chapter 1 - system overview

8/8/2019 Chapter 1 - System Overview

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SystemOverviewBefore attemptingany troubleshooting nalysison a system, ou must irst understand he systemconfigurationand how the systemworks. In thischapter, ou will approach he IBM PC from thesystemsevel and becomeacquaintedwith eachfunctionalelement n the microcomputer.Fromyour introductory computer instruction, oulearned hat everycomputersystem as ive basicparts Fig.1-1):

r An arithmetic ogic unitr A control unitr A memoryunitr An input unitr An output unit

The arithmetic logic unit (ALU) does headding,subtracting,multiplying,dividing,com-paring,and ogic operations.A controlunit regu-lates he operationof the completemachine. Itfetchesand nterprets nstructions, nd t causescertainpartsof the circuitry to respondaccordingto those nstructions. The ALU and control unitcan be considered he nerve elementsof thecomputer's rain. Important to the computer sadequate ower and a goodsystem lock.

MEMOFY UNIT

Fig.1-1.Basic arts f the BM PCcomputer.The memory unit is the rememberingpartof the machine. It storesprograms,data, calcu-lations, and results. Two types of memory areincluded n the PC: temporary memory calledrandom-access emory(RAM) and permanentmemory calledread-only memory (ROM). TheROM chips are permanently programmed orwritten into during manufacturewith computerinstructionsand specialdata. Sinceone of the

INPUTUN T I OUTPUTUN TRA M RO M


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IBM ROMs has an operatingsystem rogrampermanentlystored n it (hardware),we call itttfirmware.ttRAM is sometimes alled "main memory."Information stored n RAM existsonly as ong aspower is applied to the computer. When poweris removed, he programsand data stored n theRAM are lost unless ransferred copied) toexternalpermanentmemory. Externalmemorycan be attached o the PC by connecting loppydiskdrivesor hard disk drives.Communicationwith the computeroccursthrough the input and output units. Theseperson-machine nterfaces are called "peri-pherals." A peripheralcan be ust aboutanytypeof device onnectedo the basicPC.An input unit allowscommands, rograms,and data o enter he computer-It'show the user"talks" to the computer. Keyboards,oysticks,game paddles, graphics tablets, l ight pens,microphones, nd analog-to-digital onverters(ADCs) are examples f input devices.The computer communicateswith ourenvironmentand us via an output unit. Mono-chromeand color displays, r inters,plotters,aspeaker,and digital-to-analog onverters DACs)are examples f output devices.Somedevices re used or both nput andoutput. These nclude he mass torage evices(the floppy disk drives, he hard disk drives,andarchivalstorage ape systems) nd MOdulator-DEModulators (MODEMs) which enablecom-puter to computer communications ver longdistances.The arithmetic ogic unit and control unitcanbe combined nto a single ntegrated ircuitcalleda "central processing nit" as shown nFig. l-2. Microprocessors re also calledcentral

processing nits (CPUs) because hey can bedesignedo do the same unctionsas he centralprocessing ni t in a largecomputer. In the IBMPC, an 8088 ntegratedcircuit (IC) is the systemscentral processing nit. I t accesses emory(fetches an instruction), nterprets what theinstructionmeans,and does he actions equiredby the instruction, and then fetches he nextinstruction in the program and repeats thesequence.This sequentialprocesswasdescribed

by mathematician ohnVon Neumann n the late1940s. Today,everydesktop or laptop personalcomputer operates as a sequential "VonNeumann" machine. In the future, microcom-puter systemswill include multiple processorsand operateon many nstructionsat the sametime in a parallel "non Von Neumann" archi-tecture.

INPUTUNIT I OUTPUTUNITMEI.4ORY

Fig. 1-2. The control unit and arithmeticunittogether orm the centralprocessing nit CPU.

THEBASIC BMPCINTEGRATEDNTOA SYSTEMTaken as a whole,an IBM PC system ould ooklike the configuration shown n Fig. 1-3.A typical PC system s comprisedof thesystemunit with keyboard,a displayunit, twodisk drives,and a printer. The next few pageswill describeeachof theseunits.

SystemUnitThe systemunit shown n Fig. 1-4 s the maincomponentof the computer. It houses hesystem oard (motherboard)with its expansionslots for external nterfaceperipheral devices, heswitchingpower supply,and two standard-height,double-density isk drives. On the right side,near the back of the systemunit, is the on-offpowerswitch.


