計(jì)算機(jī)科學(xué)與技術(shù) 外文翻譯 英文文獻(xiàn) 中英對照

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1、附件1：外文資料翻譯譯文 大容量存儲器 由于計(jì)算機(jī)主存儲器的易失性和容量的限制, 大多數(shù)的計(jì)算機(jī)都有附加的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，大的容量儲存系統(tǒng)的優(yōu)點(diǎn)是易失性小，容量大，低成本, 并且在許多情況下, 為了歸檔的需要可以把儲存介質(zhì)從計(jì)算機(jī)上移開。 術(shù)語聯(lián)機(jī)和脫機(jī)通常分別用于描述連接于和沒有連接于計(jì)算機(jī)的設(shè)備。聯(lián)機(jī)意味著，設(shè)備或信息已經(jīng)與計(jì)算機(jī)連接，計(jì)算機(jī)不需要人的干預(yù)，脫機(jī)意味著設(shè)備或信息與機(jī)器相連前需要人的干預(yù)，或許需要將這個(gè)設(shè)備接通電源，或許包含有該信息的介質(zhì)需要插到某機(jī)械裝置里。 大量儲存器系統(tǒng)的主要缺點(diǎn)是他們典型地需要機(jī)械

2、的運(yùn)動因此需要較多的時(shí)間，因?yàn)橹鞔鎯ζ鞯乃泄ぷ鞫加呻娮悠骷?shí)現(xiàn) 。 1. 磁盤 今天，我們使用得最多的一種大量存儲器是磁盤，在那里有薄的可以旋轉(zhuǎn)的盤片，盤片上有磁介質(zhì)以儲存數(shù)據(jù)。盤片的上面和（或）下面安裝有讀/寫磁頭，當(dāng)盤片旋轉(zhuǎn)時(shí)，每個(gè)磁頭都遍歷一圈，它被叫作磁道，圍繞著磁盤的上下兩個(gè)表面。通過重新定位的讀/寫磁頭，不同的同心圓磁道可以被訪問。通常，一個(gè)磁盤存儲系統(tǒng)由若干個(gè)安裝在同一根軸上的盤片組成，盤片之間有足夠的距離，使得磁頭可以在盤片之間滑動。在一個(gè)磁盤中，所有的磁頭是一起移動的。因此，當(dāng)磁頭移動到新的位置時(shí)，新的一組磁道可以存取了。每一組磁道稱為一個(gè)柱面。 因?yàn)橐粋€(gè)磁道能

3、包含的信息可能比我們一次操作所需要得多，所以每個(gè)磁道劃分成若干個(gè)弧區(qū)，稱為扇區(qū)，記錄在每個(gè)扇區(qū)上的信息是連續(xù)的二進(jìn)制位串。傳統(tǒng)的磁盤上每個(gè)磁道分為同樣數(shù)目的扇區(qū)，而每個(gè)扇區(qū)也包含同樣數(shù)目的二進(jìn)制位。（所以，盤片中心的儲存的二進(jìn)制位的密度要比靠近盤片邊緣的大）。 因此，一個(gè)磁盤存儲器系統(tǒng)有許多個(gè)別的磁區(qū), 每個(gè)扇區(qū)都可以作為獨(dú)立的二進(jìn)制位串存取，盤片表面上的磁道數(shù)目和每個(gè)磁道上的扇區(qū)數(shù)目對于不同的磁盤系統(tǒng)可能都不相同。磁區(qū)大小一般是不超過幾個(gè)KB; 512 個(gè)字節(jié)或 1024 個(gè)字節(jié)。 磁道和扇區(qū)的位置不是磁盤的物理結(jié)構(gòu)的固定部分，它是通過稱為磁盤格式化或初始化形成的，它通常是由磁盤的廠家完

4、成的，這樣的盤稱為格式化盤，大多數(shù)的計(jì)算機(jī)系統(tǒng)也能執(zhí)行這一個(gè)任務(wù)。因此, 如果一個(gè)磁盤上的信息被損壞了磁盤能被再格式化，雖然這一過程會破壞所有的先前在磁盤上被記錄的信息。 磁盤儲存器系統(tǒng)的容量取決于所使用盤片的數(shù)目和所劃分的磁道與扇區(qū)的密度。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(gè)名稱是floppy disk，強(qiáng)調(diào)它的靈活性。 (現(xiàn)在直徑3.5英寸的軟盤封裝在硬的塑料盒子里，沒有繼續(xù)使用老的為5.25英寸的軟盤的柔軟紙質(zhì)包裝）軟盤很容易插入到相應(yīng)的讀寫裝置里，也容易讀取和保存，因此，軟盤通常用于信息的脫機(jī)存儲設(shè)備，普通的3.5英寸軟盤的容量是1.44MB，而特殊的軟盤

