below the polycarbonate plastic. The emitted laser beam travels through the polycarbonate plastic, reflected back from the aluminium layer and comes back towards photo detector. To write 0, the laser beam is not turned on and hence, no pit is burnt. The surface where there is no pit is called land.
CD size is 600 MB to 700 MB.DVD size is 4.7 GB to 17 GB
• What is auxiliary memory?
• Magnetic memory
– Magnetic Disks: Hard Disks and Floppy Disks
– Magnetic Tapes
• Optical disks
CD (Compact Disk) or CD-ROM
CD-R (CD Recordable)
DVD (Digital Versatile Disk)-ROM
• Used to overcome the limitations of primary
• Unlimited capacity because the cost per bit of
storage is very low.
• Larger capacity than main memory.
• Used to store large volumes of data on a
• It is Non-volatile in nature.
• Also known as Secondary Memory.
• Permanent type memory.
• Data can store on the
magnetic coated surface
• Two techniques of data
sensor is used to read
• Older disks has single
• Read/Write data from/to its surface on platters.
• Surface divided into tracks and sectors.
• Supports direct access mechanism.
• Has a disk controller.
• Has read/write head.
Tracks and Sectors
Constant angular velocity
Multiple zones recording
Functions of Disk Controller
• To interface a disk system to the CPU.
• Disk drive selection, in case of multiple disks.
• Track and sector selection.
• To issue commands to the disk drive system to
perform read/write operations.
• Error detection.
Types of Magnetic Disks
• Hard Disks
• Floppy Disks
• On-line storage devices.
• Used to store bulk (mass) amount of information.
• Made up of aluminium with a thin coating of
magnetic material (iron oxide).
• Size 3.5 inch used in desktop computers.
• Size 2.5 inch used in portable computers.
Storage Capacity of Hard
Storage capacity = Number of recording surfaces X Number of
tracks per surface X Number of sectors per track X Number of
bytes per sector
Access Mechanism of Hard
Types of Hard Disks
• Made up of plastic material (Mylar) coated with
magnetic material (iron oxide or barium ferrite).
• Not a hard plate, very thin piece of flexible plastic.
• Also called diskette.
• It is removable type device and used as backup
Popular Floppy Disks
– Encased in square,
flexible vinyl jacket.
– 1.2 MB size.
• 31/2 inch Diskette
– Encased in square,
hard plastic jacket.
– 1.44 MB size.
FDD (Floppy Disk Drive)
• Used as backup memory.
• Sequential access device.
• Made up of flexible polyester coated with
magnetizable material (iron oxide or chromium
• Size varies from 3 mm to 12.7 mm.
• Available in real, cartridge and cassette form.
Storage capacity of Magnetic
Storage capacity = Data recording density X Length.
Physical organization of data
on Magnetic Tapes
• Tracks are parallel to tape length.
• Horizontal rows called tracks or channel.
• Vertical columns called frames.
• Used for backup memory.
• Circular disk, coated with a thin metal or some other
material that is highly reflective.
• Read/Write operation can perform using laser
• Has very high storing capacity, compares to
magnetic floppy disks.
• Direct access device.
Storage Organization of
• Has one long spiral track.
• Track divided into equal length sectors.
Track on disk
Sectors on disk
Storage Capacity of Optical
Storage capacity = Number of sectors X Number of
bytes per sector.