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Lesson 4.9: Magnetic Storage
When you have completed this chapter you will:
• Explain the operating principles of magnetic storage media
• Do axiologeis performance data storage
• Identify the main types of floppy and explain the function of different parts of
• To describe the mechanism of reading and writing the driver floppy and hard disk
• Explain the internal organization of data on floppy disks and hard drives.

4.9.1. Introduction

The biggest disadvantage is that RAM memory loses its data when interrupt power to the computer. On the other hand, although ROM memory holds data is not rewritable and capacity, such as memory and RAM, limited to a few tens of MB.
The solution to these problems gave early on storage media.
The first instruments used for permanent storage of data tapes were.
The magnetic films have dominated the field of computers and the tithes 1960-1970 were the only way to permanently store large amounts of data. The magnetic tapes although seating area accommodates around a lot of data had two main drawbacks was massive and the reading was quite slow.
Then came the floppy. The disk was small in volume and much faster than magnetic tape. Moreover, the disk as magnetic tape, belongs to the media that we can remove from the reader / writer and reuse or later, or to another computer (removable data volume). Obviously, this way we may transfer our data from one computer to another, and increased data storage capacity by simply changing disks.
The disk, like the tape has some disadvantages. The disk has little capacity to store data. The most important disadvantage is the low reliability, since they may spoil the disk and the user to lose data.
Very quickly appeared and the first hard drives. Hard disks can not be transferred as floppy disks, but have significantly greater data storage capacity. Hard drives are also considerably faster and more reliable.
In recent years made their appearance and optical storage media (CD). These instruments have very large storage capacity, is extremely reliable and quite cheap. The only drawback is that they can register only once.
The rest of the course that follows will deal with the operating principles of magnetic media and examine in detail the floppy disk, hard disk and other magnetic media. In the next chapter we will examine the optical media.
4.9.2. Characteristics of Storage Media
Before proceeding to describe the magnetic storage media will refer to sizes that characterize the various storage devices:
• The capacity (capacity), as in the case of memories, tells us the maximum amount of data that can be stored on a storage medium. The capacity is measured in bytes and multiples of 1 KB = 1024 bytes, 1 MB = 1024 KB and 1GB = 1024 MB.
• The access time (access time) is an indication of the speed at which the storage unit responds to the commands of KME. This time is the time it takes the storage unit from the time you get a command read / write from the CPU to read / write the first bit. This time includes all the delays of the device that reads from or writes on the storage medium by:
• to process the command read / write
• go to the point where the data onto the storage medium and
• To begin the read / write them.
Specifically, the time it takes for the device to locate the right spot from which to read (or respectively to write) the data are called data search time (seek time).
The access time is not the same every time you give a command but depends, as we shall see, the "position" data onto the storage medium. For this reason manufacturers give either a mean value of this time, or more rarely the maximum value.

• The data transfer rate (data transfer rate) is a measure of the speed of a storage unit. The rate is equal to the number of data (bytes) to read / write from a storage unit in a second. This rate does not include the delay due to the average access time, but count the bytes we can read or write on every second (bytes / s) since the device is operational and has reached the position are data on the storage medium.
Suppose for example that a storage unit is 40 ms access time and achieves data transfer rates 2 KB / s. Time to read a KME 200 bytes from this storage unit is equal to the sum of access time plus the time it takes for the transfer of 200 bytes from the storage unit. In other words, the time is 40 ms + 200 Bytes / 2 (KB / s) = 40 ms + 200 / (2 x 1024) sec  140 ms.
4.9.3. Magnetic materials
As we know, a magnet has the ability to attract some materials lying around, like other magnets or pieces of iron. In a magnet distinguish two poles. The north pole (N) and south pole (S). ¨ A coil, when passed over by current and it behaves as a magnet. The direction of current determines which side will appear on the coil terminals.
Let us now examine the behavior of various materials when they are near a magnet or a coil. The materials are divided into two main categories depending on their behavior in magnetic field. Since we have a paramagnetic and diamagnetic materials such as wood and the other the ferromagnetic materials like iron.
Materials such as iron have the capacity to turn themselves into magnets when placed in a magnetic field, called magnetic or ferromagnetic materials. The magnetization in these materials remains after removal of the magnet.
As such, the permanent magnetization of ferromagnetic materials exploited when recording to magnetic storage media. With the help of a coil, a magnet is called head over small areas in a ferromagnetic medium, corresponding to the bit of information.
The reading of magnetic storage media based on the phenomenon of induction.
When the phenomenon of induction, if you move a magnet in a coil, then the ends of the coil voltage is displayed. This property we use when reading magnetic storage media. As the media (tape or disk of magnetic material) we pass beneath the coil, which is the read head, the ends of the coil appear small electric pulses, corresponding to the bit of information.

