In a narrow sense, secondary storage is the hard disk we usually talk about.Scientifically speakingExternal memory(It needs to exchange data with the I/O system, also known asAuxiliary memory）。
Large storage capacity, low cost, slow access speed, and can permanently save information offline.It mainly includes magnetic surface memoryFloppy disk storage, tape storage device, optical disk storage device.
Magnetic surface memory
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The advantages of magnetic surface memory are large storage capacity, low unit price, reusable recording media, long-term storage of recorded information without loss, and even offline archiving, non-destructive readout, which does not require regenerative information.Of course, magnetic surface memory also has disadvantages, mainly because of its slow access speed, complex mechanical structure and high requirements for the working environment.Because of its large storage capacity and low unit cost, magnetic surface memory is mostly used incomputer systemAs an auxiliarya large capacity memoryUsed to store a large number of programs and data such as system software, large files and databases.
Magnetic surface memory can be divided into tape memory andDisk storageTwo categories.Tape storageIt is a sequential access device,Access timeLong, but with large storage capacity, easy to carry and low price, it is a main auxiliary memory.The contents of the tape areTape driveTo read and write, pressTape driveThe reading and writing modes of are divided into start stop anddata streamThere are two types.
·Storage density.Storage density Channel density, bit density and area density.Track density refers todiskOn unit length in radius directionTrackNumber, unit: track/inch.Bit density isTrackRecordable per unit lengthBinary codeNumber of digits.The unit is bit/inch.Area density is the product of bit density and trace density, and the unit is bit/square inch.
·Storage capacity.OneDisk storageWhat can be storedbyteThe total number, called the storage capacity of disk storage.Storage capacity includes formatted capacity andUnformatted capacityPoints.Formatted capacityIt refers to the total amount of information that can be stored according to a specific record format, that is, the capacity that users can actually use.Unformatted capacity is the total number of magnetizing units available on the magnetic recording surface.When disk storage is used in a common computer system, it must be formatted before users can record information.Formatted capacity is generally 60%~70% of unformatted capacity.In the early 21st century, 3.5-inch hard disk drivecapacityUp to 2000 GB.
·AverageAccess time。Access timeIt refers to the time after the read/write command is issued from.The time required for the head to move from a starting position to a new recording position and start reading or writing information from the disc table.This period is determined by two values, one is to position the head to the required positionTrackThe time required is called positioning time orSeek time；The other is the time between the completion of the seek and the arrival of the information to be accessed on the track under the head, which is called the waiting time. These two times are random, so they are often represented by an average value.averageAccess timebe equal toAverage seek timeAndAverage waiting timeThe sum of.Average seek timeIt is the average of the maximum seek time and the minimum seek time. At the beginning of the 21st century, the average seek time is 10 ms~20 ms.Average waiting timeIt is related to the speed of the disk. It is expressed as half of the time required for the disk to rotate for one cycle.fixeddiskThe speed is up to 15000 rpm, soAverage waiting time5 ms.
·Data transmission rate。Disk storageThe number of bytes of data transmitted to the host in unit time, calledData transmission rateThe transmission rate is related to the storage device and host interface logic.Considering the host interface logic, it should be fast enoughTransmission speedReceive/send information to the device.Considering the storage device, assume that the disk rotation speed is n revolutions/secondTrackIf the capacity is N bytes, thenData transmission rateDr=n N（Bps）。It can also be written as Dr=D × v (Bps), where D is the bit density and v is the linear speed of disk rotation.The development of disk memory in the early 21st centuryData transmission rateUp to tens of megaBps.
Floppy disk storage
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Short name of floppy disk memoryfloppy diskBecause the disc is made of flexible material similar to plastic film record.The floppy disk memory is composed offloppy disk driver, floppy disk controller and floppy disk slice.
softDisk storageThe storage principle and recording mode are basically the same as those of hard disk storage, but there are some differences in structure and performance.
·Hard disk speedHigh, up to 15000 revolutions per minute, fast access speed;The speed of the floppy disk is low, only 300 revolutions per minute, and the access speed is slow.
