Limited equipment used to compensate for the capacity of computer main memory
Mass storage is to make up for computersMain memoryLimited capacity, but configured with large capacityAuxiliary memory。It mainly includes disk, tape and optical disk.
mass storage；mass memory；bulk storage；mass storage device; bulk memory; large memory；large capacity storage；massstorage device
Advantages and disadvantages
Announce
edit
The advantage of mass storage system over main memory is that it is nonvolatile and has large storage capacity. In many cases, the storage medium can be removed from the machine and saved as archives.
capacitystorageThe main disadvantage is that they generally need mechanical movement, and computers that are all electrically operatedMain memoryIn contrast, they have longer response times.
Disk drive
Announce
edit
Disk drive is the most common type of large capacity memory, and its internal magnetic coating, rotating thin disk is used to save data.The read/write head is placed above or below the disk so that when the disk rotates, each head rotates around the upper or lower surface of the diskTrack(Track).By changing the position of the read/write head, different concentric tracks can be accessed.In many cases, oneDisk storageThe system is composed of several discs installed on a common rotating shaft, one on the top of the other, with enough space left between them for the read/write head to slide.In this case, these read/write heads move synchronously.Each time the read/write head changes position, a new set of tracks (calledcylinderDylinder) becomes accessible.
A track contains more information than is routinely processed at any one timeTrackIs divided into arc segments (calleda sectorSector），The information on the sector is recorded as a continuous string of bits (Figure 1).In a disk system, each track contains the same number of sectors, and each sector contains the same number of bits.This means that the bits in the sector on the track near the center of the disk are denser than those on the outer track.
A disk storage system is composed of many separate sectors, and each sector can be accessed as an independent bit string.With different disk systems, the number of tracks on each disk and thesector numberThere are great changes.The capacity of a sector tends to be no more than a few kB. A 512 byte or 1024 byte sector is common.
Tracks and sectors are located on the diskPhysical structureChina is not a permanent part.They are magnetically positioned through a process called formatting (or initializing) the disk.The result of this process is a formatted disk.Most computer systems can accomplish this task.Therefore, if the formatting information on the disk is damaged, the disk can be reformatted. After this process, all the information previously recorded on the disk is eliminated.
The information on the disk is accessed in blocks. An information block can be aa sectorOr multiple sectors.Disk drive andMain memoryWhen exchanging information, the disk surface number, track number, sector number of the disk and the length of the access information block should be given. These parameters are actually accessDisk storageAddress of the.In this way, when the host accesses the disk storage, first start the read/write head on the disk according to the given disk number, and thenStepper motorMove the above head to the givenTrackThe disk rotates under the drive of the drive motor on track Noa sectorWhen sector No. enters below (or above) the above head, the head can be used to access information until all information of a given length is accessed.
Record density is also calledStorage density, for general useTrack densityAnd bit density.Track density refers to the number of tracks per unit length along the radius direction of the disk, and its unit is track/inch.Bit densityRefers to the edgeTrackThe number of bits contained in the direction and unit length, in bits/inch.
② Storage capacity
The storage capacity of a disk drive refers to the total amount of useful information it can store, and its unit is Byte.The storage capacity can be calculated by the following formula:
Addressing time refers to the total time for the head to reach the required read/write position from the start position, includingSeek time(Seek time) andAverage waiting time(Rotation delay or latency time).Seek time refers to the purpose of the headTrackTime required;The average waiting time refers to the rotation of the required read/write sectors toheadUsed belowaverage time。Because the time spent by the head waiting for different sectors is different, the average waiting time is half of the time spent by the disk turning a week.Obviously, the average waiting time is related to the rotational speed of the disk. The seek time is determined by the performance of the disk and is given by the disk manufacturer.
Data transmission rate refers to the number of bytes read or written per second after the head finds the address.Disk storageThe system is divided into hard disk drive andfloppy diskMachine.The hard disk used in the hard disk drive is based on an aluminum alloy disk, whose surface is coated withmagnetic material 。Most of the read/write heads on the hard disk are floating, which can move radially along the disk surfaceheadThe distance from the disk surface even dust particles are not allowed to enter, so its shockproof and dustproof are very important to prevent the disk scratching accident caused by the contact between the head and the high-speed rotating disk.The common hard disks areWinchesterDisk (short forTemperature disk）Its diameters are 14 inches, 8 inches, 5.25 inches and 3.5 inches.The warm disk is made of hard disk pack, magnetic head pack, motor and drive parts in a sealed box, so it has small size, light weight, good dust resistance, and large hard disk capacity (up to gigabitbytegigabyte）、Access speedFast (disc speed can reach more than 2400r/min).andfloppy diskFloppy disk is a disk made of plastic disk as the substrate and coated with magnetic materials on the upper and lower sides.It is soft and is enclosed in a special plastic or paper case.The commonly used 3.5-inch is packaged in a plastic box with a capacity of 1.44MB. The 8-inch and 5.25-inch paper bags are outdated.There is also a kind of Zip disk, whose capacity can reach several hundred MB in a single box.
Optical disk storage
Announce
edit
Optical disk is a storage medium that uses optical and electrical principles to read/write information.It is composed ofReflective materialIt is made by making some changes on its surface to store information.When the disc rotates, the laser beam on the top shines on the reflective surface of the stored information. According to the intensity change of the generated reflective light, the stored information can be identified, so as to achieve the purpose of reading the information on the disc.And on diskTrackThe information of the disc is recorded on a single spiral track, which extends from the inside to the outside of the disc.This track is also divided intoa sectorEach sector is 2kB.
① CD-ROM（CD-ROM：Compact Disk-Read Only Memory）。The disc of this kind of optical disk memory is pre written by the manufacturer to program or data, which can only be read by the user but not written or modified.
② CD WORM: Compact Disk Write Once Read Many.The disc of this kind of optical disc memory can be written by the user, but only once.After writing, the information will be permanently saved on the disc and can be read out for many times, but cannot be rewritten or modified.
The two above are differentOptical disk storageThe principle of reading/writing is: when writing information, focus the laser beam into a laser beam with a diameter of no more than 1 μ mrecording medium It is heated locally to melt the medium to form a small pit, which changes the optical characteristics;When reading out the information, the photoelectric inspection circuit aims at the light according to the medium that has been burned by the laser and the medium that has not been burnedreflectivityDifferent, the stored information can be read out.
③ Rewritable optical disk memory.
This kind of optical disk memory is similar to the disk, which can be read and written repeatedly. The principle of writing and reading information varies with the media materials used.For example, the principle of recording information with magneto-optical materials is to record information by changing the direction of the local magnetic field on the medium with the thermal effect of the laser beam, and then read the information using the magneto-optical effect.
Optical disk storage has the following outstanding advantages:
First, the storage capacity is large, such as one pieceCD-ROMThe format disc can store 650MB information, and oneDVD formatIts capacity can reach the level of 10GB.Therefore, such CDs are particularly suitable for multimedia applications.If you use a DVD disc, you can store a whole movie.
Second, it is highly reliable. For example, it is almost impossible to lose the information on non rewritable optical disks (CD-ROM, CD-WORM), which is especially suitable for file management.
