3.3.6. Permanent Memory

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Permanent memory for its designated for a WAREHOUSE-libraries of all types of programs or data that the user used in the work. Technology development and program support of this decade, the magnetic and optical systems as the major carriers for data storage.

General requirements posed to the long term memory are:

            1.) Possibility of writing, erasing and reading,
                user programs (files).
            2.) Quick access to files.
            3.) Large capacity.

Certain types of media used today does not meet all three of these general demands, but in some unequaled.

Magnetic tape

Large magnetic tapes can still be encountered in older systems that are still in use. Can store large amounts of data and it is their most important feature.

Tapes are kept in rings or similar music cassettes, depending on the design and capacity of the system. The principle of operation of large slipring system is shown in Figure 3.3.15a.

 Magnetic tape
Figure* 3.3.15 System with a magnetic tape. ( +/- )

Reel with magnetic tape imposed on the shaft motors controlled by control electronics so that the tape is still suspended between the rings and the magnetic head. Suspension strip controlled electro-optical sensors that send position data tapes electronics electric motors. Offset strip is necessary to prevent the effects of inertia electric tape to record and read data as it is because of the weight and inertia reel their motors coils can not make fast enough and stop. Promoting bands below the magnetic head mali speed control motors negligible inertia, which can easily move the magnetic tape head at a constant speed because they are exempt from duties wrap tape on the rings.

Small surface tape (magnetic domains) magnetized in one direction or the other contains written "0" or "1", then keeps the data size of 1 bits. Domain determines the size of the surface density of the tracks on the tape and is expressed in bits/cm. Usually amounts to several thousand bit-a/cm. The bar under the magnetic head promotes speed of 5 m/s Usually it is about 25 mm wide wound on rings, six inches in diameter. Data is written to multiple tracks simultaneously, for example 9 tracks to record eight-bit words of data. The ninth track control serves to mark the position of the tape. Each track has its own magnetic head to read and write.

Special software support provides guidance electromechanical systems on which place to locate the tape, based on the data read control trace. Length of the bar is normally about 700 meters, which is sufficient to type a few hundred MB of data. Data is written in blocks, groups of a few hundred bytes, each of which contains about controlling the overall recorded checksum of all bytes blocks written down about regularity. The beginning of each block and place checksums will mark for the purpose intended records, such as the ninth track.

Today's systems use a cassette tape like music or video tape (Figure 3.3.15b) , but in relation to them and have a tighter construction precise guiding strips and better mechanics. Quality tapes bottom panels the metal in order to reduce as much as possible their twisting. Capacities were in some versions, thanks to the development of technology and the various methods of data compression (lossless) and improved mechanism for controlling the errors, most of the older rail systems described above, and are more reliable. Data is entered along the strip in more traces or diagonally. They use different devices and different formats of writing data to the tape as DDS (Digital Data Storage), DAT (Digital Audio Tape), DLT (Digital Linear Tape) and LTO (Linear Tape-Open), with bands of a few dozen to a dozen GB or TB and durability of data retention over 20 years in adequate conditions. For smaller firms could support the DDS or DAT tape devices using the width of 4 and 8 mm, and the Data Center will be used with a DLT or LTO tape systems using the width 1/2". Standalone server without DAT or similar devices and disks in the RAID array is incomplete and unreliable.

Joint rings (which are no longer) and the cassette is rather slow data access and their primary purpose security requirement (BACKUP) data hard disk, so that in the event of damage to the data on the disc (physical, viruses, etc.) or the need to reach already archived data, the data could restore from tape to disk. Bad quality recording on the tape was not very good strength media and 'flipping' records during winding to rings which causes gradual loss of data (aka rotting bits). Therefore, it is necessary to periodically update the records, and it is good to change periodically strip them eventually replace them because of the physical strain and damage. Of course, manage the factory instructions on the machine and the media. How often to change the tape depends primarily on the activities of a company or an institution. One bank deliberately kept safely daily, weekly, monthly and even yearly archives on separate tapes, clearly marked appropriately filing season. An academic institution, not keeping the rate of students, this rigorous approach to archiving data is not needed.

Magnetic disks

On the surface of discs (discs) is applied to the magnetic layer on which data is recorded using a magnetic head in concentric circular tracks (track) during disc rotation. As data follow one after another is necessary in order to determine their locations as clues from the center to the edge, to the beginning of each track, tracks divided into sectors by them individually into smaller blocks and each of them mark the beginning and end. The block itself is still enrolled in the same amount of data as 512 B.

