Chapter-7計算機(jī)科學(xué)技術(shù)史

1、Chapter 7,Peripheral,Peripheral,A computer peripheral is any external device that provides input and output for the computer. For example, a keyboard and mouse are input peripherals, while a monitor and printer are output peripherals. Computer peripherals, or peripheral devices, are sometimes called

2、 I/O devices because they provide input and output for the computer. Some peripherals, such as external hard drives, provide both input and output for the computer.,7.1 Paper Tape,In 1837, the American inventor Samuel Finley Breese Morse developed the first American electric telegraph, which was bas

3、ed on simple patterns of dots and dashes called Morse Code being transmitted over a single wire. The telegraph quickly proliferated(擴(kuò)散) thanks to the relative simplicity of Morses system. However, a problem soon arose in that operators could only transmit around ten words a minute, which meant that

4、they couldnt keep up with the publics seemingly insatiable desire to send messages to each other. This was a classic example of a communications bottleneck.,Uses Paper Tape to Store Data,In 1857, only twenty years after the invention of the telegraph, Sir Charles Wheatstone (the inventor of the acco

5、rdion（手風(fēng)琴）) introduced the first application of paper tapes as a medium for the preparation, storage, and transmission of data.,Uses Paper Tape to Store Data,Sir Charles paper tape used two rows of holes to represent Morses dots and dashes. Outgoing messages could be prepared off-line on paper tape

6、and transmitted later. By 1858, a Morse paper tape transmitter could operate at 100 words a minute.,Uses Paper Tapes as Data Presentation,In a similar manner to Sir Charles telegraph tape, the designers of the early computers realized that they could record their data on a paper tape by punching row

7、s of holes across the width of the tape. The pattern of the holes in each data row represented a single data value or character.,The Original Computer Tapes,The original computer tapes had five channels, so each data row could represent one of thirty-two different characters. However, as users began

8、 to demand more complex character sets, including the ability to use both uppercase characters (A, B, C, .) and their lowercase equivalents (a, b, c, .), the number of channels rapidly increased, first to six and later to eight.,IBMs Tape,This illustration represents one of the more popular IBM stan

9、dards - a one-inch wide tape supporting eight channels (numbered from 0 to 7) with 0.1 inches between the punched holes.,Paper Tape Readers,The first paper tape readers accessed the data by means of springy wires (one per channel), which could make electrical connections to conducting plates under t

10、he tape wherever a hole was present. These readers were relatively slow and could only operate at around fifty characters per second. Later models used opto-electronic（光電子） techniques, in which a light source was placed on one side of the tape and optical cells located on the other side were used to

11、 detect the light and thereby recognize the presence or absence of any holes.,7.2 Disks,There are two basic types of disks: magnetic disks and optical disks. On magnetic disks, data is encoded as microscopic magnetized needles on the disks surface. You can record and erase data on a magnetic disk an

12、y number of times, just as you can with a cassette tape. Optical disks record data by burning microscopic holes in the surface of the disk with a laser. To read the disk, another laser beam shines on the disk and detects the holes by changes in the reflection pattern.,Magnetic Disks,floppy disk : A

13、typical 5-inch floppy disk can hold 360K or 1.2MB (megabytes). 3-inch floppies normally store 720K, 1.2MB or 1.44MB of data. hard disk : Hard disks can store anywhere from 20MB to more than 200GB. Hard disks are also from 10 to 100 times faster than floppy disks.,7.2.1 Floppy Disk,Floppy disk is cal

14、led floppy because it flops if you wave it. Unlike most hard disks, floppy disks (often called floppies or diskettes) are portable, because you can remove them from a disk drive. Disk drives for floppy disks are called floppy drives. Floppy disks are slower to access than hard disks and have less st

15、orage capacity, but they are much less expensive. And most importantly, they are portable.,Three basic types,8-inch: The first floppy disk design, invented by IBM in the late 1960s and used in the early 1970s as first a read-only format and then as a read-write format. The typical desktop/laptop com

16、puter does not use the 8-inch floppy disk. 5-inch: The common size for PCs made before 1987 and the predecessor to the 8-inch floppy disk. This type of floppy is generally capable of storing between 100K and 1.2MB (megabytes) of data. The most common sizes are 360K and 1.2MB.,Three basic types,3-inc

17、h: Floppy is something of a misnomer for these disks, as they are encased in a rigid envelope. Despite their small size, microfloppies have a larger storage capacity than their cousins - from 400K to 1.4MB of data. The most common sizes for PCs are 720K (double-density) and 1.44MB (high-density). Ma

18、cintoshes support disks of 400K, 800K, and 1.2MB.,Comparisons,Following are the three types developed, from newest to oldest, and their raw, uncompressed storage capacity. Housing Capacity Capacity Range Creator 3.5 rigid 1.44MB 400KB - 1.44MB Sony 3.5 rigid 2.88MB (See ED.) IBM 5.25 flexible 1.2MB

19、100KB - 1.2MB Seagate 8 flexible 500KB 100 - 500KB IBM,Images,7.2.2 Hard Disk,The primary computer storage device. Like tape, it is magnetically recorded and can be re-recorded over and over. Disks are rotating platters with a mechanical arm that moves a read/write head between the outer and inner e

20、dges of the platters surface. It can take as long as one second to find a location on a floppy disk to as little as a couple of milliseconds on a fast hard disk.,The disk surface is divided into tracks (circles within circles). The thinner the tracks, the more storage. The data bits are recorded as

21、tiny magnetic spots on the tracks. The smaller the spot, the more bits per inch and the greater the storage.,Tracks are further divided into sectors, which hold a block of data that is read or written at one time; for example, READ SECTOR 782, WRITE SECTOR 5448. In order to update the disk, one or m