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SystemOverview 3

Fig.1-3.Exampleof an BMPC svstem.

Fig.1-4. The system nit sitsbehinda detachableevboard.

KeyboardThe detachable eyboardhas 83 keys hat cangenerateall 128American StandardCode orInformation nterchange ASCII) characters. tcan alsogenerate pecialsymbols nd graphicshapes. In al l , the keyboard can cause hemachine o generateand display or print) 256characters, hapes, r symbols.The numer ic keypadon the right side of thekeyboardhassomekeys hat doubleas cursor-control keys. Ten programmable unction keysare mounted on the left side of the keyboard oexecutespecificprogramsor to initiate specialsoftware routines. The functions can beprogrammedby the softwaredesigner. Manyspecialkeysare provided ncludingShift , Up-

Down-Right-Left arrow keys, Caps Lock,Number Lock, Scroll Lock, Backspace, nter (orReturn), Home, Page Down, Page Up, End,Delete, nsert,Print Screen, ab, Control,andAlternate. The functionsof thesekeysand keycombinations re describedn the IBM Guide oOperationsmanual.All83 keyshaveautomatic epeatand a 10-character ype-ahead uffer to let you type atrapid speedwithout getting aheadof the com-puterprocessing f eachkey stroke.Each of the 83 concave eyshave a tactilefeel with an audi ble cl ick to provide posit ivefeedback hat key action has been completed.Inside the keyboardare electroniccircuits hatenhance ey operationand permit keys o beredefined or increasedprogramming lexibility.On the bottom sideof the keyboard re woplastic eet that allow the keyboard o be tilted intwo positions or best ypingcomfort. A plasticridge above he top row of keyscan hold a bookor report between he keyboardand the displayscreen. It can alsohold templates or specialapplication software. A 6-foot coiled cableconnects he keyboard o the rear of the systemunit.Output nitTwo outputunits hat make he computersystemcompleteare the displayunit and a printer.

COLORDISPLAYDAISY-WHEELPRINTER

IB MPERSONAL OMPUTER(64-640K)SYSTEMUNITASSETTE

VOICEINPUT


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Many displayunitscanconnect o the IBMincludingmonochrome nd color monitors. Adisplayunit connects o the computerat the rearof the systemunit. If a radio irequency RF)modulator s connectedo thevideoadapter'cardinside he systemunit, a standard elevision anbe used or a monitor.Two video adaptersare available or thePC: a monochromedisplayadapter hat supportstext, and a colorfgraphicsadapter that supportstextandcolor graphics.The monochrome adapter enables hesystem o generateand display25 rows of g0characterseach n white on btc[ (greenon blackwith the IBM monochromemonilor), blackonwhite (or black on green), bl inking, in highintensity,or underlined. This adaptercardalsohasa connection or the IBM dot_matrix rinter.Color is availableusing he color_graphicsadaptercard. This interfaceprovidesopiion, fo.two typesof text (25 rowsof 40 characters, r 25rows of 80 characters)and three types ofgraphics low resolution,medium esolution, ndhigh resolution). Only mediumresolution ndhigh resolutionare supportedby the ROM. The6845CRT controller C on the adapterboardmustbe directly addressed y customsoftware oenableow-resolution raphics.Low-resolution raphics efers o 100 owsof 160- ixels(pictureelements)or dotseach, nany of 16standard olors. Medium_resolutiongraphics an produce200 owsof 320 pixelsperrow in any of four colors. Additional colorscanbe generatedwhen dotsof differentcolorsarejuxtaposed. High-resolutiongraphicsproduces200rowsof 640pixelspe. .ow1n blackandwhite(or.black and green). Text can be posit ionedwithin graphic shapesenablingwinjow opera_tions.