Thus, each block of data preceding marks beginning of his association with a particular sector and the trail and before an end tag is entered and the check sum regarding regularity loaded. Analogous to the above, each block can be thought of as a room in which to place the data, which belongs to a particular apartment on one floor of a building in a street of the city. If the positions are not previously marked (city name, street, house number, floor and flat) will not know the position of the individual blocks of data. Labeling of the block and mark his belonging to a certain sector and track precedes the data recording and presents PREPARING media for data.

Process for the preparation of media for data sharing media on separate fields marked with its own unique address records by using a special kind of designed for this purpose is called FORMATTING. During the format at the same time review the accuracy of magnetic media and any damaged blocks will be recorded on the appropriate place on the disk that is still not used.

The procedure itself is formatted media can be done in its entirety, that is done prior preparation of all media (MS-DOS systems), or whether the process can take place basis so as to prepare the media as is actually necessary for the data (OS / 2, Novell NetWare systems), which depends on the concept of a computer system.

Of course, the process of formatting the media properly implemented and for systems with a magnetic strip and a long-term basis from a diskette. Preparation gauges done by manufacturers.

Magnetic media disc is basically produced in two versions:

     I.) Media with FLEXIBLE substrates - diskette (floppy disk - FDD)
    II.) Media with HARD surface -------- (hard disk drive - HDD)

Images are mostly from the PC using the standard disk with magnetic coating on both sides of the flexible substrate (DS - double side), double or high density disk (Double - High Density):

     1.) floppy disk 3.5" capacity ---  720 kB (label DS/DD)
                                   --- 1.44 MB (label DS/HD)
     2.) floppy disk 5.25" capacity --  360 kB (label DS/DD)
                                    -- 1.20 MB (label DS/HD)

Diskette (floppy disk) 5.25" are no longer in use. It still uses 3.5" floppies. It is popular because it allows easy distribution of data or protective storage (archiving) data, because it is very affordable and inexpensive portable media. In addition to the above format is used with other formats, depending on the type of computer and software support. The above values are typical of a PC. The principle of operation is based on magnetic recording and reading through the two magnetic head which physically adhere to both surfaces of the rotating disk with particle magnetic media on the disk surface. The disk is turning a few hundred rpm. As the magnetization of the medium takes place at ambient temperature with the present mechanical contact of the head to the surface, the stability of long-term data retention is not due to 'forgetting' or damage the media. Fortunately floppies is very inexpensive medium for data storage.

Floppy disk as the medium for data storage is almost at the end of his being in the computing world. But the installation procedure of Windows XP operating system provides for the use of keys <F6> and load the additional driver to a diskette. Yet not so suddenly in 'retirement'. The firm is used to distribute the additional installation drivers (driver), and users still like to use for the distribution or transmission of electronic documents. Her place is slowly but surely taking over the memory stick.

Example VI

Organization Drive capacity 1.44 MB for PC.

 Diskette for FDD  E-Skola (Fizika)
Figure* 3.3.16 Diskette of 3.5" PC system.

Diskette drive stepper motor allows the selection of 80 different circular tracks (TRACK). Each track is divided into 18 clips (SECTOR), each of which contains 512 bytes of data. How to use both sides of the disk, i.e., there are two magnetic heads, the total capacity of the disk would be:

        C = (2 sides)(80 tracks)(18 sectors)(512 byte) =
          = 1'474'560 B = 1'440 kB = 1.44 MB

Therefore, the data is placed in 2'880 blocks of 512 B, each of which has its own unique address (number), and when you save the file to a floppy disk writes in a separate table (FAT - File Allocation Table) in which the block started writing data files in which the blocks continues. This table is in the first few blocks of floppy-disk drive.

Start searching the disk size 5.25" is determined by the optical sensors that register them on a floppy holes marked the position where the head is at the beginning of the disk traces. With floppy 3.5" sensor monitors the position of the motor and the rotor disk is placed on the motor shaft bearing especially with asymmetric discharges to disk relative to the rotor and the engine does not spin in a magnetic tracks always have the same initial position to the rotor regardless of which device the floppy diskette inserted. Standard is thoroughly defined in contrast to a USB stick for example.