22、ore sectors are read into the computer, changed and written back to disk. The operating system figures out how to fit data into these fixed spaces.,The First Hard Disk,In 1956, IBM introduced the RAMAC hard disk with platters 24” in diameter that held the equivalent of 100,000 bytes. In the 1980s, d

23、esktop computer hard disks were introduced with 5MB using 5.25 platters. Todays entry level drives have at least 8,000 times more capacity. Platter size was reduced to 3.5 for desktops, 2.5 for laptops and 1 for handhelds. In 2004, Toshiba introduced the 0.85 drive.,The First Hard Disk,In 1956, IBMs

24、 RAMAC was the first machine with a hard disk, which was extraordinary technology of the times.,The First Desktop Hard Disk,The first hard disk drive for personal computers, introduced in 1979 by Seagate. It was used in drives of 40MB and less and transferred data at 625 KBytes/sec.,Tracks and Secto

25、rs,Tracks are concentric circles on the disk, broken up into storage units called sectors. The sector, which is typically 512 bytes, is the smallest unit that can be read or written.,7.2.3 Removable Cartridge,PocketZip,Jaz,LS-120,HiFD,SyJet,7.3 Keyboard,The main inventor of the first commercial type

26、writer, Christopher Latham Sholes, obviously wished to make their typewriters as fast as possible in order to convince people to use them. However, one problem with the first machines was that the keys jammed when the operator typed at any real speed, so Sholes invented what was to become known as t

27、he Sholes keyboard.,The original Sholes keyboard is interesting for at least two other reasons: first, there was no key for the number 1, because the inventors decided that the users could get by with the letter I; and second, there was no shift key, because the first typewriters could only type upp

28、er case letters.,The First Shift-Key Typewriter,The first shift-key typewriter (in which uppercase and lowercase letters are made available on the same key) didnt appear on the market until 1878, and it was quickly challenged by another which contained twice the number of keys, one for every upperca

29、se and lowercase character.,The Dvorak Keyboard,Almost anyone who spends more than a few seconds working with a keyboard quickly becomes convinced that they could do a better job of laying out the keys. Many brave souls have attempted the task, but few came closer than efficiency expert August Dvora

30、k in the 1930s.,Finding the Keyss Optimal Placement,Dvorak took the opposite tack to Sholes, and attempted to find the optimal placement for the keys based on letter frequency and human anatomy(解剖學(xué)). That is, he tried to ensure that letters which are commonly typed together would be physically close

31、 to each other. The result of these labors was the Dvorak Keyboard, which he patented in 1936.,The Dvorak Keyboard,The Dvorak Keyboard,Unfortunately, Dvorak didnt really stand a chance trying to sell typewriters based on his new keyboard layout in the 1930s. Apart from the fact that existing typists

32、 didnt wish to re-learn their trade, America was in the heart of the depression years, which meant that the last thing anyone wanted to do was to spend money on a new typewriter. In fact, the Dvorak keyboard might have faded away forever, except that enthusiasts in Oregon, USA, formed a club in 1978

33、, and theyve been actively promoting Dvoraks technique ever since.,7.4 Two Types Of Monitors,The common way of classifying monitors is in terms of the type of signal they accept: analog or digital. Nearly all modern monitors accept analog signals, which is required by the VGA, SVGA, and other high-r

34、esolution color standards.,Analog,Analog Also spelled analogue, describes a device or system that represents changing values as continuously variable physical quantities. A typical analog device is a clock in which the hands move continuously around the face. Such a clock is capable of indicating ev

35、ery possible time of day. In contrast, a digital clock is capable of representing only a finite number of times (every tenth of a second, for example). In general, humans experience the world analogically. Vision, for example, is an analog experience because we perceive infinitely smooth gradations

36、of shapes and colors.,Analog,When used in reference to data storage and transmission, analog format is that in which information is transmitted by modulating a continuous transmission signal, such as amplifying a signals strength or varying its frequency to add or take away data. Computers, which ha

37、ndle data in digital form, require modems to turn signals from digital to analog before transmitting those signals over communication lines such as telephone lines that carry only analog signals. The signals are turned back into digital form (demodulated) at the receiving end so that the computer ca

38、n process the data in its digital format.,Digital,Digital describes any system based on discontinuous data or events. Computers are digital machines because at their most basic level they can distinguish between just two values, 0 and 1, or off and on. There is no simple way to represent all the val

39、ues in between, such as 0.25. All data that a computer processes must be encoded digitally, as a series of zeroes and ones. Although digital representations are approximations of analog events, they are useful because they are relatively easy to store and manipulate electronically.,Screen size,The m

40、ost important aspect of a monitor is its screen size. Like televisions, screen sizes are measured in diagonal(對角線) inches, the distance from one corner to the opposite corner diagonally. A typical size for small VGA monitors is 14 inches. Monitors that are 16 or more inches diagonally are often call

41、ed full-page monitors. VGA is abbreviation of video graphics array, a graphics display system for PCs developed by IBM and has become one of the standards for PCs in fact.,Resolution,The resolution of a monitor indicates how densely packed the pixels are. In general, the more pixels (often expressed

42、 in dots per inch), the sharper the image. Most modern monitors can display 1024 by 768 pixels, the SVGA standard. Some high-end models can display 1280 by 1024, or even 1600 by 1200.,Bandwidth,The amount of data that can be transmitted in a fixed amount of time. For digital devices, the bandwidth is usually expressed in bits per second (bps) or bytes per second. For analog