CS110ComputerArchitecture
Lecture2:IntroductiontoC,PartI
Instructor:SörenSchwertfeger
http://shtech.org/courses/ca/
School of Information Science and Technology SIST
ShanghaiTech University
1Slides based on UC Berkley's CS61C
Agenda
• EverythingisaNumber• Compilevs.Interpret• Administrivia• QuickStartIntroductiontoC• Pointers• AndinConclusion,…
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Agenda
• EverythingisaNumber• Compilevs.Interpret• Administrivia• QuickStartIntroductiontoC• Pointers• AndinConclusion,…
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BIG IDEA: Bits can represent anything!!• Characters?
– 26 letters Þ 5 bits (25 = 32)– upper/lower case + punctuation
Þ 7 bits (in 8) (“ASCII”)
– standard code to cover all the world’s languages Þ 8,16,32 bits (“Unicode”)www.unicode.com
• Logical values?– 0 ® False, 1 ® True
• colors ? Ex:
• locations / addresses? commands?
• MEMORIZE: N bits Û at most 2N things
Red (00) Green (01) Blue (11)
KeyConcepts• Insidecomputers,everythingisanumber• Butnumbersusuallystoredwithafixedsize– 8-bitbytes,16-bithalfwords,32-bitwords,64-bitdoublewords,…
• Integerandfloating-pointoperationscanleadtoresultstoobig/smalltostorewithintheirrepresentations:overflow/underflow
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NumberRepresentation
• Valueofi-th digitisd × Baseiwherei startsat0andincreasesfromrighttoleft:
• 12310=110 x 10102 +210 x 10101 +310 x 10100=1x10010 +2x1010 +3x110=10010 +2010 +310=12310
• Binary(Base2),Hexadecimal(Base16),Decimal(Base10)differentwaystorepresentaninteger– Weuse1two,5ten,10hex tobeclearer
(vs.12,48,510,1016)
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NumberRepresentation
• Hexadecimaldigits:0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F
• FFFhex =15tenx16ten2 +15tenx16ten1 +15tenx16ten0=3840ten +240ten +15ten=4095ten
• 111111111111two =FFFhex =4095ten• Mayputblankseverygroupofbinary,octal,orhexadecimaldigitstomakeiteasiertoparse,likecommasindecimal
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SignedandUnsignedIntegers
• C,C++,andJavahavesignedintegers,e.g.,7,-255:int x, y, z;
• C,C++alsohaveunsignedintegers,e.g.foraddresses
• 32-bitwordcanrepresent232 binarynumbers• Unsignedintegersin32bitwordrepresent0to232-1(4,294,967,295)(4Gig)
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UnsignedIntegers00000000000000000000000000000000two =0ten00000000000000000000000000000001two =1ten00000000000000000000000000000010two =2ten
... ...01111111111111111111111111111101two =2,147,483,645ten01111111111111111111111111111110two =2,147,483,646ten01111111111111111111111111111111two =2,147,483,647ten10000000000000000000000000000000two =2,147,483,648ten10000000000000000000000000000001two =2,147,483,649ten10000000000000000000000000000010two =2,147,483,650ten
... ...11111111111111111111111111111101two =4,294,967,293ten11111111111111111111111111111110two =4,294,967,294ten11111111111111111111111111111111two =4,294,967,295ten
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SignedIntegersandTwo’s-ComplementRepresentation
• SignedintegersinC;want½numbers0,andwantone0
• Two’scomplementtreats0aspositive,so32-bitwordrepresents232integersfrom-231(–2,147,483,648)to231-1(2,147,483,647)– Note:onenegativenumberwithnopositiveversion– Booklistssomeotheroptions,allofwhichareworse– Everycomputerusestwo’scomplementtoday
• Most-significantbit(leftmost)isthesignbit,since0meanspositive(including0),1meansnegative– Bit31ismostsignificant,bit0isleastsignificant
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Two’s-ComplementIntegers00000000000000000000000000000000two =0ten00000000000000000000000000000001two =1ten00000000000000000000000000000010two =2ten
... ...01111111111111111111111111111101two =2,147,483,645ten01111111111111111111111111111110two =2,147,483,646ten01111111111111111111111111111111two =2,147,483,647ten10000000000000000000000000000000two =–2,147,483,648ten10000000000000000000000000000001two =–2,147,483,647ten10000000000000000000000000000010two =–2,147,483,646ten
... ...11111111111111111111111111111101two =–3ten11111111111111111111111111111110two =–2ten11111111111111111111111111111111two =–1ten
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SignBit
WaystoMakeTwo’sComplement• ForN-bitword,complementto2tenN
– For4bitnumber3ten=0011two,two’scomplement
(i.e.-3ten)wouldbe
16ten-3ten=13ten or10000two – 0011two =1101two
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• Hereisaneasierway:– Invertallbitsandadd1
– Computersactuallydoitlikethis,too
0011two
1100two+1two
3ten
1101two
Bitwisecomplement
-3ten
Two’s-ComplementExamples
• Assumeforsimplicity4bitwidth,-8to+7represented
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00110010
3+25 0101
00111110
3+(-2)
1 10001
01110001
7+1-8 1000Overflow!
