Storage knowledge class (I): How disks work

Most permanent or semi-permanent computer data is done by magnetization of a small piece of metal material on the disk. Then, these magnetic maps can be converted into raw data. This is the principle of magnetic storage and the main content of this article.

History of magnetic storage

Before the emergence of magnetic storage, the primary computer storage medium was a punch card invented by Herman Hollerith in 1890.

The history of magnetic storage can be traced back to June 1949 when a group of IBM engineers and scientists started to develop new storage devices. At that time, they studied the first magnetic storage device for computers, which changed the entire industry. In May 21, 1952, IBM released an IBM 701 tape drive with the IBM 726 defense calculator, marking a change in the punch card calculator like an electronic computer.

 

Four years later, a small group of IBM engineers took the lead in introducing the first computer disk storage system to the 308 ramac (random access and control) computer.

The 305 ramac driver can store 5 million characters on 50 disks. The storage density of personal data is only 2 kb/square foot. Unlike the tape drive, the ramac recording head can directly reach the position on each disk surface without reading intermediate information. Random accessibility has effectively improved computer performance, making data storage and retrieval much faster than originally stored on tape.

In more than 60 years of development, the magnetic storage industry has been able to store 3 TB of data on 3.5 feet-size drives.

How does a magnetic field store data?

All magnetic storage devices read and write data through the electromagnetic principle. The basic physical principle is that when the current passes through the conductor, a magnetic field is generated around the conductor (see figure 1 ). Note that electrons flow from the cathode to the anode, although we generally think that electrons flow from the anode to the cathode.

 

▲Figure 1 magnetic field generated around the wire when the current passes through

Electromagnetic phenomenon was discovered in 1819 by Hans christinster, when he found that the compass could not accurately indicate the direction when approaching the electrified wire. When the current stops, the compass returns to normal.

The magnetic field produced by the wire conductor affects the magnetic material in its range. When the current direction or voltage polarity changes, the magnetic pole also changes.

In 1831, he discovered another electromagnetic effect. He found that if the conductor moves through the magnetic field, it will generate electricity. The current direction changes with the pole direction (see figure 2 ).

For example, when an alternating current generator is applied to a magnetic storage device, this bidirectional electromagnetic function can record and read data on the disk. During recording, the head changes the electric pulse into a magnetic field, and the magnetic field is converted into an electric pulse during reading.

 

▲Figure 2 wire imported when current passes through the magnetic field

The read/write head in the magnetic storage device is a U-shaped conductor. The U-shaped head is wrapped in a coil to let the current pass through (see figure 3 ). When the current passes through the coil, the magnetic field is generated in the drive head. Changing the polarity of current also changes the magnetic pole. In fact, the voltage of the head can be rapidly changed at two levels.

▲Figure 3 read/write head

 

Disks or tapes that make up the storage medium form some media, on which a layer of magnetization is stored. This substance is usually iron oxide with impurities. Each magnetic particle on the storage medium has its own magnetic field. When the medium is empty, the magnetic poles of these magnetic fields are usually messy. The magnetic field direction of each particle is random, so there is a mutual offset, and there is no obvious magnetic pole.

When the drive read/write head produces a magnetic field, the magnetic field will beat between two levels of the U-type Magnet. Because the magnetic field is easier to pass through the air than through the conductor, the magnetic field will bend outward, and then use the adjacent storage medium as the shortest path to reach the other end. The magnetic field is directly passed through the Medium to polarization the magnetic particles so that they are consistent with the magnetic field. The direction of the magnetic pole is determined by the current direction of the coil. During the development of magnetic storage devices, the distance between the read/write head and the media is significantly reduced. In this way, the recorded domain will also become smaller. The smaller the domain to be recorded, the higher the data storage density.

When the magnetic field passes through the media, the particle directions in the lower part of the head are the same. When the magnetic domains of particles are unified, they do not offset each other, so they form an obvious magnetic field. This local magnetic field, generated by many magnetic particles, will now generate a traceable cumulative Magnetic Field in a uniform manner. The reverse magnetic flux in the head will lead to the reverse of the magnetic pole of the magnetic particles on the disk.

Flux reversal changes the magnetic pole of magnetic particles on the surface of the storage medium. A drive head creates an inverted magnetic flux on the media to record data. If the drive writes any byte, it will create a positive-negative and negative-positive reverse magnetic flux on the media. The flux reversal in the conversion area is used to save the given data. This method is called encoding. According to the encoding method used, the driver logic or controller saves the data and encodes it into a series of flux reversal.

During the write process, the head is added with a voltage. Due to the change in voltage polarity, the magnetic field electrode also changes. The magnetic flux transform area is accurately written down where the polarity changes. During the reading process, the signal generated by the head is different from that generated during writing. On the contrary, the head only generates a voltage pulse or peak voltage when it passes through the flux reversal. When the positive pole changes to the negative pole, the male detected by the head is the negative pole. When the negative pole changes to the positive pole, the pulse is the peak voltage of the positive pole. This effect occurs because the conductor generates current only when it passes through the magnetic line at a certain angle. Because the head moves in parallel with the magnetic field, the only chance that the head generates voltage is when it is read through the magnetic pole or the magnetic field conversion area.

In fact, when reading information from the media, the head is converted into a detector in the magnetic flux transform area, which can generate a voltage pulse when the conversion area is used. If there is no transition, no pulse occurs. Figure 4 shows the relationship between the read waveform and the magnetic flux transform area recorded on the storage media.

 

▲Figure 4: Magnetic read/write process

You can regard the write mode as a square wave of the positive and negative voltage. When the voltage is positive, the head will generate a magnetic field, which leads the magnetic medium to the same direction. When the waveform is changed to the negative pole, the magnetic field directs the medium to the other side. When the waveform is switched from the positive pole to the negative pole, the magnetic flux on the disk also changes direction, and vice versa. During reading, the head perceives the magnetic field and generates positive and negative pulse waves. Instead of continuous waveforms. In other words, unless the voltage is zero, the head can detect the change of magnetic flux and generate corresponding positive and negative pulses. The pulse only appears when the head passes through the magnetic field of the medium. If you know how long the drive turns around, the controller circuit can determine whether the pulse degrades within the specified conversion time.

When the head passes through the storage media in Read mode, pulse current is generated, which may produce a lot of noise. The sensitive current in the driver and the Controller Assembly amplified signal and decoded the weak pulse into binary data, that is, the first data to be input.

Hard drive and other storage devices use the basic electromagnetic law to read and write data. When the current passes through the magnet (head), the magnet generation can generate a magnetic field that can be stored in the medium, and drive the write data. The head then uses the media surface to drive Data Reading. Because the magnetic head changes in the stored magnetic field, it will generate a weak current to indicate whether the magnetic field is changed.