·Hard disk has fixed head, fixed disk, disk group and other structures;floppy diskAll of them are movable heads with replaceable disc structure.
·The hard disk is read and written by floating head, and the head does not contact the disc;floppy diskThe magnetic head is a contact type read-write device.
·Hard disk systems and hard disk chips are expensive, and most disks cannot be interchanged;floppy diskLow cost, convenient storage, flexible use, and interchangeability.
·Hard disks are demanding to the environment, and should have ultra clean measures;Floppy disks are less strict with the environment.
Depending on the size of the disks used, there are 5.25-inch, 3.5-inch and 2.5-inch floppy disk drives.In terms of internal structure, it can be divided into double-sided double density, double-sided high density, etc. according to the different recording densities used.
The shape of a floppy disk is similar to that of an ordinary thin film disc.When in use, the floppy disk is inserted into the floppy disk machine together with the protective sleeve, and the drive mechanism drives the floppy disk sheet to rotate at a constant speed (the protective sleeve does not move). The magnetic head contacts the recording area on the disk sheet through the slot hole to read or write information.
In order to store information correctly, the disc must be divided intoTrackanda sector, they are calleddiskAddress.These information must be written on the disk, and the synchronization flag, verification information, interval, etc. must be added.These information together constitutedisk"Soft segmentation" information of.The so-called "soft segmentation" is to use the "index hole" as the positioning benchmarka sectorThe partition of is realized by the flag written by the software.The "index hole" is used to detect the rotation speed of the disc and divide thea sectorSection.When the hole on the disc turns to the position opposite to the hole on the plastic envelope,floppy diskThe sensor element on the machine can measure a pulse signal, which is used as the starting sign of the disc rotation cyclea sectorThe starting point of the division.Discs shall be processed before delivery“Preformat”, that is, the "soft segmentation work" is completed.The user then reformats (or initializes) with a formatter according to different models and operating systems.
Note that eachTrackuppersector numberThey are the same, but because the circumference of the inner track is smaller than that of the outer track, and they record the same amount of information, the bit density of the inner track is higher than that of the outer track.EachTrackHow much to dividea sector, depending on the record format.secondly,TrackThe address of is numbered from outside to inside, the outermost is 0, and the innermost is n.There are 40 5.25 inch discsTrackAnd 80 tracks. There is only one type of 80 tracks for 3.5-inch disks.
One dual density disc is equivalent to two single density discs. The single density is recorded by FM, and the dual density is recorded by MFM.According to the difference of recording surface and density,8 inch floppy diskThere are four forms: single-sided single density (SS, SD), double-sided single density (DS, SD), single-sided double density (SS, DD) and double-sided double density (DS, DD). Double densityfloppy diskBoth are marked with 2D or DD marks.Both 5.25-inch and 3.5-inch floppy disks are double-sided dual density and high-density (4x density, QD).
floppy diskThe magnetic head and the disk surface are contact read-write, and there is not only radial but also circumferential friction when addressing.When a disc rotates for more than 106 times, it shall be scrapped to avoid exceeding the specified bit error rate.
floppy disk driverIt is mainly composed of driving mechanism, magnetic head, positioning mechanism and read-write circuit.floppy diskThe controller interprets commands from the host and sends them tofloppy disk driverSend variouscontrol signalAt the same time, it also needs to detect the state of the drive, and read and write data to the drive according to the specified data format.
Tape storage device
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Tape driveIts storage principle is basically the same as that of a magnetic disk drive, except that its magnet is a ribbon plastic, called a tape.When writing, the information code can be recorded on the magnetic tape through the magnetic head.When the tape with code is moving under the head, the electromotive force is sensed on the head coil to read the information code.Tape storage devices areTape driveAnd tape.
There are many kinds of tapes. PressbandwidthThere are 1/4 inch and 1/2 inch;2400 feet, 1200 feet and 600 feet according to the belt length;According to the shape, it can be divided into open reel tape and cassette tape;According to the recording density, there are 800 bits/inch, 1600 bits/inch and 6250 bits/inch;Parallel recording by stripTrackThere are 9, 16 and so on.computer systemThe 1/2-inch open reel tape and 1/4-inch cartridge tape are mostly used in. They are standard tapes.