11011110
-3+(-2)
-5 11011
10001111
-8+(-1)+7 10111
CarryintoMSB=CarryOutMSB
CarryintoMSB=CarryOutMSB
Overflow!
Overflowwhenmagnitudeofresulttoobigtofitintoresultrepresentation
Carryin=carryfromlesssignificantbitsCarryout=carrytomoresignificantbits
0to+31
-16to+15
-32to+31
B
C
D
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Supposewehada5-bitword.Whatintegerscanberepresentedintwo’scomplement?
A
0to+31
-16to+15
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Supposewehada5-bitword.Whatintegerscanberepresentedintwo’scomplement?
B
C
D
A -32to+31
Agenda
• EverythingisaNumber• Compilevs.Interpret• Administrivia• QuickStartIntroductiontoC• Pointers• AndinConclusion,…
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Processor
Control
Datapath
ComponentsofaComputer
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PC
Registers
Arithmetic&LogicUnit(ALU)
MemoryInput
Output
Bytes
Enable?Read/Write
Address
WriteData
ReadData
Processor-MemoryInterface I/O-MemoryInterfaces
Program
Data
GreatIdea:LevelsofRepresentation/Interpretation
lw t0,0(s2)lw t1,4(s2)sw t1,0(s2)sw t0,4(s2)
HighLevelLanguageProgram(e.g.,C)
AssemblyLanguageProgram(e.g.,RISC-V)
MachineLanguageProgram(RISC-V)
HardwareArchitectureDescription(e.g.,blockdiagrams)
Compiler
Assembler
MachineInterpretation
temp=v[k];v[k]=v[k+1];v[k+1]=temp;
0000 1001 1100 0110 1010 1111 0101 10001010 1111 0101 1000 0000 1001 1100 0110 1100 0110 1010 1111 0101 1000 0000 1001 0101 1000 0000 1001 1100 0110 1010 1111
LogicCircuitDescription(CircuitSchematicDiagrams)
ArchitectureImplementation
Anythingcanberepresentedasanumber,
i.e.,dataorinstructions
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Wearehere!
IntroductiontoC“TheUniversalAssemblyLanguage”
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IntrotoC• Cisnota“veryhigh-level”language,nora“big”one,andisnotspecializedtoanyparticularareaofapplication.Butitsabsenceofrestrictionsanditsgeneralitymakeitmoreconvenientandeffectiveformanytasksthansupposedlymorepowerfullanguages.
– KernighanandRitchie• Enabledfirstoperatingsystemnotwritteninassemblylanguage:UNIX- AportableOS!
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IntrotoC
• WhyC?:wecanwriteprogramsthatallowustoexploitunderlyingfeaturesofthearchitecture– memorymanagement,specialinstructions,parallelism
• Candderivatives(C++/Obj-C/C#)stilloneofthemostpopularapplicationprogramminglanguagesafter>40years!
