【導(dǎo)讀】稱為大容量存儲系統(tǒng)的存儲設(shè)備,包括有磁盤、CD和磁帶。相對于主存儲器，術(shù)語聯(lián)機和脫機通常分別用于描述連接于和沒有連接于計算機的設(shè)備。有該信息的介質(zhì)需要插到某機械裝置里。時間，因為主存儲器的所有工作都由電子器件實現(xiàn)。盤片，盤片上有磁介質(zhì)以儲存數(shù)據(jù)。當(dāng)磁頭移動到新的位置時，新的一組磁道可以存取了。每一組磁道稱為一個柱面。劃分成若干個弧區(qū)，稱為扇區(qū)，記錄在每個扇區(qū)上的信息是連續(xù)的二進制位串。盤系統(tǒng)可能都不相同。磁區(qū)大小一般是不超過幾個KB;512個字節(jié)或1024個。多數(shù)的計算機系統(tǒng)也能執(zhí)行這一個任務(wù)。磁盤能被再格式化，雖然這一過程會破壞所有的先前在磁盤上被記錄的信息。低容量的系統(tǒng)僅有一張塑料盤片組成,稱為軟磁盤或軟盤，另一個名稱是。floppydisk，強調(diào)它的靈活性。因此，硬盤系統(tǒng)出廠前已被密封在盒子里。它平均為盤片旋轉(zhuǎn)。存取時間，尋道時間和等待時間之和。塑料帶的磁介質(zhì)涂層上，而塑料帶則圍在磁帶盤上作為存儲器，要存取數(shù)據(jù)時，

　　

【正文】 erent sectors can be accessed by short movements of the read/write head. In turn, tape systems are not popular for online data storage Instead, magic tape technology is used in offline archival data storage applications where its high capacity, reliability, and cost efficiency are beneficial, although advances in alternative technologies such as DVD are rapidly challenging this last vestige of magic tape. File Storage and Retrieval Information is stored in mass storage systems in large units called files. A typical file may consist of a plete text document, a photograph, a program, or a collection of data about the employees in a pany. The physical properties of mass storage devices dictate that these files be stored and retrieved in multiple byte units. For example, each sector on a magic disk must be manipulated as one continuous string of bits. A block of data conforming to the physical characteristics of a storage device is called a physical record. Thus , a file stored in mass storage will typically consist of many physical records. In contrast to this division into physical records, a file often has natural divisions determined by the information represented. For example, a file containing information regarding a pany’s employees would consist of multiple units , each consisting of the information about one employee. These naturally occurring blocks of data are called logical records. Moreover, logical records often consist of smaller units called field. For example ,a logical record containing information about an employee would probably consist of fields such as name, address, employee identification number, etc Logical record sizes rarely match the physical record size dictated by a mass storage device. In turn, one may final several logical records residing within a singe physical record or perhaps a logical record split between two or more physical records. The result is that a certain amount of unscrambling is associated with retrieving data from mass storage systems. A mon solution to this problem is to set aside an area of main memory that is large enough to hold several physical records and to use this memory space as a regrouping area, That is ,blocks of data patible with physical records can be transferred between this main memory area and the mass storage system, while the data residing in the main memory area can be referenced in terms of logical records. An area of memory used in this manner is called a buffer. In general , a buffer is a storage area used to hold data on a temporary basis, usually during the process of being transferred from one device to another. For example, modern printers contain memory circuitry of their own, a large part of which is used as a buffer for holding portions of a document that have been received by the printer but not yet printed. Finally we should note that main memory, magic disks, pact disks, and magic tape exhibit decreasing degrees of random access to data. The addressing system used in main memory allows rapid random access to individual bytes of data. Magic disks provide random access only to entire sectors of data. Moreover, retrieving a sector involves seek and rotation delays. Compact disks also provide random access to individual sectors, but the delays encountered are greater than those for magic disks due to the additional time required to locate the spiraling track and to adjust the rotation speed of the disk. Finally, magic tape offers little in the way of random access. Modern tape systems mark positions on the tape so that different segments of the tape can be retrieval time for segments far down the tape will be significant.