Hard disk drive (HDD) is placed in a compact, closed, and a hard case and there are very precise mechanics. It may therefore denser save data from disk and is understandably much greater capacity. Placed inside the computer case, and in principle is not a portable medium. The hard disk is essentially a permanent memory of a computer and work it takes to be constant communication regarding enrollment and reading data. Usually in casing has more plate attached disks on the same axle. Read and record on magnetic disks applied to the surface of the magnetic head performed on both sides of discs that float on a cushion of air over the surface occurred due to high speed drives (about 5000 rev/min and up).

Today's technology allows enough flat surface plate to disk head hovers around 25 nm away from the plate. If one takes into account that the human hair thick around 100'000 nm, and the grain of dust around 1000 nm, it is clear that the technology process of the disk performs exceptionally clean environment. Therefore, the devices are closed and sealed enclosures are well, and often have a 'garbage collector', a small plate with the disc that collects arise through tiny scrap thrown by centrifugal force to the drive plate. Due to the small distance between the head and the plate susceptible to shaking during operation. Should therefore be handled with care especially in performances with removable outer casing. The usual disk diameter is 3.5", while laptops are installed with dimensions smaller units with a diameter disk 2.5", which are the lower capacity, slower and designed with greater resistance to shaking.

The development of technology has enabled the development of disk capacity over 100 MB which can be read in devices that operate on the principles of inter diskette drive (interchangeability) and hard disk (precision), but too expensive for mass use (ZIP drive, etc.). But the development of USB devices in particular 'USB stick' to implement them is out of common use. With 'USB stick' increasing popularity have handy mobile hard drives of 2.5" in the casing with a USB interface, and usually do not need a separate power source, which makes them very popular. Floppy device from 3.5" still installed in personal computers, but it is intended more as a device in this box - 'first aid'. In addition, in support of an earlier program is designed as a device for archiving or sharing and delivering data via floppy disks. Maybe it's just this, and the small cost of the media, the reason for his 'tough' survival.

Example VII

The mechanical performance of the hard disk drive with two plates PC.

 Parts of hard disk - HDD  E-Skola (Fizika)
Figure* 3.3.17 Performance hard drive with multiple rotating plates. ( +/- )

Although the image of the hard disk contains two tables and four heads, the entire surface of the disk can be logically divided into different sections, so for example with a PC hard drive to Figure 3.3.17 can have 8 board with 16 sides that reads 16 heads. Circular disk traces formed in the shape of the body apparently CYLINDER. Some tracks are divided into clips (SECTOR), each of which normally contains 512 bytes (LDS - Log Data Sector) followed by the checksum data to correct errors regarding the proper reading data from the disk. A group of sectors along the cylinder, which are in the same track and the clip but on different surfaces e.g. all above board surface, it forms a larger cluster of data - cluster (CLUSTER), the smallest unit available data sets that the operating system can recognize. The hard disk of 16 sides, 684 cylinders, 38 sectors per cylinder and 8 sectors per cluster has the capacity:

        C = (16 sides)(684 cylinders)(38 sectors)(512 byte) =
          = 212'926'464 B = 207'936 kB = 203.0625 MB

The total number of clusters is:

        Clusters = (684  38  16) / 8 = 51'984

The number of clusters on the disk depends on the characteristics of the OS and BIOS. Maximum number of clusters recorded with 16-bit addressing term data on the disk (FAT or FAT 16) is 65'536 for DOS, Windows 3.11, and Windows 95 Increase disk capacity increases and the size of clusters in steps 0.5 KB, 1 KB, 2 KB, 4 KB, 8 KB, 16 KB and 32 KB. Therefore, the maximum capacity of one logical disk partitions:

        32 kB  65536 = 2'097'152 kB = 2'048 MB = 2 GB

The maximum size of the cluster in NT systems with FAT 16 address scheme is 64 kB and then the maximum capacity of one logical partition (volume) twice.

OS based on a 32-bit disk access (FAT 32), such as Windows 95b, Windows 98, and others, have a cluster size of 0.5-32 KB, depending on capacity, but the total possible number of clusters is much larger and has the highest possible logical disk (volume) size of 32 GB. NTFS file system type in the described brings significant improvements, but more about that in the description of the Windows XP operating system.

