Secondary Storage Media 辅助存储介质 Secondary storage or external memory may sound at first like a misnomer, since there are at least two or three faster forms of memory in every machine. Secondary memory is clearly slower than either registers or main memory, so why not tertiary memory? In practice, registers are not generally thought of as storage devices more like working locations. Main memory is the primary storage location of data during execution of a program. Cache memory [1] is a relatively recent invention (also one that is frequently invisible to the programmer in most models of computation). External memory is secondary to RAM [2] as an important visible depository of data. It sits at one end of the memory hierarchy: usually at least a few orders of magnitude larger than main memory, several orders of magnitude slower, and an order or two less expensive. The first and last of these properties provide its principal advantage; the middle property, its principal disadvantage. Secondary memory has two features that can not be provided by RAM memory as discussed up to this point. First, while RAM memory may not be large enough to hold all of the data that a program needs, secondary storage can be arbitrarily large. One million bytes of RAM sounds like a considerable quantity. However, consider a matrix designed to hold relationships between any two of 5000 data items. One advantage of a computer is that once data have been entered, they can be stored on the machine and accessed repeatedly. Generating address labels for a magazine is a good example. Instead of retyping all the labels for each edition, subscriber data are input once, stored, and then dumped from storage whenever necessary. Programs provide another example. Like the subscriber data, they are stored on the computer and accessed on demand. Where exactly are the data and the programs stored? The obvious answer is main memory, but main memory is expensive, and the supply on most machines is limited. Another problem is its volatility; main memory loses its contents when the power is cut. We need a fast, accurate, inexpensive, high-capacity, nonvolatile extension of main memory. and secondary storage fills this need. Ⅰ. Semiconductor Memory Storage Semiconductor memory uses semiconductor-based integrated circuits to store information. A semiconductor memory chip may contain millions of tiny transistors or capacitors. Both volatile and non=volatile forms of semiconductor memory exist. In modern computers, primary storage almost exclusively consists of dynamic volatile semiconductor memory or dynamic random access memory. Since the turn of the century, a type of non- volatile semiconductor memory known as flash memory has steadily gained share as off-line storage for home computers. Non-volatile semiconductor memory is also used for secondary storage in various advanced electronic devices and specialized computers. Examples of semiconductor memory include static random access memory (RAM), which relies on transistors, and dynamic random access memory, which uses capacitors to store the bits. Flash memory [3] is a non-volatile computer memory that can be electrically erased and reprogrammed. It is a technology that i.s primarily used in memory cards and USB [4] flash drives for general storage and transfer of data between computers and other digital products. It is a specific type of EEPROM [5] (Electrically Erasable Programmable Read- Only Memory) that is erased and programmed in large blocks; in early flash the entire chip had to be erased at once. Flash memory costs far less than byte-programmable EEPROM and therefore has become the dominant technology wherever a significant amount of non- volatile, solid state storage i.s needed. Example applications include PDAs [6] (personal digital assistants), laptop computers, digital audio players, digital cameras and mobile phones. It has also gained popularity in the game console market, where it is often used instead of EEPROMs or battery-powered SRAM [7] for game save data. Flash memory is non-volatile, which means that no power is needed to maintain the information stored in the chip. In addition, flash memory offers fast read access times (although not as fast as volatile DRAM [8] memory used for main memory in PCs) and better kinetic shock resistance than hard disks. These characteristics explain the popularity of flash memory in portable devices. Another feature of flash memory is that when packaged in a "memory card [9] , " it is enormously durable, being able to withstand intense pressure, extremes of temperature, and even immersion in water. U-Disk is a flash memory card and is superior to the similar types of flash memory cards such as Compact Flash, Smart Media and so forth. U-Disk International Association consists of 89 IT companies around the world. With the promotions done by U-Disk International Association, U-Disk will create a new standard in the world' s mini-memory storage market. Ⅱ. Paper Data Storage Paper data storage refers to the storage of data on paper. This includes writing, illustrating, and the use of data that can be interpreted by a machine or is the result of the functioning of a machine. A defining feature of paper data storage is the ability of humans to produce it with only simple tools and interpret it visually. Though it is now mostly obsolete, paper was once an important form of computer data storage. The earliest use of paper to store instructions for a machine was the work of Basile Bouchon [10] who, in 1725, used punched paper rolls to control textile looms. This technology was later developed into the wildly successful Jacquard loom [11] . The 19th century saw several other uses of paper for data storage. In 1846, telegrams could be prerecorded on punched tape. The data tabulation industry and computer revolution of the 20th century led to several more uses of paper as a data storage medium. Hollerith [12] 's company later became IBM and his cards were widely used with computers through the 1970s. Other technologies were also developed that allowed tabulating machines and computers to work with marks on paper instead of punched holes. This technology was widely used for tabulating votes and grading standardized tests. Barcodes made it possible for any object that was to be sold or transported to have some computer readable information securely attached to it. Ⅲ. Magnetic Storage Magnetic storage and magnetic recording are terms from engineering referring to the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetization in a magnetizable material to store data and is a form of non-volatile memory. The information is accessed using one or more read/write heads. As of 2007, magnetic storage media, primarily hard disks, are widely used to store computer data as well as audio and video signals.  In the field of computing, the term magnetic storage is preferred and in the field of audio and video production, the term magnetic recording is more commonly used. The distinction is less technical and more a matter of preference. · Magnetic Cassette The least expensive of the secondary storage media is magnetic cassette, one of the most common backup media. Data are output to a tape recorder. By playing the "recording" back, the material is restored to main memory. Cassettes are inexpensive and compact, but they are also relatively slow and error prone. They are used on some small home computer systems or for archival storage. ·Diskette The most common microcomputer secondary storage medium is diskette or floppy disk, a thin circular piece of flexible polyester coated with a magnetic material. Data are recorded on one or both flat surfaces. Because contact with dust, or even a human finger can destroy the data, each diskette has its own protective jacket. A diskette drive works much like a record turntable. The round hole in the center of the disk allows the drive mechanism to engage and spin it; an access mechanism, analogous to the tone arm [13] , reads and writes the surface through the window visible near the bottom. The data are recorded on a series of concentric circles called tracks. The access mechanism steps from track to track, reading or writing one at a time. The tracks are subdivided into sectors; it is the contents of a sector that move between the diskette and main memory. To distinguish the sectors. they are addressed by numbering them sequentially 0, 1,2, and so on. Although diskette is certainly faster than cassette, data access still means a delay of at least a fraction of a second. Many common personal computer applications involve only limited disk access, so the delay is hardly noticeable. On other applications, however, the delay can be intolerable. The solution is often a hard disk. ·Hard Disk A diskette drive spins only when data are being read or written. The drive must be brought up to operating speed before the read/write heads can be moved and the data accessed, and that [14] takes time. A hard disk, in contrast, spins constantly. Since it is not necessary to wait for the drive to reach operating speed before moving the access mechanism, seek time is significantly reduced, often to a few thousandths of a second. Further improvements are gained by spinning the disk more rapidly (5000 revolutions per minute or more) , which reduces rotational delay. Data stored on hard disk can be accessed far more rapidly than data stored on diskette. Another advantage of hard disk is its storage capacity. A typical double-sided diskette might hold 1440,000 characters. A hard disk for a microcomputer system might store 1 to 10 billion characters. With slow diskette drives, the access mechanism rides directly on the disk surface. At 1000 revolutions per minute, however, any physical contact between the disk surface and the read/write head would quickly destroy both: thus, a hard disk's access mechanism rides on a cushion of air,