Peripheral device - part of the hardware, structurally separated from the main unit of the computer system, the composition and characteristics of which largely depend on the possibility and efficiency of the application of the PC. So, by appointment, the following types of peripheral devices can be distinguished:
• external storage devices, or external PC memory;
• Input devices;
• information output devices;
• Information transfer devices.
Peripheral devices operate on the instructions of the central processor and are designed for external data processing, their preparation, input, storage, management, protection, output and transmission via communication channels. Peripheral devices of the PC include external memory devices, information output devices, input devices, printers, remote data transmission devices, audio systems.
The peripheral device can be either physically external (printer, scanner, external modem) and be in the system unit (data storage or internal modem).
For long-term storage of information, the computer is equipped with external storage devices that are classified according to different criteria:
• by type of access:
- with random access (disks, flash cards);
- with sequential access (magnetic tapes);
• on the technology used to write/read information:
- with magnetic carriers (HDD, NGDH);
- with optical media (CD, DVD);
- with magneto-optical carriers (magneto-optical disks);
- using flash memory (flash cards);
• by media type:
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- with a permanent medium (hard disks);
- with removable media (flexible, optical and magneto-optical disks, flash drives, tape drives, removable hard drive packages).
The hard disk drive (HDD, hard disk, hard drive, English Hard Disk Drive, HDD) is an information storage device based on the principle of magnetic recording.
Typically, the HDD is not available to the user, since it is fixed permanently inside the computer in a sealed enclosure along with electronic control circuits (Figure 3.13, a). Consists of a package of magnetic disk plates mounted on one axis Figure 3.13, b). The motor rotates the discs at speeds from 3600 to 10,000 rpm. A special drive (stepper motor) positions the read/write heads.
The main characteristics of storage media:
• capacity, i.e. the maximum amount of data that can be written to the media, today's hard drives have hundreds of gigabytes;
• speed, which is determined by the time of access to the data and the speed of their transmission. In HDDs, the positioning speed of the head over the disk space with the desired data is about 10 ms, and the data transfer rate ( p ) depends on the number of bytes in the sector ( D, the number of seconds -
Fig. 3.13. Hard disk drive:
a - with the cover removed; b - tracks, sectors, plates, heads
tori on the track ( n 2) and the rotation speed of the disks (w in min): . The speed of information transfer increases when installing in the hard drive cache memory (2, 4, 8 or 16 MB).
Information is recorded on plates covered with a layer of ferromagnetic material under the influence of a magnetic field of a changing property so that two stable states that correspond to binary digits 0 and 1 are realized.
First, carriers are subjected to the process of markup - formatting (hard disks, optical media, flash-based devices) before the computer is installed in the computer to create and address data access structures. When formatting, the old information on the media is lost or destroyed.
Formatting of hard disks is performed in several stages.
Low-level formatting is performed at the factory. The surface of the disks is magnetized to concentric tracks, the path is divided into sectors (see Figure 3.13, b). Each sector is recorded with service information, which determines the beginning of the sector and its number for positioning the heads. In the future, the sector will store the saved data, and at the end of the sectors - checksums to verify the integrity of the data.
The information is recorded and read by the heads along the circumferences of the paths. The collection of tracks on the whole bundle disks located at the same distance from the axis, forms a cylinder.
High-level formatting is to create a master boot record (Master Boot Record) on the disk with a partition table. Each of the partitions (these are logical disks) are assigned their own name (letters of the English alphabet). At the same time, a Boot Record is created, which states that the operating system is installed on the disk, and the logical structure of the file system. The clusters are marked - the minimum space (several adjacent sectors), the information allocation units for the files on the disk. Data exchange between the disk and the main memory is carried out in sequence by an integer number of sectors.
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When writing and reading information, the disc rotates around the axis, and the head control mechanism brings it to the track chosen for recording or reading data. Discs refer to data carriers with arbitrary (direct) access, because (unlike a tape) you can immediately go to the place where you write the information you need.
There are IDE hard drives (Integrated Drive Electronics), and SCSI (Small Computer System Interface).
Advantages of the SCSI interface: the hard drive is controlled by an adapter installed on the system board, and it can also manage CD drives, scanners, tape drives, other internal and external devices.
In addition to the hard disks installed inside the system unit, there are mobile external disks connected to the computer via the USB port.
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