 
4.9.4. The flexible disk
In the field of personal computers used two types of disk:
• The 51/4'' with a capacity of 360 KB and 1.2 MB and
• The 3 1/2'' with a capacity of 720 KB and 1.44 MB

 
Figure 4.9.1 The disk 5 ¼'' and 3 ½''
Both types of disks consisting of a flexible circular disk coated with a suitable magnetic material on which written data. A plastic cover protects the disc from dust and from the environment. The accretion of dust or grease, on the surface of the disc can make it impossible to record or read so the user can lose data.
To read or write data to disk using the floppy drive (disc drive). This device turns the floppy disk with the help of a small engine and can write or read data on the floppy disk.
The floppy 5 ¼'' so named because they have width 5 ¼'' inches is equal to 13.3 cm. In these disks, there are:
1. Opening (1) axis in the center of the disk where the driver rotates the flexible disk drive.
2. The opening (2) to cover from where the driver disk reads and writes data onto the magnetic medium
3. The opening (3) used by the floppy drive as a reference point on the floppy disk. At some point in the floppy disk there is a corresponding opening. When the disk rotates, opening the floppy disk passes through the corresponding opening of the cover disk of activating an optical switch. In this way the driver diskette to know when each turn passes this point of the optical switch. This information is used by the floppy driver as a reference point to find the information you want on the disc.
4. Finally on the disk we see the write-protect notch (4). The notch that protects the instrument from unintentional writes. When Nick is closed, an optical switch prohibits any recording on the magnetic medium. To allow writing should be the slot that is open to not turn the internal switch of the driver diskette.
The 3 ½'' floppy disks are called so because they have width 3 ½'' inches is equal to 8,9 cm. These disks except more capacity have more credibility because of the careful and streamlined design. The casing is hard and rigid protecting the best floppy disk inside. There are no openings where dust can penetrate easily into the medium.
1. The opening in the center of which rotates the disk has been replaced with a metal circular disc (1) above applies where the axis of rotation of the driver diskette. Metal pin on the two notches. One, located in the center of the disk, is used to rotate the engine by flexible drive while
2. the other marks a point of reference (2) onto the floppy disk.
3. The opening from which writes and reads the head and this has been covered by a metal cover (3) that opens with the help of a mechanism, when the disk is in the driver floppy. The cover is restored when the disk comes out of the driver.
4. Similarly the notch protection and has a plastic part (4) that allows either state to have write-protect (Write protect) or situation allows records. When the plastic part allows the switch, ie the slot is open then the disk is write protected, otherwise where the groove is closed, the records are allowed.
5. Finally, 3 ½'' floppy disks have an extra notch (5) shows that if the disk has a capacity of 1.44 MB (when available slot) or 720 KB (when there is no indentation).
4.9.5. Floppy
The driver disk is the device we use to write and read a flexible disk.
This device includes a motor that rotates the floppy disk with angular speed 360 rpm or equivalent 6 full turns per second.
The recording and reading of data is done using the read / write heads. There are two heads one on each side of the flexible disk drive based on an arm that can move forward - backward, as shown in Figure 4.9.2
 
Figure 4.9.2 The floppy
By moving the arm and forth - back and turning the floppy disk heads can be in any position on the floppy disk. The movement of the arm from the inner to the outer diameter lasts at most 200 ms and a full rotation of the floppy disk is a 1/6 s = 167 ms. We conclude, given that both movements can be done simultaneously, that to find the head over some part of the very need to drive 200 ms. This is where the head should be moved from the outer diameter of the internal disk or vice versa. This time is the maximum seek time and data to guide the flexible disk is about 200 ms.
The most representative size of the driver's speed is the average search time data. For flexible disks is about 85 ms.
 
