计算机科学与技术 外文翻译 英文文献 中英对照

附件1：外文资料翻译译文

大容量存储器

由于计算机主存储器的易失性和容量的限制, 大多数的计算机都有附加的称为大容量存储系统的存储设备, 包括有磁盘、CD 和磁带。相对于主存储器，大的容量储存系统的优点是易失性小，容量大，低成本, 并且在许多情况下, 为了归档的需要可以把储存介质从计算机上移开。

术语联机和脱机通常分别用于描述连接于和没有连接于计算机的设备。联机意味着，设备或信息已经与计算机连接，计算机不需要人的干预，脱机意味着设备或信息与机器相连前需要人的干预，或许需要将这个设备接通电源，或许包含有该信息的介质需要插到某机械装置里。

大量储存器系统的主要缺点是他们典型地需要机械的运动因此需要较多的时间，因为主存储器的所有工作都由电子器件实现。

1. 磁盘

今天，我们使用得最多的一种大量存储器是磁盘，在那里有薄的可以旋转的盘片，盘片上有磁介质以储存数据。盘片的上面和（或）下面安装有读/写磁头，当盘片旋转时，每个磁头都遍历一圈，它被叫作磁道，围绕着磁盘的上下两个表面。通过重新定位的读/写磁头，不同的同心圆磁道可以被访问。通常，一个磁盘存储系统由若干个安装在同一根轴上的盘片组成，盘片之间有足够的距离，使得磁头可以在盘片之间滑动。在一个磁盘中，所有的磁头是一起移动的。因此，当磁头移动到新的位置时，新的一组磁道可以存取了。每一组磁道称为一个柱面。

因为一个磁道能包含的信息可能比我们一次操作所需要得多，所以每个磁道划分成若干个弧区，称为扇区，记录在每个扇区上的信息是连续的二进制位串。传统的磁盘上每个磁道分为同样数目的扇区，而每个扇区也包含同样数目的二进制位。（所以，盘片中心的储存的二进制位的密度要比靠近盘片边缘的大）。

因此，一个磁盘存储器系统有许多个别的磁区, 每个扇区都可以作为独立的二进制位串存取，盘片表面上的磁道数目和每个磁道上的扇区数目对于不同的磁盘系统可能都不相同。磁区大小一般是不超过几个KB; 512 个字节或1024 个字节。

磁道和扇区的位置不是磁盘的物理结构的固定部分，它是通过称为磁盘格式化或初始化形成的，它通常是由磁盘的厂家完成的，这样的盘称为格式化盘，大多数的计算机系统也能执行这一个任务。因此, 如果一个磁盘上的信息被损坏了磁盘能被再格式化，虽然这一过程会破坏所有的先前在磁盘上被记录的信息。

磁盘储存器系统的容量取决于所使用盘片的数目和所划分的磁道与扇区的密度。低容量的系统仅有一张塑料盘片组成,称为软磁盘或软盘，另一个名称是floppy disk，强调它的灵活性。(现在直径3.5英寸的软盘封装在硬的塑料盒子里，没有继续使用老的为5.25英寸的软盘的柔软纸质包装）软盘很容易插入到相应的读写装置里，也容易读取和保存，因此，软盘通常用于信息的脱机存储设备，普通的3.5英寸软盘的容量是1.44MB，而特殊的软盘会有较高的容量，一个例子是INMEGA公司的ZIP盘，单盘容量达几百兆。

大容量的磁盘系统的容量可达几个GB，它可能有5-10个刚性的盘片，这种磁盘系统出于所用的盘片是刚性的，所以称为硬盘系统，为了使盘片可以比较快的旋转，硬盘系统里的磁头不与盘片是表面接触，而是依靠气流“浮”在上面，磁头与盘片表面的间隙非常小，甚至一颗尘粒都会造成磁头和盘片卡住，或者两者毁坏（这个现象称为划道）。因此，硬盘系统出厂前已被密封在盒子里。

评估一个磁盘系统的性能有几个指标：

（1）寻道时间，读/写磁头从当前磁道移到目的磁道（依靠存取臂）所需要的时间。

（2）旋转延迟或等待时间，读/写磁头到达所要求的磁道后，等待盘片旋转使读/写磁头位于所要存取的数据（扇区）上所需要的时间。它平均为盘片旋转一圈时间的一半。

（3）存取时间，寻道时间和等待时间之和。

（4）传输速率，数据从磁盘上读出或写入磁盘的时间。

硬盘系统的性能通常大大优于软盘，因为硬盘系统里的读/写磁头不接触盘片表面，所以盘片旋转速度达到每分种几千转，而软盘系统只有每分300转。因此，硬盘系统的传输速率通常以每秒MB数目来标称，比软盘系统大得多，因为后者仅为每秒数KB。

因为磁盘系统需要物理移动来完成它的们的操作，因此软盘系统和硬盘系统

都难以与电子工业线路的速度相比。电子线路的延迟时间是以毫微秒或更小单位度量的，而磁盘系统的寻道时间，等待时间和存取时间是以毫秒度量的，因此，从磁盘系统检索信息所需要的时间与电子线路的等待时间相比是一个漫长的过程。

2. 光盘

另一种流行的数据存储技术是光盘，盘片直径是12厘米（大约5英寸），由反射材料组成，上面有光洁的保护层。通过在它们反射层上创建反射偏差的方法在上面记录信息，这种信息可以借助激光束检测出来，因为在CD旋转时激光束监视它的反射面上的反射偏差。

CD技术原来用于音频录制，采用称为CD-DA（光盘数字音频）的记录格式，今天作为计算机数据存储器使用的CD实际上使用同样的格式。CD上的信息是存放在一条绕着CD的螺旋形的磁道上，很象老式唱片里的凹槽；与老式唱片不同的是，CD上的磁道是从里向外的，这条磁道被分成称为扇区的单元。每个扇区有自己的标识，有2KB的数据容量，相当于在音频录制时1/75的音乐。

