通曉信息技術(shù)技能、概念與能力教參chapter

1、Learning ObjectivesExplain what a software stack represents and how it is usedDescribe how the Fetch/Execute Cycle works, listing the five stepsUnderstand the function of the memory, control unit, arithmetic/logic unit (ALU), input unit and output unit, and a program counterDiscuss the purpose of an

2、 operating systemExplain the purpose of a compilerDescribe how large tasks are performed with simple instructionsExplain why integration and photolithography are important in integrated circuitsComputer OverviewComputers are used throughout your daywe are continually using computationThey are laptop

3、s, cameras, tablets, iPods, desktops, GPS navigators, TVs and all the other electronic devices that we use9-*Cast of CharactersProcessor: follows the programs instructionsOperating System: program that performs common operations, and makes your computer a useful deviceSoftware: programs Instructions

4、: tell the processor what to doFetch/Execute Cycle: executes the instructionsMemory: stores the dataHardware: the physical parts of the computer9-*SoftwareWhen you get a new app it is really a long sequence of bitsA team of programmers created the bits but did not type them one by oneThe bits were p

5、roduced from a program as discussed later in the chapterThe program is a series of instructions expressing an algorithmLines are sparse, with a few symbols eachSome regular English words name available facilitiesSometime youSeeWordsRunTogether9-*SoftwareA programs meaning may not usually be clear wi

6、thout specialized trainingEven so, you might be able to guess that this instruction in a splash image display programif (this.Opacity 1.0) this.Opacity += 0.02;will make something more opaque if its at least partially transparent The smallest typo can create a bug9-*Software LayersSystem software is

7、 arranged in layersEach layer uses services from the layer belowEach layer provides services to the layer aboveShared effort: new programs reuse existing workOnce written, a feature can be used in many applications.Consistent behaviorTo fix a bug or make an improvement, only one place to change9-*In

8、struction Execution EngineA computer constantly executes instructionsIt does this by cycling through a series of operationsSeries is called: Fetch/Execute CycleGet the next instructionFigure out what to doGather the data needed to do itDo itSave the result, and Repeat (billions of times/second)!A Fi

9、ve-Step CycleThese operations are repeated in a never-ending sequenceThe step names suggest the operations described in the previous slideAnatomy of a ComputerAll computers, regardless of their implementing technology, have five basic parts or subsystems:Memory, Control unit, Arithmetic/logic unit (

10、ALU),Input unit, and Output unitPrincipal Subsystems of a Computer1. MemoryMemory stores both the program while it is running and the data on which the program operatesProperties of memory:Discrete locationsMemory is organized as a sequence of discrete locationsIn modern memory, each location is com

11、posed of 1 byte (8 bits)1. MemoryAddressesEvery memory location has an address, whole numbers starting at 0ValuesMemory locations record or store valuesFinite capacityMemory locations have a finite capacity (limited size),Data may not “fit” in the memory locationByte-Size Memory LocationCommon visua

12、lization of computer memoryDiscrete locations are shown as boxes holding 1-byte eachAddress of location is displayed above the box and the contents of location is shown in the boxByte-Size Memory LocationThat 1-byte memory location can store one ASCII character or a number less than 256Blocks of fou

13、r bytes are used as a unit so often that they are called memory wordsRandom Access MemoryComputer memory is called random access memory (RAM)“Random access” is out-of-date and simply means that the computer can refer to the memory locations in any orderRAM is measured in megabytes (MB) or gigabytes

14、(GB)Lots of memory is need to provide the space required for programs and data2. Control UnitThe control unit of a computer is where the Fetch/Execute Cycle occursIts circuitry fetches an instruction from memory and performs the other operations of the Fetch/Execute Cycle on it3. Arithmetic/Logic Un

15、it (ALU)“Does the math”A circuit in the ALU can add two numbers The circuit uses logic gates or simpler circuits that implement operations like AND and OR There are also circuits for multiplying, for comparing two numbers, etc.The ALU carries out each machine instruction with a separate circuit4. An

16、d 5. Input and Output UnitsThese two components are the wires and circuits through which information moves into and out of a computerA computer without input or output is uselessThe PeripheralsPeripherals connect to the computer input/output (I/O) portsThey provide input or receiving its outputThey