Figure 4.9.3: The head of the driver diskette
On the head there is a small coil which can write data on the floppy disk. The same coil is used when reading data.
When we want to write a bit at some point on the disk, then moved the head over this point and applies a pulse current in the coil head. The magnetic field created, captures a small region of the flexible disc just below the coil. So at this point created a small magnet. The direction of the magnet depends on the direction of current in the coil at the time of registration. Depending on the direction of current so we can have two elementary magnets corresponding to two different bits. This way we can write data onto the disk.
When we want to read the data written in one area on the floppy disk, enough to pass the coil over the head of this region. Then the elementary magnets of the flexible disk induce a small voltage to the coil. If this trend strengthened quite recovered data written to disk.

4.9.6. Organization of data on the floppy disk
The data is not written anywhere on the floppy disk but are organized into groups.
 
Figure 4.9.4: Organization floppy on trails and areas
In rigid magnetic disks followed by the organization shown in Figure 4.9.4. Data is placed on concentric circles called tracks (tracks). The paths are numbered. The trail is located on the outer diameter of the disc and cover a full circle is the first track (Track 0), while the path with the smallest diameter is the largest number of the disk path. The number of tracks on the disc is different depending on the type of floppy disk. The more paths contains a flexible disk, the greater is the capacity of the rule.
Each path is divided into additional sectors (sectors). These areas are specific angle arcs. The separation of the fields and paths in a unique way for all types of floppy disk. In Table 4.9.1 we see the organization of the disks used in personal computers.
Disk Size Path Number Capacity Number of sectors per track
5 ¼ '360 KB 40 9
5 ¼ '1.2 MB 80 15
3 ½ '720 KB 80 9
3 ½ '1.44 MB 80 18
Table 4.9.1 Organization Diskette
As the trails so the numbered areas (Figure 4.9.4). The sector with the number 1 (sector 1), ie the first sector in each path is the one that starts from the notch that serves as a reference point on the disc.
4.9.7. Connecting the floppy drive on a PC
The personal computer support at least two floppy disk drivers. The flexible disk drivers connected to the parent with the help of a special permit extension called flexible disk controller driver (Floppy Disc Controller). The controller is now incorporated into the comprehensive support of motherboard (chipset).
The connection driver is a special cable.
 (Figure ribbon floppy)
The cable are two pairs of connectors. Each pair has a plug connector for floppy disk 3 ½ 'and a floppy drive for 5 ¼'. In each pair, we can connect only one floppy drive.
4.9.8. Hard disk
The hard disk has prevailed in the computer world as the fundamental unit of persistent storage for data and programs. The capacity and speed reading of data is increasing while the cost is constantly decreasing.
 