5、會有較高的容量，一個(gè)例子是INMEGA公司的ZIP盤，單盤容量達(dá)幾百兆。 大容量的磁盤系統(tǒng)的容量可達(dá)幾個(gè)GB，它可能有5-10個(gè)剛性的盤片，這種磁盤系統(tǒng)出于所用的盤片是剛性的，所以稱為硬盤系統(tǒng)，為了使盤片可以比較快的旋轉(zhuǎn)，硬盤系統(tǒng)里的磁頭不與盤片是表面接觸，而是依靠氣流“浮”在上面，磁頭與盤片表面的間隙非常小，甚至一顆塵粒都會造成磁頭和盤片卡住，或者兩者毀壞（這個(gè)現(xiàn)象稱為劃道）。因此，硬盤系統(tǒng)出廠前已被密封在盒子里。 評估一個(gè)磁盤系統(tǒng)的性能有幾個(gè)指標(biāo)： （1）尋道時(shí)間，讀/寫磁頭從當(dāng)前磁道移到目的磁道（依靠存取臂）所需要的時(shí)間 。 （2）旋轉(zhuǎn)延遲或等待時(shí)間，讀/寫磁頭到達(dá)所要求的磁道

6、后，等待盤片旋轉(zhuǎn)使讀/寫磁頭位于所要存取的數(shù)據(jù)（扇區(qū)）上所需要的時(shí)間。它平均為盤片旋轉(zhuǎn)一圈時(shí)間的一半。 （3）存取時(shí)間，尋道時(shí)間和等待時(shí)間之和。 （4）傳輸速率，數(shù)據(jù)從磁盤上讀出或?qū)懭氪疟P的時(shí)間。 硬盤系統(tǒng)的性能通常大大優(yōu)于軟盤，因?yàn)橛脖P系統(tǒng)里的讀/寫磁頭不接觸盤片表面，所以盤片旋轉(zhuǎn)速度達(dá)到每分種幾千轉(zhuǎn)，而軟盤系統(tǒng)只有每分300轉(zhuǎn)。因此，硬盤系統(tǒng)的傳輸速率通常以每秒MB數(shù)目來標(biāo)稱，比軟盤系統(tǒng)大得多，因?yàn)楹笳邇H為每秒數(shù)KB。 因?yàn)榇疟P系統(tǒng)需要物理移動來完成它的們的操作，因此軟盤系統(tǒng)和硬盤系統(tǒng)都難以與電子工業(yè)線路的速度相比。電子線路的延遲時(shí)間是以毫微秒或更小單位度量的，而磁盤系統(tǒng)的尋道時(shí)間

7、，等待時(shí)間和存取時(shí)間是以毫秒度量的，因此，從磁盤系統(tǒng)檢索信息所需要的時(shí)間與電子線路的等待時(shí)間相比是一個(gè)漫長的過程。 2. 光盤 另一種流行的數(shù)據(jù)存儲技術(shù)是光盤，盤片直徑是12厘米（大約5英寸），由反射材料組成，上面有光潔的保護(hù)層。通過在它們反射層上創(chuàng)建反射偏差的方法在上面記錄信息，這種信息可以借助激光束檢測出來，因?yàn)樵贑D旋轉(zhuǎn)時(shí)激光束監(jiān)視它的反射面上的反射偏差。 CD技術(shù)原來用于音頻錄制，采用稱為CD-DA（光盤數(shù)字音頻）的記錄格式，今天作為計(jì)算機(jī)數(shù)據(jù)存儲器使用的CD實(shí)際上使用同樣的格式。CD上的信息是存放在一條繞著CD的螺旋形的磁道上，很象老式唱片里的凹槽；與老式唱片不同的是，CD