Many types of printers can connectandfunction with the pC. Both dot-matrixand fullcharacter rinters are commonly onnectedothis machine. The recent ntroiuction of con_sumer aserprinters hasgivenvery high qualityhard copyoutput capability to pC users. AII ofthese nterconnections re achieved singa spe_cial expansion lot adapterboardas the inierface.Someusershaveboth typesof printersconnected

at the same time-dot_matrix fo r drafts andworkingcopies, nd etter quality or documents.. A final outputdevice hat is part of the pCis a small (2-inch)8-ohmspeakermountedat theleft sideof the system oard nside he systemunit chassis. This device can produce thefamiliar beep,arcade,and musicsounds. t canalsoproducecrudespeech.types of bidirectional data storagedevicesare usedwith the IBM pC: tapecasserreand minifloppy drives. While cassette torage sslow,magnetic apesprovide an excellentwav toprovide archival -and storage fo r the largeamountsof data that are generated verydiy.Many more files or pagesof information ian bestoredon a goodaudiocassetteape than can bestored on a 5r/z inch floppy disk. In fact, oneform oftape archivalstorageusesvideo tape forlong erm computer-generatedata.The typical syst"em sesminifloppy disksthat operate n a diskdriveunit as he mass tor-agemedium or the pC. Up to sixdiskdrivescanbe connected o the pC usingnon_IBMhard_ware.Each40-trackdiskcanbe single_ r double_sided,double-density epending-on he driveused. The disksare magnetically ectoredduring

formatting into 512-byte ,"ttorc providin!163,840 ytesof storage or single_sided,oublel9:5llt disks184,320ytes itfipc_ooS2.x)or3.27,680ytesof storageor double_sided,ouLle_densitydisk drivessystems36g,640 ytes or pC_DoS 2.x).ConnectionsFigure 1-5shows he connectionson the rear ofthe PC. A femaleconnectorprovidespower toan externalmonochromedisplay. The male con_nctor o the right is for the powercord. To theright of the round fan air exhaustport is a 5_pincircular connector or the keyboardcable. Nextis a 5-pin circularconnector or cassette atainput/output. The five slots align with fiveexpansion ockets n the system oard nside.These slots are for connectingother displayoutputs,diskdrives, lotters,printers,and otherperipherals.


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SvstemOveruiew 5

b"=_-----*"----'--'dililililllllltllllllllllllll lllltllllllililillillllllllllilllllllll lllllllllll

Fig. -5. The connectionsoundon thebackof thePC svstem nit.

InternalGomponentsFigure 1-6 shows he posit ion of subsystemsinside the 5.5 inch by 19.6 nch by 16.1 nchsystemunit. On the left is the system oard,ormotherboard.At the top of this board are thefive expansion slots to connect peripheraldevices.On the ower eft of the system oardare he memorychips.

SWITCHINGPOWERSUPPLY

The top right portion of the systemunit isthe switching owersupply. The lower two-thirdsof the system nit houseswo floppy disk drives.Figure 1-7 s a block diagram of the IBMPC system. Dashed ines allocate he systemelementsnto the basicpartsof any computer.Eachpart of this diagramwill be covered ndetail n the nextchapter.

SystemBoardThe most mportant part of the computer s theCPU and he circuitrysurroundinghe CPU. Onthe PC, the CPU and ts associated ircuitry aremounted on a denselypacked printed-circuitboard called he systemboard, or motherboard.Two system oardshavebeen produced or theIBM PC. The board shown n Fig. 1-8 wasdesignedor systemshat contained etween16kand64K of RAM.A photograph f the system oard used nnewer64K to 256K RAM systemss shown nFig. 1-9. Theseparameters efine he amountofRAM that canbe installed n the system oardbefore expansion memories can be used.COMPUTERFACTS pages27 and 34 provideanother iewof the newerPC system oard. Thetraceside s shownon CF pages 6 and35.As noted,most of the ICs on theseboardsare mounted n a commondirectionwith pin 1 ofeach chip facing the samedirection. This isimportant when these boards are repairedbecause t helps o preventyou from mountingthe chip backwards.Chips are markedwith thepackageposit ioned n the sameway. If youinadvertently oldera replacement C into theboardwith pin 1 facing he wrong way, you canquickly tel l by the upside down lettering andnumbering hat it's mounted ncorrectly.Each component C has a correspondingcode(U23,U36, and so forth) stampedon thesystemboard. In addition, he chip locationsaremarked n increasingorder from top to bottomand right to left. This is helpful in quicklylocating Cs.In Chapter2, al l systemboard chipswil lbe discussedn detail. For consistency,nly the

AIR KE CASSETTE PERIPHERALINPUT/OUTPUT OPTIONSLOTSOWER EXHAUST CONNECTOR

silrJ)

Fig. 1{. Inside view of systemunitOISK ORIVEANALOG CARD


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SvstemOveniew 7

Fig. 1-8. Photoof the16K-64K BM PCsystem oard.

