# Presentation: 532495

- Professional
- Training

## Welcome 

- Introduction

## Policy and Procedures 

-  Attendance

-  Breaks

-  Equipment handling and usage

-  In Class behavior

## Lesson 5:Storage Devices 

**Overview**
- Computers need a way to store information when their power is turned off. They also need to store larger amounts of information than can fit into main memory at one time. The solution to these problems is a variety of storage devices. In this lesson we discuss floppy disk drives, hard disk drives, CD-ROMs, tape drives, and other removable types of storage media.

## Lesson 5:

**Objectives**
- At the end of this lesson, you should be able to:
- Identify 3.5" and 5.25" floppy disks and the major components of floppy drives.
- Understand low-level formatting, partitioning, and high-level formatting of hard disks.
- Understand the general categories of hard disk interfaces.
- Know the factors affecting hard disk performance.
- Understand how CD-ROMs are formatted and how data is read.
- Describe five other options for removable storage.

**Storage Devices **

## Lesson 5:

_**Floppy Disks**__ _
- The first type of disk drive for the PC was the floppy drive. The floppy disk has evolved over time from the 5.25" floppy disk with a storage capacity of 360 KB to the high-density 3.5" floppy disk with a storage capacity of 1.44 MB.

**Storage Devices **

## Lesson 5:

**Storage Devices **

- The actual carrier of information for the floppy disk is a thin, circular, and very pliable sheet of plastic which is coated with a layer of magnetic material. The thickness of the magnetic coating is only about 8E-5 inches

## Lesson 5:

- The case of the 3.5" floppy disk is much more rigid than the 5.25" floppy.
- The disk inside is just as pliable or "floppy" as the 5.25" disk, but is much better protected.
- The 3.5" size is also more convenient than the 5.25" size -- the 3.5" size easily fits in a shirt pocket.
- These features, along with a higher storage capacity, have made the 3.5" floppy diskette more popular than the 5.25" format. Most new computers just have a 3.5" floppy drive.

**Storage Devices **

## Lesson 5:

_**Physical Format of a Floppy Disk**__ _
- A floppy disk has a very thin coating with small iron particles embedded. The disk drive has a recording head which is a tiny electromagnet. The electromagnet has a particular polarity (alignment of its north and south poles) which causes the particles in the area below the head to have the same magnetic alignment. This magnetic alignment in a specific direction is also called "flux." Changing the direction of the current through the head changes the flux on the disk underneath the head. The drive controller interprets the pattern of flux reversals as zeroes and ones.

**Storage Devices **

## Lesson 5:

- Before a new disk can store data, it must be "low-level" formatted (physically formatted, or hard formatted). This operation is usually done at the factory. Low-level formatting creates concentric circles on the disk called tracks with small gaps between each track. The outermost track is called track 0. The track numbers increase as you go toward the center of the disk.

**Storage Devices **

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**Storage Devices **

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- The number of tracks depends on the format of the disk. For example, a 3.5" floppy disk has either 40 or 80 tracks. The actual width of a track is about .01 inch.
- The tracks are divided into smaller sections called sectors. Each sector is the same length, so the gaps between sectors grow larger from the center of the disk to the outside, forming a pie shape. Each sector has header and trailer information written to it and the data area is filled with dummy byte values. Each sector is sized to hold 512 bytes of data.

**Storage Devices **

## Lesson 5:

_**Hard Disk Drives**__ _
- A hard disk drive gets its name from the fact that the disks are rigid. Hard disks are usually made from aluminum. The disks have a coating containing iron oxide, like the coating used for floppy disks.
- A hard disk drive usually consists of several disks which are arranged vertically and rotate around a common axis. The hard disks rotate at a much faster speed than floppy disks. Modern hard disk drives rotate at 4,500 to 7,200 revolutions per minute (RPM), whereas a 5.25" floppy disk rotates at 360 RPM and a 3.5" floppy disk rotates at 300 RPM.

**Storage Devices **

## Lesson 5:

**Storage Devices **

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- The set of tracks that are precisely aligned above and below each other are called a "cylinder." Information which is on the same cylinder can be read very quickly, because the heads don't have to move to access it. They just read the information as the disk rotates underneath (or above) them.

**Storage Devices **

## Lesson 5:

_**Components of a Hard Disk Drive**__ _
- In addition to the disks themselves, a hard disk drive contains the following groups of components:
- Drive motor
- Read/write heads
- Stepper motor
- Control circuitry

**Storage Devices **

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**Storage Devices **

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_**Drive Motor**__ _
- The drive motor causes the hard disks to rotate. Unlike floppy diskettes, which only start rotating when data is being read or written, hard drives spin all the time the computer is on. This is necessary because it takes too long to bring the disks up to full speed.