8、上的磁道是從里向外的，這條磁道被分成稱為扇區(qū)的單元。每個(gè)扇區(qū)有自己的標(biāo)識，有2KB的數(shù)據(jù)容量，相當(dāng)于在音頻錄制時(shí)1/75的音樂。 CD上保存的信息在整個(gè)螺旋形的磁道是按照統(tǒng)一的線性刻度，這就意味著，螺旋形磁道靠邊的環(huán)道存放的信息比靠里邊的環(huán)道要多。所以，如果盤片旋轉(zhuǎn)一整圈，那么激光束在掃描螺旋形磁道外邊時(shí)讀到的扇區(qū)個(gè)數(shù)要比里邊多。因而，為了獲得一致的數(shù)據(jù)傳輸速率，CD-DA播放器能夠根據(jù)激光束在盤片上的位置調(diào)整盤片的旋轉(zhuǎn)速度。但是，作為計(jì)算機(jī)數(shù)據(jù)存儲器使用的大多數(shù)CD驅(qū)動器都以一種比較快的、恒定的速度旋轉(zhuǎn)盤片，因此必須適應(yīng)數(shù)據(jù)傳輸速率的變化。 這種設(shè)計(jì)思想就使得CD存儲系統(tǒng)在對付長而連續(xù)的

9、數(shù)據(jù)串時(shí)有最好的表現(xiàn)，如音樂復(fù)制。相反，當(dāng)一個(gè)應(yīng)用需要以隨機(jī)的方法存取數(shù)據(jù)時(shí)，那么磁盤存儲器所用的方法（獨(dú)立的、同心的磁道）就勝過CD所用的螺旋形方法。 傳統(tǒng)CD的容量為600~700MB。但是，較新的DVD的容量達(dá)到幾個(gè)GB。DVD由多個(gè)半透明的層構(gòu)成，精確聚焦的激光可以識別不同的層。這種盤能夠儲存冗長的多媒體演示，包括整個(gè)電影。 3． 磁帶 一種比較老式的大容量存儲器設(shè)備是磁帶。這時(shí)，信息儲存在一條細(xì)薄的的塑料帶的磁介質(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(shí)，磁帶裝到稱為磁帶驅(qū)動器的設(shè)備里，它在計(jì)算機(jī)控制下通?？梢宰x帶，寫帶和倒帶，磁帶機(jī)有大有小，從小的盒式磁帶機(jī)

10、到比較老式的大型盤式磁帶機(jī)，前者稱為流式磁帶機(jī)，它表面上類似于立體聲收錄機(jī)，雖然這些磁帶機(jī)的存儲容量依賴于所使用的格式，但是大多數(shù)都達(dá)幾個(gè)GB。 現(xiàn)代的流式磁帶機(jī)都將磁帶劃分為許多段，每段的標(biāo)記是格式化過程中磁化形成的，類似于磁盤驅(qū)動器。每一段含有若干條縱向相互平行的磁道，這些磁道可以獨(dú)立地存取，因而可以說，磁帶是由許多單獨(dú)的二進(jìn)制位串組成的，好比磁盤的扇區(qū)。 磁帶技術(shù)的主要缺點(diǎn)是：在一條磁帶上不同位置之間移動非常耗費(fèi)時(shí)間，因?yàn)樵诖艓Ь磔S之間要移動很長的磁帶，于是，磁帶系統(tǒng)的數(shù)據(jù)存取時(shí)間比磁盤系統(tǒng)的長，因?yàn)閷τ诓煌纳葏^(qū)，磁盤的讀/寫磁頭只要在磁道之間作短的移動，因此，磁帶不是流行的聯(lián)機(jī)的

11、數(shù)據(jù)存儲設(shè)備，但是，磁帶系統(tǒng)常使用在脫機(jī)檔案數(shù)據(jù)應(yīng)用中，原因是它具有容量大，可靠性高和性價(jià)比好等優(yōu)勢。雖然例如DVD非傳統(tǒng)技術(shù)的進(jìn)展正迅速向這磁帶的最后痕跡提出挑戰(zhàn)。 4. 文件存儲和檢索 在大容量存儲系統(tǒng)中，信息是以稱為文件的大的單位儲存的，一個(gè)典型的文件可以是一個(gè)全文本的資料，一張照片，一個(gè)程序或一組關(guān)于公司員工的數(shù)據(jù)，大容量存儲系統(tǒng)的物理特性表明，這些文件是按照許多字節(jié)為單位存儲的檢索的，例如，磁盤上每個(gè)扇區(qū)必須作為一個(gè)連續(xù)的二進(jìn)制位串進(jìn)行操作，符合存儲系統(tǒng)物理特性的數(shù)據(jù)塊稱為物理記錄，因此存放在大容量存儲系統(tǒng)中的文件通常包含許多物理記錄。 與這種物理記錄劃分相對的是，一個(gè)文件通