(socketU3). A closerview s provided n Fig.1-11. The 8088 s an Intel designedmicro-processor hat operateson a 4.77MHz clock as a16-bit machine internally, using the sameinstructionset as the 16-bit ntel 8086micro-processor, ut with an 8-bit data bus. It supports16-bit operations ncluding multiply and divideand has a 20-bit address us so t can access vera million memory ocations.Internally, he 8088handles16-bit ormatsso the CPU expandsts internaladdress ord to20 bits at its output usinga segmentationcheme.Memory addresses re ogicallysubdividedntospecial segmentsof 64K bytes each. Eachsegment an be allocated o special egisters nthe 8088. Then bytes within a segmentareaddressed singa 16-bitoffsetaddresswhich s

added o a 16-bitsegment ddresso generatephysicaladdress.This addressing chemewill bedescribedn detail n Chapter2.

8087NUMERIC ATAPROCESSORCOPROCESSOR)The 8088can alsooperate n maximum modewith an optiona l 8087numeric data processorfunctioningas a coprocessor.This configurationgreatly increases omputationalspeed. Thesocket us t below U3 is fo r the 8087 Ua). The8087 s a high-speed, wo-channel /O control-ler/coprocessor hat extends he 8088 nstructionset to include arithmetic and ogic operations. Itdoesn'tchange he way the systemoperates, ut


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I Chapter1

Fig. 1-9. Photoof the 64K-256KBM PCsystem oard.

i t does greatly reduce the t ime required forcertain mathematical unctions. This IC canoperateon floating point and multiple digit BCDnumbersup to 18digits n length.The 8087numericdataprocessor andlesthe data transfersbetween t and the 8088.Permanently stored in the 8087 are microinstructions or add,subtract,mult iply,divide,absolute alue,arctangent,angent, quare oot,and other operations. Its unique "number-crunching" architectureenables alculations100times aster than the 8088. When nstalled,aswitch on the systemboard enables he 8087CPU to quickly downloadmathematicopera-t ions to its 8088 coprocessor,dramaticallyreducingexecution imesof thesealgorithms.

8259PROGRAMMABLETNTERRUPTONTROLLERPrC)Justabove he 8088shown n the ayout diagramof Fig. 1-11 s the 8259PIC (U2) that producesspecialsignals hat are used or externaldevicecommunicationwith the CPU. The PC is aninterrupt driven machine n which peripheraldevices ommunicate ith the CPU by interruptsignals hat cause he CPU to stop what it wasdoing and service their request. Up to eightexternaldevices anrequestPIC U2 to produceaCPU interrupt signal. By knowing which deviceis requesting he interrupt, U2 puts a specialcodeout on the databus for CPU recoqnit ionandaction.


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SystentOveniew 9

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8237

8255

Fig.1-10.Thesystem oardchip ayout

8288CLOCKGENERATORIn the lower right of Fig. 1-10 s an 8288clockgenerator (U11) that receivesa power goodsignal rom the power supplyand a 14.31818MHz signal from an attached crystal andproduces heresetpulseand clocksignalso startthe CPU operatingand awaken he circuitry ofthe system oard nto electronic ife.Oncepowered, his IC continuously ro-ducesseveralclock signals hat pulse hroughoutthe PC. A 4.772727MHz signal spassedo the8088CPU and out onto the system oard as hesystemclock. The clock generatoralsoproducesa 2.386MHz signal hat is dividedby two andused o refresh he dynamic RAM on the systemboard, and to update he time-of-day nternal PCclock. It also s used o activate he speaker. Fig 1-11. The8088centralprocessing nit.


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8253 PROGRAMMABLETNTERVALTMERPtT)The 8253PIT (U34) near the lowermiddle of thesystemboard (Fig. 1-11) eceives he divided2.386MHz clock signalanddevelops pecial imeof day and date signalswhich are maintainedaslong as power s applied o the machine.Thesetime and date signals analsobe stored n massmemory with a file you havedeveloped o youknow which version of a prog ram you areaccessing hen you reload he file into temporarymemoryon the system oard. t alsoproducesspeaker ulsesor generating ound.