 Figure 4.9.5 Hard Disk
The hard disk contains, just like the floppy disks coated with magnetic material for storing data. But unlike the floppy disks are stiff and each device contained hard disk rather than over one. The number of disks varies from 2 to 16.
The diameter of the discs is different for each hard disk. The diameter plays an important role in the performance of hard. The smaller the diameter of the disk, the faster the heads placed over a specific area. At the same time the smaller diameter results in less surface area and hence less disk capacity.
For each disk has two heads one on each side. All heads are mounted on an arm and a move together and not independently of each other. When the disks rotate, heads hover a short distance from the discs ( 0.0003mm) on a thin layer of air. If the disks are not rotating at the full speed, then the layer of air is not enough and heads resting on the magnetic means destroying it. For this reason, when the disks are not spinning, the heads are placed at a point (LANDZ) disks, where no data. This process is called the park and is done automatically by the auditors of modern hard disks.
The performance of hard drives is really enviable. The data access time is minimal milliseconds (msec), while the data transfer rates ranging from a few MB per second up to tens of MB per second. Typical values for the data access time (access time) is 5 ms and the data transfer rate (transfer rate) 5 MB / sec. The performance of hard drives vary and the model of the device manufacturer.
4.9.9. Hard Disk Organization
As in the case of floppy disks, hard disks, organize the data into the fields and paths. In the case of hard drives no form of standardization in the number of paths and areas, so these numbers can vary even for same capacity disks. The organization depends solely on the manufacturer.
On hard disks instead of paths used interchangeably the notion of cylinders. A cylinder is defined as all paths are the same diameter but different drives. Obviously what we have paths on the surface of a hard disk has many cylinders and the disk.
One last parameter used to describe the geometry of the hard drives are the heads containing the hard disk. The number of warheads is not necessarily equal to twice the number of magnetic hard disk containing the device, since the manufacturer can not have two heads placed on each magnetic disk.
For example prices for these items for a hard disk capacity 3.1 GB. This disk has 16 heads, 6136 cylinders and 63 sectors per track.
4.9.10. Connect the hard disk on a PC
The PC supports four hard drives. The hard disk controller is built into the drive. So the hard drives connected directly to the motherboard IDE connectors or SCSI, depending on their type.
 Each channel supports two IDE hard disk, of which one is the main channel (master) and the other the secondary (slave).
4.9.11. The LS-120.
This is for many, the successor to the floppy. This device accepts special 3 ½'' floppy disks with a capacity of 120 MB. The main advantage is that the device can read and simple 3 ½'' floppy disks with a capacity of 1.44 MB or even earlier with a capacity of 720 KB.
The increased capacity due to a clever head positioning mechanism on the floppy disk. During construction of the floppy disk marked trails on the average. This information is used to precise positioning of the head. In this way we can have very dense placement of trails on the average, thus increasing capacity.
The LS-120 is connected either via an IDE channel or via a parallel port on the PC. The performance of LS-120 is quite improved over the ancestor of the floppy drive 3 ½''. The average search time data (seek time) is 70 ms versus 85 ms in the case of the floppy disk, and data transfer rates reach up to 485 KB / sec.
4.9.12. Zip-Drives
This is a device much like the LS-120. The Zip drive disks accept special size 3 ½'' capacity and 100 MB.
The performance of the Zip drive is much better than the LS-120. The average search time data is only 29 ms while the data transfer speed reaches and 1,4 MB / sec.
4.9.13. Jaz-Drives
The devices use Jaz drives instead of flexible disks. The capacity of these disks is either 1GB or 2G.
With instrument data access time equal to 16 ms and average data transfer rates equal to 7 MB / sec, the Jaz drives are not inferior at all in relation to a hard disk with the additional advantage that we can change the disks containing the data . The major disadvantage of these devices is very high cost of replacement hard drives and the fact that the disks are quite sensitive to environmental conditions.

What I learned:
• The capacity of a storage medium is equal to the number of bytes we can store it.
• The data transfer rate of a reader / writer a storage medium is equal to the number of data bytes that can write or read this device per unit time.
• The data access time is equal to the time it takes the reader / writer from the moment you accept a command read / write to read / write the first bit.
• The floppy 5 ¼'' exist in two versions with capacities of 360KV and 1.2MV respectively.
• The 3 ½'' floppy disks come in two versions with capacities of 720KV and 1.44MV respectively.
• The internal disks have a flexible magnetic disk on which data are written.
• The floppy disk has two heads one on each side.
• The organization of data on the disk is on trails and areas.
• Hard drives have several rigid disks coated with magnetic material on which written data.
• Usually for each disk has two heads one on each side.
• The organization of data on the hard disk is on trails and areas.
• As the cylinder is composed of paths with the same diameter located on different disks and hard disk. Obviously the number of cylinders is equal to the number of paths.
• Cylinders, heads and the number of sectors per track defines the internal structure of a hard disk.
Terminology
• Head
• Data transfer rate
• Time Data Access
• Seek time data
• Path
• Domain
• Cylinder

Control knowledge

1. When a magnet closer to a _______________ material then it becomes a magnet.
2. If you move a magnet near a coil, then the ends of the coil shown __________________.
3. The 3 ½'' floppy disks have a capacity of ______ and _______.
4. To write data to a disk should the write-protect notch is __________.
5. The _________________ is the device that reads or writes data to disk.
6. The time it takes the hard drive once we give the command to read a file until you read the first bit is equal to the time of that hard ________.
7. If a hard drive reads at 1ms 1024 Bytes then the _________________ of the specific hard data is equal to 1000 KB / sec.
8. To LS-120 diskettes receive special ________ to ______ 120 MB but can read and simple disks with a capacity of _______ or _____.
9. To ZIP receives special diskettes ______ with _________ 100 MB and can not read simple disks.

Problem:
1. With a special program we give the command to the hard disk to read a file of 120 KB and take it to memory. After time 60 ms file in memory. If the data access time for that hard is 4 ms, to calculate the rate of data transfer.

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