12、常有一種由它所表示的信息決定的自然劃分，例如，一個(gè)關(guān)于公司員工信息的文件由許多單元組成，每個(gè)單元由一個(gè)員工的信息組成。這些自然產(chǎn)生的數(shù)據(jù)塊稱為邏輯記錄，其次，邏輯記錄通常由更小的稱為字段的單元組成，例如，包含員工信息的記錄大概由姓名，地址，員工標(biāo)識號等字段組成。 邏輯記錄的大小很少能夠與大容量存儲系統(tǒng)的物理記錄相匹配，因此，可能許多個(gè)邏輯記錄可以存放在一個(gè)物理記錄中，也可能一個(gè)邏輯記錄分成幾個(gè)物理記錄，因此，從大容量存儲系統(tǒng)中存取數(shù)據(jù)時(shí)需要一定的整理工作，對于這個(gè)問題的常用解決方法是，在主存儲系統(tǒng)里設(shè)置一個(gè)足夠大的存儲區(qū)域，它可以存放若干個(gè)物理記錄并可以通過它重新組織數(shù)據(jù)。（以符合邏輯記錄

13、（讀）或物理記錄（寫）的要求）也就是說，在主存儲器與大容量存儲系統(tǒng)之間傳輸?shù)臄?shù)據(jù)應(yīng)該符合物理記錄的要求。同時(shí)位于主存儲器區(qū)域的數(shù)據(jù)按照邏輯記錄可以被查閱。 主存儲器中的這種存儲區(qū)域稱為緩沖區(qū)，通常，緩沖區(qū)是在一個(gè)設(shè)備向另一個(gè)設(shè)備傳輸數(shù)據(jù)時(shí)用來臨時(shí)保存數(shù)據(jù)的，例如，現(xiàn)代的打印機(jī)都有自己的存儲芯片，其大部分的作為緩沖區(qū)，以保存該打印機(jī)已經(jīng)收到但還沒有打印的那部分?jǐn)?shù)據(jù)。 由此可知，主存儲器，磁盤，光盤和磁帶依次表示隨機(jī)存取程度降低的設(shè)備，主存儲器里所用的編址系統(tǒng)可允許快速隨機(jī)地存取某個(gè)字節(jié)。磁盤只能隨機(jī)存取整個(gè)扇區(qū)的數(shù)據(jù)。其次，檢索一個(gè)扇區(qū)涉及尋道時(shí)間和旋轉(zhuǎn)延遲，光盤也能夠隨機(jī)存取單個(gè)扇區(qū)，但是

14、延遲時(shí)間比磁盤長一些，因?yàn)榘炎x/寫頭定位到螺旋形磁道上并調(diào)準(zhǔn)盤片的旋轉(zhuǎn)速度需要的時(shí)間較長，最后，磁帶幾乎沒有隨機(jī)存取的機(jī)制，現(xiàn)代的磁帶系統(tǒng)都在磁帶上做標(biāo)記，使得可以單獨(dú)存取磁帶上指定的段，但是磁帶的物理結(jié)構(gòu)決定了存取遠(yuǎn)距離的段需要花費(fèi)比較多的時(shí)間。 附件2：外文原文（復(fù)印件） Mass Storage Due to the volatility and limited s

15、ize of a computer’s main memory, most computers have additional memory devices called mass storage systems, which include magnetic disks,CDs,and magnetic tapes. The advantages of mass storage systems over main memory include less volatility, large storage capacities, low cost, and in many cases, the

16、 ability to remove the storage medium from the machine for archival purposes. The terms on-line and 0ff-line are often used to describe devices that can be either attached to or detached from a machine. On-line means that the device or information is connected and readily available to the machine w

17、ithout human intervention. Off-line means that human intervention is required before the device or information can be accessed by the machine-perhaps because the device must be turned on, or the medium holding the information must be inserted into some mechanism. A major disadvantage of mass stor