At 1.44 MB diskette format and 1.2 MB notion of clusters and sectors of the same, which means that in Figure 3.3.16 is not properly drawn. Error is intentionally designed to be easier to understand the concept of sectors and clusters. The development of technology in hard drives is still quite agile with respect to the occurrence of solid-state drives. Capacities continuously growing, up to 2 terabytes as much as possible to identify 32-bit operating systems are based on accounts with a sector size of 512 B. It was stated that the disc forced producers are increasingly using disk sector size of 4 kB, which reduces the number of sectors on the disk of the same capacity and the space occupied sector checksum data to correct errors in reading (ECC - Error Correction Code / Error Correcting Circuits). Increasing the size of the sector for eight times does not increase so much space and ECC error correction, which means that the useful disk space increases, but most of the disc. ECC algorithm ensures that properly corrects up to several hundreds of illegible bits. It seems that the hard drive is still 'doing well' because of the increasing reliability and huge capacity. Newer operating systems, such as Windows 7, will know to correctly identify the sector increased, while the elderly need a program to 'translate' records (Windows XP) or the new core (Linux). ECC is needed because reading and writing data on a magnetic medium do not makes perfect magnetic poles of zeros and ones.

Drive components, motor disc plate, a system to move the head and running writing and reading data, and other elements (thermal recalibration, cache management, and other), managed by the CONTROLLER. In addition to the above conversion is done divide logical disk into physical division of the disc allowing it to different drives designed to 'out' to see the logical divisions that identify the system and the physical parameters of the disk are of no significance. As a rule, each hard drive has the added cache - buffer SRAM type, where the task is to accelerate the transfer of data from the disk to the computer's bus. Usually the order of a few MB and works on the principle of transferring a certain amount of data from disk to the buffer, which were last in the environment around the retrieved data. This significantly speeds up the reading process. Buffer can be used in the process of writing, but then there is a risk that in the event of a power outage data arrive from the write buffer to disk. Therefore, usually in the BIOS or the OS settings, determines that the buffer is only used in the process of reading the data (write cache disable).

Interface with which the disk is connected to a computer or terminal to its control (controller) on the motherboard, the ATA (Advanced Technology Attachment), SATA (Serial Advanced Technology Attachment) or SCSI (Small Computer Systems Interface) type. ATA is a very old standard specification for IDE (Integrated / Intelligent Drive Electronics), repeatedly promoted, based on the parallel data transfer from disk via wire cable 40 (or 80 if the mass is not common for every line) and signal communication of 5 V, data rates up to 133 Mb/s, as shown in Figure 3.3.17. The compensation was renamed PATA (Parallel) in order to emphasize the difference compared to SATA. Replaced by faster serial communication - SATA, which takes place over 7 lines (4 for communication) signal of 0.5 V, the transfer speed of 1.5 Gb/s (version I), 3.0 Gb/s (version II) and 6.0 Gb/s (version III) with a view to further development. The standard is based on a fast serial transmission via separate receiving and transmitting pairs per 8B/10B coding, i.e. the set of 8-bit and added two more to check the errors, transmission control and synchronization. Newer versions of standards used 128B/130B coding faster thanks to the improvement of the synchronization of data transfer. Convert serial data format on receipt of the IDE specifications ensure compatibility with PATA devices. Of course there is also the power connector. SCSI is described in the next Chapter. Apart from the above computer systems have used other types of sites, but they are mainly described that preserve uniqueness PC computer systems. One of the examples is fresh as ATA interface hard disk size 2.5" and 3.5", which is now reduced to the same form of the introduction of SATA specifications. SATA specification provides for lower voltage devices, and connector for a different power.

SATA NCQ technology means Native Command Queuing, way of reading content from disk plate, which is not based on the sequential collection of data from a set of observed following the trail by a mark on the disc, but the gathering of data from the set that you're encountering when you rotate the disk no matter where the trail. Namely, if one wants to retrieve some data from the disk located in different traces (circles), the disk head does not follow strictly 'circle' so that after reading the first moves into the next circle and beyond, but it moves 'zig-zag' of the circle to circle that they would not wait for a new revolution to retrieve the following information. Of course, it allows for very precise and rapid homing head bracket using the control mechanism (actuator) whose work is based on controlling the magnetic field strength in the coil built-in in the carrier tape heads, and which is located in a strong magnetic field of the permanent control mechanism. Modern electronics much more accurately monitor the position of the magnetic head in this way, compared to the stepper motor device used by the FDD, and much easier to perform temperature compensation in respect of stretching plates drive from warming.


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