17、are not considered part of the processor:They are only specialized gadgets that encode or decode information between the processor and the physical worldThe PeripheralsThe keyboard encodes our keystrokes into binary form for the computerThe monitor decodes information from the computers memory and d

18、isplays it on a screenThe peripherals handle the physical part of the operationPortable Memory & Hard DrivesSome peripherals are used by computers for both input and output:USB memoryHard disks/drivesThey are storage devicesThe hard disk is the alpha-peripheral, being the most tightly linked device

19、to the computerA Device Driver for Every PeripheralMost peripheral devices are “dumb”They provide only basic physical translation to or from binary signals.Additional information from the computer is needed to make it operate “intelligently”Added processing by software called a device driver gives t

20、he peripheral its standard meaning and behaviorEvery device needs a device driverMachine InstructionsThe computers instructions are more primitive than what programmers typeADD 4000, 2000, 2080Commands the computer to add the numbers stored in memory locations 2000 and 2080 and then store that in th

21、e memory location 4000Computer instructions encode the memory addresses, not the numbers themselvesIndirect reference: referring to a value by referring to the address in memoryMachine InstructionsADD 4000, 2000, 2080Looks like those three numbers should be added togetherWhat it really means is that

22、 whatever numbers are stored in memory locations 2000 and 2080 be added together, and the result be stored in location 4000Illustration of a single instructionThe Program Counter:The PCs PCHow does the computer determine which instruction it should execute next?Address of the Next Instruction The in

23、struction is stored in memory and the computer has its addressComputers use the address (known as the program counter or PC) to keep track of the next instructionThe Program Counter:The PCs PCAfter one instruction, the computer must get ready for the nextIt assumes that the next instruction is the n

24、ext instruction in sequenceBecause instructions use 4 bytes of memory, the next instruction must be at the memory address PC + 4, so the processor increases the PC by 4Branch and Jump InstructionsNot all instructions are in a strict sequenceThe instruction may include a memory location (address) to

25、go to nextThis changes the PC, so instead of going to PC+4 automatically, the computer jumps or branches to the specified location to continue executionThe Fetch/Execute CycleA five-step cycle:Instruction Fetch (IF)Instruction Decode (ID)Data Fetch (DF) / Operand Fetch (OF)Instruction Execution (EX)

26、Result Return (RR) / Store (ST)ADD 4000, 2000, 2080ADD the values found in memory locations 428 and 884 and store the result in location 800Instruction Fetch (IF)Execution begins by moving the instruction at the address given by the PC (PC 800) from memory to the control unitBits of instruction are

27、placed into the decoder circuit of the CUOnce instruction is fetched, the PC can be readied for fetching the next instructionIFInstruction Decode (ID)ALU is set up for the operationDecoder finds the memory address of the instructions data (source operands)Most instructions operate on two data values

28、 stored in memory (like ADD), so most instructions have addresses for two source operandsThese addresses are passed to the circuit that fetches them from memory during the next stepInstruction Decode (ID)Decoder finds the destination address for the Result Return step and places the address in the R

29、R circuitDecoder determines what operation the ALU will perform (ADD), and sets up the ALUIDData Fetch (DF)The data values to be operated on are retrieved from memoryBits at specified memory locations are copied into locations in the ALU circuitryData values remain in memory (they are not destroyed)

30、DFInstruction Execution (EX)For this ADD instruction, the addition circuit adds the two source operands together to produce their sumSum is held in the ALU circuitryThis is the actual computationEXReturn Result (RR)RR returns the result of EX to the memory location specified by the destination addre

31、ss. Once the result is stored, the cycle begins againRRThe Computer ClockComputers are instruction execution engines. Since the computer does one instruction per cycle in principle, the speed of a computer depends on the number of Fetch/Execute Cycles it completes per second.The Computer ClockThe ra

32、te of the Fetch/Execute Cycle is determined by the computers clock, and it is measured in megahertz, or millions (mega) of cycles per second (hertz). A 1,000 MHz clock ticks a billion (in American English) times per second, which is one gigahertz (1 GHz)One Cycle per Clock TickA computer with a 1 GH