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Disclaimer
• YouwillnotlearnhowtofullycodeinCintheselectures!You’llstillneedyourCreferenceforthiscourse– K&Risamust-have• Checkonlineformoresources
• KeyCconcepts:Pointers,Arrays,ImplicationsforMemorymanagement
• WewilluseANSIC89– original”oldschool”C– BecauseitisclosesttoAssembly
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Compilation:Overview
• CcompilersmapCprogramsintoarchitecture-specificmachinecode(stringof1sand0s)– UnlikeJava,whichconvertstoarchitecture-independentbytecode
– UnlikePythonenvironments,whichinterpretthecode– Thesediffermainlyinexactlywhenyourprogramisconvertedtolow-levelmachineinstructions(“levelsofinterpretation”)
– ForC,generallyatwopartprocessofcompiling.c filesto.o files,thenlinkingthe.o filesintoexecutables;
– Assemblingisalsodone(butishidden,i.e.,doneautomatically,bydefault);we’lltalkaboutthatlater
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CCompilationSimplifiedOverview(morelaterincourse)
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foo.c bar.c
Compiler Compiler
foo.o bar.o
Linker lib.o
a.out
Csourcefiles(text)
Machinecodeobjectfiles
Pre-builtobjectfilelibraries
Machinecodeexecutablefile
Compiler/assemblercombinedhere
Compilation:Advantages
• Excellentrun-timeperformance:generallymuchfasterthanSchemeorJavaforcomparablecode(becauseitoptimizesforagivenarchitecture)
• Reasonablecompilationtime:enhancementsincompilationprocedure(Makefiles)allowonlymodifiedfilestoberecompiled
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Compilation:Disadvantages
• Compiledfiles,includingtheexecutable,arearchitecture-specific,dependingonprocessortype(e.g.,MIPSvs.RISC-V)andtheoperatingsystem(e.g.,Windowsvs.Linux)
• Executablemustberebuiltoneachnewsystem– I.e.,“portingyourcode”toanewarchitecture
• “Change® Compile® Run[repeat]”iterationcyclecanbeslowduringdevelopment– butMaketoolonlyrebuildschangedpieces,andcandocompilesinparallel(linkerissequentialthough->Amdahl’sLaw)
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CPre-Processor(CPP)
• Csourcefilesfirstpassthroughmacroprocessor,CPP,beforecompilerseescode
• CPPreplacescommentswithasinglespace• CPPcommandsbeginwith“#”• #include“file.h”/*Insertsfile.h intooutput*/• #include/*Looksforfileinstandardlocation*/• #defineM_PI(3.14159)/*Defineconstant*/• #if/#endif /*Conditionalinclusionoftext*/• Use-save-tempsoptiontogcc toseeresultofpreprocessing• Fulldocumentationat:http://gcc.gnu.org/onlinedocs/cpp/
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foo.c CPP foo.i Compiler
TypedVariablesinCint variable1 = 2;float variable2 = 1.618;
char variable3 = 'A';
• Mustdeclarethetypeofdataavariablewillhold– Typescan'tchange
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Type Description Examplesint integernumbers,includingnegatives 0,78,-1400unsignedint integernumbers(nonegatives) 0,46,900long largersignedinteger -6,000,000,000char singletextcharacterorsymbol 'a','D','?’float floatingpointdecimalnumbers 0.0,1.618,-1.4double greaterprecision/bigFPnumber 10E100
Integers:Pythonvs.Javavs.C
• C:int shouldbeintegertypethattargetprocessorworkswithmostefficiently
• Onlyguarantee:sizeof(long long)≥sizeof(long)≥sizeof(int)≥sizeof(short)– Also,short >=16bits,long >=32bits– Allcouldbe64bits 29
Language sizeof(int)Python >=32bits(plainints),infinite (longints)Java 32bitsC Dependsoncomputer;16 or32or64
Consts andEnums inC
• Constantisassignedatypedvalueonceinthedeclaration;valuecan'tchangeduringentireexecutionofprogramconst float golden_ratio = 1.618;const int days_in_week = 7;
• YoucanhaveaconstantversionofanyofthestandardCvariabletypes
• Enums:agroupofrelatedintegerconstants.Ex:enum cardsuit {CLUBS,DIAMONDS,HEARTS,SPADES};enum color {RED, GREEN, BLUE};
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B:Canassignto“PI”butnot“pi”
C:Coderunsatsamespeedusing“PI”or“pi”
A:Constants“PI”and“pi”havesametype
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Compare“#define PI 3.14”and“const float pi=3.14”– whichistrue?
Agenda
• EverythingisaNumber• Compilevs.Interpret• Administrivia• QuickStartIntroductiontoC• Pointers• AndinConclusion,…
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Administrivia• Findapartnerforthelab.InformyourlabTAaboutyourpartnerselectionduringlab1.Partnerteamsshouldbe2persons– therecanbeexactlyone3personlabteamperlab.
• Labsstartnextweek!Checkyourschedule!Youcannotgetcheckedwithoutapartner!
• ThetasksforLab1arepostedonthewebsite.Prepareforitovertheweekend.
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Administrivia• WehaveregisteredyouforAutolab yesterdaynight.