**Storage Devices **

## Lesson 5:

_**Read/Write Heads **_
- The head is a tiny electromagnet which reads or writes a pattern of flux reversals in small areas on the disk called "bit cells" or "transition cells." The drive controller interprets the pattern of flux reversals as zeroes or ones. There is a read/write head for each surface of each disk. The drive shown in Figure 8 has three platters, so there are six read/write heads. All the heads move together as a unit, but data can only be read from one disk surface at a time. As a result of this architecture, multiple hard disks are required for the computer to perform read/write operations in parallel. This improves total throughput to your disk system.

**Storage Devices **

## Lesson 5:

_**Stepper Motor**_
- The stepper motor moves the read/write heads from one track to another. The average amount of time it takes the heads to move to the correct track is called the "average seek time." This is an important element in the performance of a hard drive.

**Storage Devices **

## Lesson 5:

_**Control Circuitry**__  _
- The control circuitry makes sure that the rotational speed of the disk does not vary by more than a half percent. It also coordinates actions of the drive with the disk controller.

**Storage Devices **

## Lesson 5:

_**Hard Disk Interfaces**__ _
- The actual method by which data is transferred to the hard disk drive and recorded there (or read from there) is called the "interface." The interface is managed by the hard disk controller. It is essential that the interface used by the hard disk match the interface used by the controller.
- The most common hard disk interfaces used by the PC are:
- IDE
- SCSI

**Storage Devices **

## Lesson 5:

_**IDE Interface**__ _
- IDE (pronounced "eye-dee-ee") stands for "Integrated Drive Electronics." An IDE drive allows more sectors on outer tracks than inner tracks (remember the pie shaped inter-sector gaps in Figure 3.4), thus allowing for a higher drive capacities. The maximum size of an IDE drive is 528 MB. An IDE controller can support two drives.
- An interesting feature of IDE drives is integrated error correction. Each track has one additional sector that is held in reserve to automatically replace bad sectors. (A "bad sector" is one that the drive can't reliably write to or read from.)
- The next step in interface evolution was the Enhanced IDE (EIDE) interface. EIDE allows for larger disk sizes (many gigabytes) and up to four separate drives per controller.

**Storage Devices **

## Lesson 5:

_**SCSI**__ _
- SCSI (pronounced "scuzzy") stands for Small Computer System Interface. It is a separate bus system that allows many different intelligent devices to be attached to the same interface card. which in turn connects to the PC's main bus. The devices may be hard drives, CD-ROMs, scanners, or tape drives. One SCSI controller can manage up to 8 devices (including itself). On a SCSI hard drive, the actual disk controller is built into the hard drive's on-board circuitry.

**Storage Devices **

## Lesson 5:

_**Preparing a Hard Drive to Receive Data**__ _
- The steps required for a hard disk to receive data are:
-         Low-level formatting
-         Partitioning
-         High-level formatting

**Storage Devices **

## Lesson 5:

_**Low-Level Formatting**__ _
- A hard drive is usually low-level formatted at the factory. This means that tracks and sectors are magnetically laid out on the surface of the disk before you purchase the drive. (Tracks, sector, and cylinders are described earlier in this lesson.)

**Storage Devices **

## Lesson 5:

_**Partitioning**__ _
- The next step in preparing a drive to receive data is to separate the physical drive into "partitions." (Partitions are also called "logical drives.") DOS cannot recognize a hard drive for formatting unless it has at least one partition. The minimum size of a partition is 1 MB.

**Storage Devices **

## Lesson 5:

**Storage Devices **

## Lesson 5:

_**High-Level Formatting**__ _
- High-level formatting means writing to the drive the actual logical structures that DOS uses to read and write information.
- DOS divides each logical drive into allocation units called "clusters." A cluster is the smallest unit that DOS can access on the drive. The size of a cluster is chosen when you format the drive using the FORMAT command. The smallest cluster that DOS can use is 2,048 bytes (4 sectors).

**Storage Devices **

## Lesson 5:

_**Factors Affecting Hard Drive Performance**__ _
- The main factors affecting a hard drive's overall performance are:
- Average seek time
- Spin rate
- Data transfer rate
- Cache size
- Data fragmentation

**Storage Devices **

## Lesson 5:

_**Average Seek Time**__ _
- Average seek time is the average amount of time for the read/write head to move to a specific track. Typical values for new IDE drives are 10 to 15 milliseconds. SCSI drives tend to be somewhat faster with average seek times from 8 to 10 milliseconds.