33、z clock has one billionth of a secondone nanosecondbetween clock ticks to run the Fetch/Execute Cycle. In that amount of time, light travels about one foot (30 cm).A simple processor might use five ticks to complete one instruction (five steps)One Cycle per Clock TickModern computers try to start a

34、new instruction each clock tickThis is done using a pipelineA pipeline is similar to an automobile assembly lineThe CPU fetches an instruction and passes it along the pipelineIt is then free to fetch the next instruction on the next tickOne Cycle per Clock TickThis is more complicated than car assem

35、blySome instructions must wait for results from previous instructionsVarious other complicationsTherefore, it is not quite true that 1,000 instructions are executed in 1,000 ticksSchematic Fetch/Execute CycleMany, Many Simple OperationsComputers “know” very few instructionsThe decoder hardware in th

36、e controller recognizes, and the ALU performs, only about 100 different instructions (with a lot of duplication)There are only about 20 different kinds of operations. Everything that computers do must be reduced to some combination of these primitive, hardwired instructionsCycling the Fetch/Execute

37、CycleADD is representative of the complexity of computer instructionssome are slightly simpler, some slightly more complexComputers achieve success at what they can do with speed. They show their impressive capabilities by executing many simple instructions per secondTranslationA programmer writes s

38、ource code, such as: this.Opacity += 0.02The bits the processor needs are known as object code, binary code, or just binarySource code is translated into assembly code, then into binaryAssembly languageA primitive programming languageUses words instead of 0s and 1sADD Opacity, TwoCths, OpacityTo con

39、vert source code into assembly, the source code must be compiled by a compilerAssembly languageA compiler is a computer program that translates another computer program into assembler languageEach language requires its own compilerThe assembly language is converted to machine language by yet another

40、 program called an assemblerIntegrated Circuits (ICs)MiniaturizationComputer clocks can run at GHz rates because their processor chips are so tinyElectrical signals can travel one foot (30 cm) in a nanosecondEarly computers (the size of whole rooms) could never have run as fast because their compone

41、nts were farther apart than one footMaking everything smaller has made computers fasterIntegrationEarly computers were made from separate parts (discrete components) wired together by handThere were three wires coming out of each transistor, the two wires from each resistor, the two wires from each

42、capacitor, and so onEach had to be connected to the wires of another transistor, resistor, or capacitorIntegrationActive components and the wires that connect them are manufactured together from similar materials by a single (multi-step) processIC technology places transistors side by side in the si

43、licon, along with the wire(s) connecting themResult is small and reliablePhotolithographyICs are made with a printing process called photolithography:Begin by depositing a layer of material (like aluminum) on the siliconCover that layer with a light-sensitive material called photoresist, and place a

44、 mask over itThe mask has a pattern corresponding to the features being constructedExposure to uv light causes open areas to hardenUnexposed areas do not and can be washed away leaving the patternHot gases etch the original layerWhen the remaining photoresist is removed, the pattern from the remains

45、TransistorsA transistor is a connector between two wires that can be controlled to allow a charge to flow between the wires (conduct) or notThe transistor is a MOS (Metal Oxide Semiconductor) transistorHow Semiconductor Technology WorksSilicon is a semiconductor: sometimes it conducts electricity an

46、d sometimes it does notThe ability to control when semiconductors do and dont conduct electricity is the main process used in computer constructionThe Field EffectThe silicon in the channel can conduct electricity when it is in a charged fieldCharging the gate positively creates a field over the cha

47、nnelElectrons are then attracted from the silicon into the channel, causing it to conductIf the field is removed, the electrons disperse into the silicon, the channel doesnt conductOn-Again, Off-AgainThe transistor controls signals in the computerOn-Again, Off-AgainTransistors can be combined to com

48、pute A AND BImplementing ALU OperationsLogical operations such as AND and OR can be built from transistorsMore complicated combinations can be made from theseArithmetic, memory and control units can be made combining these partsEventually, you have an ALU, and then a full processorCombining the IdeasStart with an information-processing task.Task is performed by an application implemented as a large programThe programs commands are c