18、age systems is that they typically require mechanical motion and therefore require significantly more time to store and retrieve data than a machine’s main memory, where all activities are performed electronically. Magnetic Disks One of the most common forms of mass storage in use today is the mag

19、netic disk, in which a thin spinning disk with magnetic coating is used to hold data. Read/write heads are placed above and/or below the disk so that as the disk spins, each head traverses a circle, called a track, around the disk’s upper or lower surface. By repositioning the read/write heads, diff

20、erent concentric tracks can be accessed. In many cases, a disk storage system consists of several disk mounted on a common spindle, one on top of the other, with enough space for the read/write heads to slip between the platters In such cases, the read/write heads move in unison. Each time the read

21、/write heads are repositioned, a new set of tracks-which is called a cylinder becomes accessible . Since a track can contain more information than we would normally want to manipulate at any on time, each track is divided into arcs called sectors on which information is recorded as track is divided

22、 into arcs called sectors on which information is record as a continuous string of bits. Each track on a traditional disk contains the same number of sectors, and each sector contains the same number the center of bits. (Thus the bits within a sector are more compactly stored on the track nearer the

23、 center of the disk than those on the tracks near the outer edge.) Thus, a disk storage system consists of many individual sectors, each of which can be sectors per track vary grealy from one disk system to another. Sector sizes tend to be no more than a few KB; sectors of 512 bytes or 1024 bytes a

24、re common. The location of tracks and sectors is not a permanent part of a disk’s physical structure. Instead, they are marked magnetically through a process called formatting (or initializing ) the disk. This process is usually performed by the disk’s manufacturer , resulting in what are known as

25、formatted disks. Most computer systems can also perform this task. Thus, if the format information on a disk is damaged, the disk can be reformatted, although this process destroys all the information that was previously recorded on the disk. The capacity of a disk storage system depend on the numb

26、er of number of disks used and the density in which the tracks and sectors are placed. Lower-capacity systems consist of a single plastic disk known as a diskette or, in those cases in which the disk is flexible, by the less prestigious title of floppy disk. (today’s 3 1／2-inch diameter floppy disks

27、 are housed in rigid plastic cases, which do not constitute as flexible a package as their older 5 1／4-inch diameter cousins that were housed in paper sleeves.) Diskettes are easily inserted and removed from their corresponding read/write units and are easily stored. As a consequence, diskettes are

28、often used for off-line storage of information. The generic 3 1／2-inch diskette is capable of holding 1.44MB of data but nongeneric diskettes are available with much higher capacities. An example is the Zip disk system from Iomega Corporation, which provides storage capacities up to several hundred

29、MB on a single rigid diskette. High-capacity disk systems, capable of holding many gigabytes, consist of perhaps five to ten rigid disks mounted on a common spindle. The fact that the disks used in these systems are rigid leads them to be known as hard-disk systems, in contrast to their floppy coun

30、terparts. To allow for faster rotation speeds, the read/write heads in these systems do not touch the disk but instead “float”just off the surface. The spacing is so close that even a single particle of dust could become jammed between the head and disk surface, destroying both (a phenomenon known a

31、s a head crash), Thus hard-disk systems are housed in cases that are sealed at the factory. Several measurements are used to evaluate a disk system’s performance:(1)seek tome (the time required to move the read/write heads from one rack to another);(2)rotation delay or latency time (half the time r

32、equired for the disk to make a complete rotation, which is the average amount of time required for the desired data to rotate around to the read/write head once the head has been positioned over the desired track); (3)access time (the sum of seek time and rotation delay);and (4)transfer rate (the ra

33、te at which data can be transferred to or from the disk). Hard-disk systems generally have significantly better characteristics than floppy systems. Since the read/write heads do not touch the disk surface in a hared-disk system, one finds rotation speeds of several thousand revolutions per minute,

34、 whereas disks in floppy-disk systems rotate on the order of 300 revolutions per minute. Consequently, transfer rates for hard-disk systems, usually measured in megabytes per second, are much greater than those associated with floppy-disk systems, which tend to be measured in kilobytes per second.