• HW1isavailableonAutolab.DuenextThursday.• Websiteisdown.Hopefullywillbeupsoon.Slideswillbepostedonpiazzafornow.MaybeLab1,too.
• Registerinpiazza!Willbepartofyourgrade!– Therearealsoappsforyourphone…
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Agenda
• EverythingisaNumber• Compilevs.Interpret• Administrivia• QuickStartIntroductiontoC• Pointers• AndinConclusion,…
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TypedFunctionsinC
int number_of_people (){return 3;
}
float dollars_and_cents (){return 10.33;
}
int sum ( int x, int y){
return x + y;}
• Youhavetodeclarethetypeofdatayouplantoreturnfromafunction
• ReturntypecanbeanyCvariabletype,andisplacedtotheleftofthefunctionname
• Youcanalsospecifythereturntypeasvoid– Justthinkofthisassayingthatnovalue
willbereturned• Alsonecessarytodeclaretypes
forvaluespassedintoafunction• VariablesandfunctionsMUSTbe
declaredbeforetheyareused
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Structs inC• Structs arestructuredgroupsof
variables,e.g.,
typedef struct {int length_in_seconds;int year_recorded;
} Song;
Song song1;
song1.length_in_seconds = 213;song1.year_recorded = 1994;
Song song2;
song2.length_in_seconds = 248;song2.year_recorded = 1988;
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Dotnotation:x.y = value
AFirstCProgram:HelloWorldOriginal C:
main(){printf("\nHello World\n");
}
ANSI Standard C:
#include
int main(void){printf("\nHello World\n");return 0;
}
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CSyntax:main
• WhenCprogramstarts– Cexecutablea.out isloadedintomemorybyoperatingsystem(OS)
– OSsetsupstack,thencallsintoCruntimelibrary,– Runtime1st initializesmemoryandotherlibraries,– thencallsyourprocedurenamedmain()
• We’llseehowtoretrievecommand-lineargumentsinmain()later…
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ASecondCProgram:ComputeTableofSines
#include #include
int main(void){
int angle_degree;double angle_radian, pi, value;/* Print a header */printf("\nCompute a table of the sine function\n\n");
/* obtain pi once for all *//* or just use pi = M_PI, where *//* M_PI is defined in math.h */pi = 4.0*atan(1.0);printf("Value of PI = %f \n\n", pi);
printf("angle Sine \n");
angle_degree = 0;/* initial angle value *//* scan over angle */while (angle_degree 360 */
{angle_radian = pi*angle_degree/180.0;value = sin(angle_radian);printf (" %3d %f \n ",
angle_degree, value);angle_degree = angle_degree + 10; /* increment the loop index */
}return 0;}
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SecondCProgramSampleOutput
Compute a table of the sine function
Value of PI = 3.141593
angle Sine 0 0.000000
10 0.173648 20 0.342020 30 0.500000 40 0.642788 50 0.766044 60 0.866025 70 0.939693 80 0.984808 90 1.000000
100 0.984808 110 0.939693 120 0.866025 130 0.766044 140 0.642788 150 0.500000 160 0.342020 170 0.173648 180 0.000000
190 -0.173648 200 -0.342020 210 -0.500000 220 -0.642788 230 -0.766044 240 -0.866025 250 -0.939693 260 -0.984808 270 -1.000000 280 -0.984808 290 -0.939693 300 -0.866025 310 -0.766044 320 -0.642788 330 -0.500000 340 -0.342020 350 -0.173648 360 -0.000000
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CSyntax:VariableDeclarations
• Allvariabledeclarationsmustappearbeforetheyareused(e.g.,atthebeginningoftheblock)
• Avariablemaybeinitializedinitsdeclaration;ifnot,itholdsgarbage!
• Examplesofdeclarations:– Correct: {
int a = 0, b = 10;...
−Incorrect: for (int i = 0; i < 10; i++)}
42NewerCstandardsaremoreflexibleaboutthis,morelater
CSyntax:ControlFlow(1/2)
• Withinafunction,remarkablyclosetoJavaconstructsintermsofcontrolflow– if-else
• if (expression) statement• if (expression) statement1else statement2
– while• while (expression)
statement• do
statementwhile (expression);
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CSyntax:ControlFlow(2/2)
– for• for (initialize; check; update) statement
– switch• switch (expression){
case const1: statementscase const2: statementsdefault: statements
}• break
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