CD上保存的信息在整个螺旋形的磁道是按照统一的线性刻度，这就意味着，螺旋形磁道靠边的环道存放的信息比靠里边的环道要多。所以，如果盘片旋转一整圈，那么激光束在扫描螺旋形磁道外边时读到的扇区个数要比里边多。因而，为了获得一致的数据传输速率，CD-DA播放器能够根据激光束在盘片上的位置调整盘片的旋转速度。但是，作为计算机数据存储器使用的大多数CD驱动器都以一种比较快的、恒定的速度旋转盘片，因此必须适应数据传输速率的变化。

这种设计思想就使得CD存储系统在对付长而连续的数据串时有最好的表现，如音乐复制。相反，当一个应用需要以随机的方法存取数据时，那么磁盘存储器所用的方法（独立的、同心的磁道）就胜过CD所用的螺旋形方法。

传统CD的容量为600~700MB。但是，较新的DVD的容量达到几个GB。DVD由多个半透明的层构成，精确聚焦的激光可以识别不同的层。这种盘能够储存冗长的多媒体演示，包括整个电影。

3．磁带

一种比较老式的大容量存储器设备是磁带。这时，信息储存在一条细薄的的塑料带的磁介质涂层上，而塑料带则围在磁带盘上作为存储器，要存取数据时，

磁带装到称为磁带驱动器的设备里，它在计算机控制下通常可以读带，写带和倒带，磁带机有大有小，从小的盒式磁带机到比较老式的大型盘式磁带机，前者称为流式磁带机，它表面上类似于立体声收录机，虽然这些磁带机的存储容量依赖于所使用的格式，但是大多数都达几个GB。

现代的流式磁带机都将磁带划分为许多段，每段的标记是格式化过程中磁化形成的，类似于磁盘驱动器。每一段含有若干条纵向相互平行的磁道，这些磁道可以独立地存取，因而可以说，磁带是由许多单独的二进制位串组成的，好比磁盘的扇区。

磁带技术的主要缺点是：在一条磁带上不同位置之间移动非常耗费时间，因为在磁带卷轴之间要移动很长的磁带，于是，磁带系统的数据存取时间比磁盘系统的长，因为对于不同的扇区，磁盘的读/写磁头只要在磁道之间作短的移动，因此，磁带不是流行的联机的数据存储设备，但是，磁带系统常使用在脱机档案数据应用中，原因是它具有容量大，可靠性高和性价比好等优势。虽然例如DVD 非传统技术的进展正迅速向这磁带的最后痕迹提出挑战。

4. 文件存储和检索

在大容量存储系统中，信息是以称为文件的大的单位储存的，一个典型的文件可以是一个全文本的资料，一张照片，一个程序或一组关于公司员工的数据，大容量存储系统的物理特性表明，这些文件是按照许多字节为单位存储的检索的，例如，磁盘上每个扇区必须作为一个连续的二进制位串进行操作，符合存储系统物理特性的数据块称为物理记录，因此存放在大容量存储系统中的文件通常包含许多物理记录。

与这种物理记录划分相对的是，一个文件通常有一种由它所表示的信息决定的自然划分，例如，一个关于公司员工信息的文件由许多单元组成，每个单元由一个员工的信息组成。这些自然产生的数据块称为逻辑记录，其次，逻辑记录通常由更小的称为字段的单元组成，例如，包含员工信息的记录大概由姓名，地址，员工标识号等字段组成。

逻辑记录的大小很少能够与大容量存储系统的物理记录相匹配，因此，可能许多个逻辑记录可以存放在一个物理记录中，也可能一个逻辑记录分成几个物理记录，因此，从大容量存储系统中存取数据时需要一定的整理工作，对于这个

问题的常用解决方法是，在主存储系统里设置一个足够大的存储区域，它可以存放若干个物理记录并可以通过它重新组织数据。（以符合逻辑记录（读）或物理记录（写）的要求）也就是说，在主存储器与大容量存储系统之间传输的数据应该符合物理记录的要求。同时位于主存储器区域的数据按照逻辑记录可以被查阅。

主存储器中的这种存储区域称为缓冲区，通常，缓冲区是在一个设备向另一个设备传输数据时用来临时保存数据的，例如，现代的打印机都有自己的存储芯片，其大部分的作为缓冲区，以保存该打印机已经收到但还没有打印的那部分数据。

由此可知，主存储器，磁盘，光盘和磁带依次表示随机存取程度降低的设备，主存储器里所用的编址系统可允许快速随机地存取某个字节。磁盘只能随机存取整个扇区的数据。其次，检索一个扇区涉及寻道时间和旋转延迟，光盘也能够随机存取单个扇区，但是延迟时间比磁盘长一些，因为把读/写头定位到螺旋形磁道上并调准盘片的旋转速度需要的时间较长，最后，磁带几乎没有随机存取的机制，现代的磁带系统都在磁带上做标记，使得可以单独存取磁带上指定的段，但是磁带的物理结构决定了存取远距离的段需要花费比较多的时间。

附件2：外文原文（复印件）

Mass Storage

Due to the volatility and limited size 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 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 without 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 storage 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 magnetic 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, different 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/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 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 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 are common.

The location of tracks and sectors is not a p ermanent 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 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 number 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 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 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 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 counterparts. 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 as 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 required 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 rate 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, 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.

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, 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 technology 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 by 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 use 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 sectors, 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 around 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 of 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 design 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 spiral 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 several 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. To 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 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 lengthwise 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 very 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 for 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 tape.

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 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 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 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 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 regrouping 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 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, 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 bytes 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 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.