35、Since disk systems require physical motion for their operation, both hard and floppy systems suffer when compared to speeds within electronic circuitry. Delay times within an electronic circuit are measured in units of nanoseconds (billionths of a second) or less, whereas seek times, latency times,

36、and access times, and access times of disk systems are measured in milliseconds (thousandths of a second). Thus the time required to retrieve information from a disk system can seem like an eternity to an electronic circuit awaiting a result. Compact Disks Another popular data storage technolo

37、gy is the compact disk (CD). There disks are 12 centimeters ( approximately 5 inches ) in diameter and consist of reflective material covered with a clear protective coating. Information is recorded on them by creating variations in their reflective surfaces. This information can then be retrieved b

38、y means of a laser beam that monitors irregularities on the reflective surface of the CD as it spins. CD technology was originally applied to audio recording using a recording using a recording format known as CD-DA (compact disk-digital audio), and the CDs used today for computer data storage us

39、e essentially the same format. In particular, information on these CDs is stored on a single track that spirals around the CD like a groove in an old-fashioned record; however, unlike old-fashioned records, the track on a CD spirals from the inside out .This track is divided into units called sector

40、s, each with its own identifying markings and a capacity of 2KB of data, which equates to 1／75 of a second of music in the case of audio recording . Information is stored on a CD at a uniform linear density over the entire spiraled track, which means that more information is stored in a loop arou

41、nd the outer portion of the spiral than in a loop around the inner portion. In turn, more sectors will be read in a single revolution of the disk when the laser beam is scanning the outer portion of the spiraled track than when the beam is scanning the inner portion. Thus, to obtain a uniform rate o

42、f data transfer, CD-DA players are designed to vary the rotation speed depending on the location of the laser beam. However, most CD drives used for computer data storage spin at a faster, constant speed and thus must accommodate variations in data transfer rates. As a consequence of such desig

43、n decision, CD storage systems perform best when dealing with long, continuous string of data, as when reproducing music. In contrast, when an application requires access to items of data in a random manner, the approach used in magnetic disk storage (individual, concentric tracks) outperforms the s

44、piral approach used in CDs. Traditional CDs have capacities in the range of 600 to 700MB. However. Newer DVD (Digital Versatile Disks),which are constructed from multiple, semi-transparent layers that can be distinguished by a precisely focused laser, provide storage capacities on order of severa

45、l GB. Such disks are capable of storing lengthy multimedia presentations, including entire motion pictures. Magnetic Tape An older form of mass storage device uses magnetic tape. Here, information is recorded on the magnetic coating of a thin plastic tape that is wound on a reel for storage. T

46、o access the data, the tape is mounted in a device called a tape drive that typically can read, write, and rewind the rape under control of the computer. Tape drives range in size from small cartridge units, called streaming tape units, which units. Although the capacity of these devices depends on

47、the format used, most can hold many gigabytes. Modern streaming tape systems divide a tape into segments, each of which is magnetically marked by a formatting process similar to that of disk storage devices. Each of these segments contains several tracks that run parallel to one another lengthwis

48、e on the tape. There tracks can be accessed independently, meaning that the tape ultimately consists of numerous individual stings of bits in a manner similar to the sectors on a disk. A major disadvantage of magnetic tape technology is that moving between different positions on a tape can be ver

49、y time-consuming owing to the significant amount of tape that must be moved between the reels, Thus tape systems have much longer data access times than magnetic disk systems in which different sectors can be accessed by short movements of the read/write head. In turn, tape systems are not popular f

50、or on-line data storage Instead, magnetic tape technology is used in off-line 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 magnetic t

51、ape. File Storage and Retrieval Information is stored in mass storage systems in large units called files. A typical file may consist of a complete text document, a photograph, a program, or a collection of data about the employees in a company. The physical properties of mass storage devices

52、dictate that these files be stored and retrieved in multiple byte units. For example, each sector on a magnetic 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

53、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 company’s employees would consist of multiple u

54、nits , 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 f

55、ields 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

56、 more physical records. The result is that a certain amount of unscrambling is associated with retrieving data from mass storage systems. A common 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 r

57、egrouping area, That is ,blocks of data compatible 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 bu

58、ffer. 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 do

59、cument that have been received by the printer but not yet printed. Finally we should note that main memory, magnetic disks, compact disks, and magnetic tape exhibit decreasing degrees of random access to data. The addressing system used in main memory allows rapid random access to individual byte

60、s of data. Magnetic 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 magnetic disks due to the additional

61、time required to locate the spiraling track and to adjust the rotation speed of the disk. Finally, magnetic 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.