**Storage Devices **

## Lesson 5:

_**Spin Rate**__ _
- Once the head is on the right track, it must wait for the right sector to pass underneath it before it can read the data you are looking for. This wait time is called "latency" and depends on how fast the disk is spinning (rotating).
- Typical new IDE hard drives rotate at 4,500 RPM. This means they make 75 complete revolutions in one second. On average, the drive must make half a rotation to bring the correct sector under the read/write head.

**Storage Devices **

## Lesson 5:

_**Data Transfer Rate**__ _
-    Once the read/write head is in the right place, how fast can data be transferred? Specifically, "data transfer rate" means how fast can data be moved from the disk to the computer's main memory. Notice that this number can be constrained by the speed of the computer's bus or by the fact that the CPU may be busy performing other tasks and not able to accept the data at the fastest possible rate. For these reasons, drives use the concept of "maximum transfer rate."

**Storage Devices **

## Lesson 5:

_**Cache Size**__ _
- Just like the CPU uses cache memory to increase its average speed of reading information from main memory, modern disk drives use their own on-board cache memory (also called a buffer). When the read/write heads read information form the disk, they save it in a buffer in addition to passing it to the disk controller. This means that the next time there is a request for data from the CPU, there is a chance that it will be in the buffer. If the data is found in the buffer, the "disk read" is as fast as reading the data from memory.

**Storage Devices **

## Lesson 5:

_**Data Fragmentation**__ _
-    Data fragmentation is not a property of the drive itself, but is a consequence of how DOS manages file clusters over time. When there is very little data on a drive, files are usually stored in adjacent clusters on the same cylinder, so only one access is required to read the data.

**Storage Devices **

## Lesson 5:

**Storage Devices **

## Lesson 5:

_**CD-ROM**__ _
-    CD-ROM (Compact Disk - Read Only Memory) was first introduced for use with computers in 1985. It uses the same physical structure as the Audio CD, which was first introduced in 1979, in fact a computer can play back audio CD's with a software player. Information on CD-ROMs is **usually** read-only, like ROM, but a CD is storage (not memory). The real benefit of a CD-ROM is its large data capacity (about 650 MB of information). Compare this with the typical 3.5" floppy which can only store about 1.4 MB.
- Rather than using a magnetic coating to store information, a CD uses small indentations in a plastic base, which is coated with a layer of reflective aluminum, to store information.

**Storage Devices **

## Lesson 5:

- A CD-ROM uses laser beams to read the information on the disk as shown in the following figure.

**Storage Devices **

## Lesson 5:

- A laser beam is split into two parts by a diffraction grating. The split beams are then focused very closely together by lenses and converge on the disk. The two beams are reflected off the aluminum coating, pass back through the focusing lenses, are partially reflected by a mirror, and are finally focused onto a photo-diode. (A photo-diode converts light into an electrical signal.)

**Storage Devices **

## Lesson 5:

_**Format**__ _
-    Instead of many tracks in the shape of concentric circles, a CD-ROM uses a single spiral track, like a phonograph record. Unlike a phonograph record, the beginning of the track is on the inner edge of the CD-ROM and it spirals outward. The sectors are laid out on this single track, one after another.

**Storage Devices **

## Lesson 5:

_**CD-R and CD-RW**__ _
-    CD-ROM drives while extremely useful for storing large amounts of data, were really only useful to software manufacturers for distributing software and other data to users. The user, himself could not record or write to a CD-ROM. Only manufacturers could afford the expensive equipment to write to a disk.
- Recently however this write technology has become available to the consumer market in the form of relatively inexpensive *CD-R (compact disk — recordable)* and *CD-RW (compact disk - rewritable)* drives. Both of these drives have the same storage capacity as a standard CD-ROM.

**Storage Devices **

## Lesson 5:

_**DVD-ROM and DVD —RW**__ _
-    A *digital versatile disk (DVD)* is similar in operation to a CD-ROM. The reflected laser signal is converted by the *read head* into electrical pulses interpreted as data. This in part contributes to the fact that a DVD has the potential to contain 8.5-17 GB of data. A DVD has four layers per side.
-    And of course there are two sides, or another effective doubling. Current DVD units only have one read head so the disk must be manually turned over to read the other side. Like a CD-ROM, a DVD is a read only device.

**Storage Devices **

## Lesson 5:

*CD-R RW (Record — rewritable)* permits writing to a CD more than one time. These units are currently fairly expensive.

**Storage Devices **

## Lesson 5:

- - Tape drives use a standard video cartridge as the storage media. They have very high storage capacities but are quite expensive.

**Storage Devices **

## Expansion Bus and Memory

- Any

- Questions

- Lesson 4:

## Expansion Bus and Memory

**Conclusion**

- Lesson 4: