DIRECT ACCESS STORAGE DEVICES

what are direct storage devices?• are devices that can directly read or write to a specific place. DASDs

can be grouped into three categories:magnetic disks, optical storage devicesflash memory.

Magnetic Disks

Fixed-Head Magnetic Disk Storage

• Looks like a large CD or DVD Covered with magnetic film Formatted Both sides(usually) in concentric circles called tracks

Data recorded serially on each track Fixed read/write head positioned over data

Advantages Fast(more so than movable head) Disadvantages High cost and reduced storage

A fixed-head disk withfour read/write heads, oneper track

Movable-Head Magnetic Disk Storage• Have one read/write head that floats over each surface of disk. -Example: computer hard drive• Disks can be a single platter, or part of a disk pack: -A stack of magnetic platters.• Disk Pack Platter -Each Platter has two recording surfaces .Exception: top and bottom platters -Each surface is formatted with a specific number concentric tracks where the data is recorded. -The number of tracks varies from manufacture to manufacture .Typically there are 1000+ on a high-capacity hard disk.

Movable-Head Magnetic Disk Storage(cont’d.)

• Disk pack platter(cont’d.) -Each track on each surface is numbered .Track zero: identifies the outermost concentric circle on each surface. .The highest-numbered tracks in the center. -Arm moves two read/write heads between each pair of surfaces. .One for the surface above it and one for the surface below. .The arm moves all of the heads in unison. They’re all positioned on the same track but on their respective surfaces creating a virtual cylinder.

A disk pack is a stack ofmagnetic platters. Theread/write heads movebetween each pair ofsurfaces, and all of theheads are moved inunison by the arm.

Movable-Head Magnetic Disk Storage(cont’d.)

• Disk pack platter (cont’d.) - It’s slower to fill a disk pack surface-by-surface than it is to fill it up track-by-track. - If we fill Track 0 of all the surfaces, we’ve got a virtual cylinder of data. - There are as many cylinders as there are tracks, and the cylinders are as tall as the disk pack.

Movable-Head Magnetic Disk Storage(cont’d.)

• Disk pack platter (cont’d.) -To access any given record, the system needs three things: .Its cylinder number, so the arm can move the read/write heads to it; .Its surface number, so the proper read/write head is activated. . Its sector number, so the read/read head knows the instant when it should begin reading and writing .

Sector stores up to 512 bytes of data

Track is narrow recording band that forms full circle on disk

Formatting prepares disk for use and marks bad sectors as unusable

How does a hard disk work?

Step 1. Circuit board controls movement of head actuator and a small motor.

Step 2. Small motor spins platters while computer is running

Step 3. When software requests a disk access, read/write heads determine current or new location of data.

Step 4. Head actuator positions read/write head arms over correct location on platters to read or write data.

OPTICAL STORAGE DEVICES

• A plastic-coated disk on which data, such as text, music, or video, is digitally stored as microscopic regions of varying reflectivity and is read by scanning the surface with a laser. Also called laser disc.

• an optical disc consists of a single spiraling track of same-sized sectors running from the center to the rim of the disk.

• Atrack has sectors, but all sectors are the same size regardless of their locations on the disc. This design allows many more sectors, and much more data, to fit on an optical disc compared to a magnetic disk of the same size.

cont

TYPES OF OPTICAL-DISC SYSTEMS

•CD Technology • CD-ROM;CD Read Only Memory. When bought can only be read but you can’t save anything on it.• CD-R(WORM);CD Write once Read many. You are able to save to this disk one time, so you can store an application or data on it ,after savng once ,you can read it many times but can’t save on it again.• CD-RW;CD Read Write. One can save data over and over again.

Advantages of CDSmall and portableVery cheap to produceMost computers can read CDs Fairly fast to access the data-quicker than a floppy disk or magnetic tape

Disadvantages

Fairly fragile, easy to scratchSmaller storage capacity than a hard drive or DVDSlower to access than the hard disk

DVD TECHONOLOGY• Although DVDs use the same design and are the same size and shape

as CDs, they can store much more data. A dual-layer, single-sided DVD can hold the equivalent of 13 CDs; its red laser, with a shorter wavelength than the CD’s red laser, makes smaller pits and allows the spiral to be wound tighter.

Blu-ray Disc Technology• A Blu-ray disc is the same physical size as a DVD or CD but the laser

technology used to read and write data is quite different.

Blue ray disc• The pits on a Blu-ray disc are much smaller and the

tracks are wound much tighter than they are on a DVD or CD.

• The disc’s format was created to further the commercial prospects for high-definition video, and to store large amounts of data, particularly for games and interactive applications via the Java programming language

STORAGE DEVICES 20

FLASH MEMORY

•Flash memory is a type of nonvolatile memory that erases data in units called blocks. A block stored on a flash memory chip must be erased before data can be written, or programmed, to the microchip. Flash memory retains data for an extended period of time whether a flash-equipped device is powered on or off

How flash memory works

A basic flash memory cell consists of a storage transistor with a control gate and a floating gate, which is insulated from the rest of the transistor by a thin dielectric material or oxide layer. The floating gate stores the electrical charge and controls the flow of the electrical current.

Flash memory evolved from erasable programmable read-only memory (EPROM) and electrically erasable programmable read-only memory (EEPROM). Devices using flash memory erase data at the block level and rewrite data at the byte level (NOR flash) or multiple-byte page level (NAND flash). Flash memory is widely used for storage and data transfer in consumer devices, enterprise systems and industrial applications.

A process called Fowler-Nordheim tunneling removes electrons from the floating gate. Either Fowler-Nordheim tunneling or a phenomenon known as channel hot-electron injection traps the electrons in the floating gate.

When erasing through Fowler-Nordheim tunneling, a strong negative charge on the control gate forces electrons off the floating gate and into the channel, where a strong positive charge exists. The reverse happens when using Fowler-Nordheim tunneling to trap electrons in the floating gate. Electrons are able to forge through the thin oxide layer to the floating gate in the presence of a high electric field, with a strong negative charge on the cell’s source and the drain and a strong positive charge on the control gate.

FLASH MEMORY

ADVANTAGES

• Faster read and write compared to traditional hard disk drives.

• Smaller size.• Less prone to damage.• Cheaper than traditional drives in

small storage capacities.• Uses less power than traditional

hard disk drives.

DISADVANTAGES

• Flash memory cells have a limited number of write and erase cycles before failing.

• Most flash drives do not have have a write-protection mechanism

• Smaller size devices, such as flash drives make them easier to lose

• Currently costs a lot more per gigabyte than traditional hard drives for large storage capacities.

• May require a special version of a program to run on a flash-based drive to protect from prematurely wearing out the drive.

References

• Understanding Operating Systems 6th Edition by Ann McHoes, Ida M. Flynn 

• Google Images