Tape driveThere are also many kinds.PressTape driveIn terms of scale, there are standard 10 inch tape drives, cartridge tape drives and massive wide tape storage.PressTape driveThere are high-speed tape drives (4m/s~5m/s), medium speed tape drives (2m/s~3m/s) and low-speed tape drives (below 2m/s).Tape drive'sData transmission rateIs c=d × v, where d is the recording density and v is the belt speed.Fast belt speed leads to high transmission rate.According to the tape recording format, there are start stop and data stream.
Tape driveIn order to find the recording area, the tape must be driven forward or backward, and the head must stop between the two recording areas after reading and writing.Therefore, it is requiredTape driveCorresponding measures shall be taken in the structure and circuit to ensure that the tape moves in a balanced way at a certain speed and starts and stops quickly.
The data flow is briefly described belowTape driveStructure of.data streamTape driveIt is to write data on tape continuouslydata blockA recording gap is inserted between the data blocks so that the tape drive does not start or stop between data blocks.It uses electronic control instead of mechanical control, thus simplifyingTape driveThe structure reduces the cost and improves the reliability.
data streamTape driveThere are 1/2-inch open disk and 1/4-inch box.The structure of the cassette tape is similar to that of the audio tape and video tape. The cassette tape is equipped with a supply reel and a receiving reel. The length of the tape is mainly 450 feet and 600 feet, and the capacity is 45 MB and 60 MB respectively.
data streamTape driveThe read/write mechanism of is different from the start/stop tape drive. The latter is multi bit parallel read/write, while the former is similar todiskBecause of their serial reading and writing mode, they have different recording formats.
CD storage device
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The optical disc uses a focused laser beam to record the high-density information on the disc media in a non-contact manner, and the changes in the optical properties of the media material (such as reflectivity and polarization direction) represent the 1 or 0 of the stored information.Its outstanding advantage is that the laser can be focused to less than 1 μ m, so the recorded area density can reach 645 Mb/square inch, which is higher than the general magnetic recording level.The storage capacity of a CD-ROM disk can reach 600 MB, which is equivalent to more than 400 1.44 MB 3.5-inch floppy disks.The disadvantages of optical disks are long access time and low data transmission rate.
According to the nature of reading and writing, optical disks are divided into three types: read-only, primary, and rewritable.
·CD-ROM
Read only discs are produced by manufacturers at high costMasterAfter a large number of re pressed out of the disc.This kind of molded recording makes the optical disc undergo permanent physical changes, and the recorded information can only be read out and cannot be modified.Typical products are as follows.
► LD: commonly known as video disk, recordingAnalog videoAnd audio information, can show 60 minutes of full bandwidth PAL TV.
► CD-DA: digital turntable, recording digital audio information, which canstorage74 minute digital stereo information.
► VCD: commonly known as small video disc, it records digital video and audio information.It can store dynamic image information compressed and encoded according to MPEG1 standard for 74 minutes.
► DVD: digital video disc.The single recording layer has a capacity of 4.7 GB, and can store 135 minutes of video image information and audio information compressed and encoded according to MPEG2 standard, equivalent to high-definition television.
► CD-ROM: mainly used as a computerExternal memory。recordabledigital dataIt can also record digital video and audio information at the same time.
·Primary disc
Users can record information on such discs, but the recorded information will permanently change the physical consistency of the media, so it can only be written once.The written information cannot be changed, but can only be read.A typical product is CD-R discs.Users can write data to a blank CD-R disc on a dedicated CD-R recorder, and the created CD-R disc can be placed on a CD-ROMDriverRead in.
·Rewritable disc
Users can randomly write, erase or rewrite information on such discs.
CD-ROM is very suitable for distributing large quantities of data to a large number of usersDriverIt has developed from a single speed (150KBps) optical drive to a 40 speed (6000 KBps) optical drive. The diameter of the optical disc is usually 5.25 inches. The density ratio of optical disc information recording isdiskHigh, with a storage capacity of approximately 650 MB.Erasable discs can be read and written repeatedly, just like ordinary discsdisk。