8237DIRECTMEMORYACCESS DMA)CONTROLLERBelow U34 in Fig. 1-11 is an 8237 Dl./.AController U35). This special urposemicro-processorenables arge data block transfersbetweenmassstorage disk drivesor other /O)and the internal memory on the systemunitboard without CPU involvement. t can handledatablocksup to 64K bytes n length.One of the four DMA channelsof U35combineswith one of the three counter/t imerchannelsof U34 to producea refreshsignal orthe temporarymemorychipsand any additionalmemory mounted on expansionboards andpluggednto oneof the five expansion ockets.

8255PROGRAMMABLEPERTPHERALNTERFACEPPt)The IC belowU35 in Fig. 1-11 s the 8255PPI(U36). This smart peripheral device allowsexternalcommunicationwith the CPU. It is aparallel I /O chip with three ports that can beconfiguredby a softrvare ommand o functionaseither input or output. The PPI is used o readthe system board configuration switches odeterminehow much memory s installed, henumber of disk drives,and the type of display

screenbeing used. It also accepts ata inputfrom the keyboard.

READONLYMEMORYROM)The column of largechips beginningwith theemptysocket or U28 holds40K bytesof ROM.The emptysocketwasoriginallydesigned o holdcassetteBASIC bu t wasn'trequired n the finalconfiguration. Four 8K ROMs contain a largecassetteBASIC high level anguagenterpreterprogram. A fif th 8K ROM chip below U28contains he ROM basic nput/output system(BIOS) that enablesCPU communication iththe system ircuitry. It handles ideo displaygraphics, t ime-of-dayclock, print ing to thescreencassetteoperations,and printer andasynchronousevicecommunications. ncludedin this chip is a self-testprogram that checks hefunctionalityof the PC duringpower-up. It alsocontainsa minifloppy disk bootstrap oader tocause a floppy drive to load mass storageprogramsonto the temporarymemory on thesystem oard.

RANDOM CCESSMEMORYRAM)The temporarymemory s the RAM mounted nfour columnsof nine chipseachat the far left topof Fig. 1-11.Eight chips n eachcolumnmakeupthe 8-bit data word. Early versionsof the PCcontained our columnsof 16Kx 1-bit Cs with asinglecolumn of chipsproviding the minimum16Ksystemand four columnsof chips providingthe maximumconfigurationof 64K RAM. Afterlate springof 1983, achPC wasdesigned o hold64K x l-bit RAM chips providing workingmemoryof between64K and256K.In both cases,he ninth chip in eachcolumnis a specialRAM used o store the parity valuefor eachbyte of storeddata. During storage,parity circuitry determines he numberof logic lsin the word andaddsenough 1 or 0) to make henumberof ls an evennumber. Thus 10010001n


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SvstemOverview 11

the storedword wouldcause he circuitry o storea I in the parity RAM thus ensuring hat thetotal number of 1s s even. When a word is readfrom RAM memory, parity check circuitrycomputes what the parity should be andcomparests value(1 or 0) with the valuestoredin the parity RAM. If the two valuesare not thesame,a non-maskablenterrupt calledparityerror occurs causing he CPU to produce adisplay hat readsPARITY CHECK and thenhaltssystem peration.

POWER UPPLYThe switchingpower supply nside he systemunit provides p to 64 wattsof energy o operatethe computer, ts internal disk drives,and theadapter cards plugged nto the systemboardexpansionslots. It can also pass 120 VAC

through o a socketon the rear of the systemunitfor poweringa monochromedisplay.The supply eceives120VAC at 50/60H2through he ine cord andgenerates 5, -5 ,+72,and -12volts asoutput. It is fused and ncludesapower sensingdevice hat automaticallycuts offpower o the PC if too much or too little voltageis detected. t alsoshutsdown f an overvoltageor overcurrentcondit ion occursbecauseof ashort on the systemboard, on an expansionboard,or in the disk drives.Two 6-pin connectorsprovide power to thesystemboard, and two connectorsprovide powerto each nternaldisk drive assembly.In the nextchapter, ou will read about herole of eachof the previouslydescribed hips nthe PC systemand gain an in-depth under-standingof how the IBM personalcomputersystem perates.Chapter2 takesyou deep ntothe circuitry o gain a detailedunderstanding fthe system ou are about o repair andmaintain.


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chapter 1 - system overview

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