CS 61C: Great Ideas in Computer Architecture Lecture 2 ...cs61c/sp16/lec/02/... · Great Ideas in...

CS61C:GreatIdeasinComputerArchitectureLecture2:IntroductiontoC,PartI

Instructors:VladimirStojanovic&NicholasWeaver

http://inst.eecs.berkeley.edu/~cs61c/

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Agenda

• EverythingisaNumber• ComputerOrganization• Compilevs.Interpret

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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– We’lluse1two,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++alsohaveunsigned integers,whichareusedforaddresses

• 32-bitwordcanrepresent232 binarynumbers• Unsignedintegersin32bitwordrepresent0to232-1(4,294,967,295)

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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;want½numbers<0,want½numbers>0,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

1/21/16 9

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

BinaryAdditionExample

00110010

3+25 1

0

0

1

1

0

0

0

0

Carry

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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☐

☐

☐

☐

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Supposewehada5-bitword.Whatintegerscanberepresentedintwo’scomplement?

0to+31

-16to+15

-32to+31☐

☐

☐

☐

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Supposewehada5bitword.Whatintegerscanberepresentedintwo’scomplement?

Agenda

• EverythingisaNumber• ComputerOrganization• Compilevs.Interpret

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ENIAC(U.Penn.,1946)FirstElectronicGeneral-PurposeComputer

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• Blazinglyfast(multiplyin2.8ms!)– 10decimaldigitsx10decimaldigits

• Butneeded2-3daystosetupnewprogram,asprogrammedwithpatchcordsandswitches

EDSAC(Cambridge,1949)FirstGeneralStored-ProgramComputer

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• Programsheldasnumbersinmemory• 35-bitbinary2’scomplementwords

Processor

Control

Datapath

ComponentsofaComputer

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PC

Registers

Arithmetic&LogicUnit(ALU)

MemoryInput

Output

Bytes

Enable?Read/Write

Address

WriteData

ReadData

Processor-Memory Interface I/O-MemoryInterfaces

Program

Data

GreatIdea:LevelsofRepresentation/Interpretation

lw $t0,0($2)lw $t1,4($2)sw $t1,0($2)sw $t0,4($2)

HighLevelLanguageProgram(e.g.,C)

AssemblyLanguageProgram(e.g.,MIPS)

MachineLanguageProgram(MIPS)

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”

• Classpre-req includedclassesteachingJava

• Pythonusedintwolabs• Cusedforeverythingelse

• “Some”experienceisrequiredbeforeCS61CC++orJavaOK

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IhaveprogrammedinC,C++,C#,orObjective-C☐

IhaveprogrammedinJava☐

IhaveprogrammedinFORTRAN,Cobol,Algol-68,Ada,Pascal,orBasic

☐

☐

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LanguagePoll!Pleaseraisehandforfirstoneoffollowingyoucansayyesto

IntrotoC• Cisnota“veryhigh-level” language,nora“big”one,andisnotspecializedtoanyparticularareaofapplication.Butitsabsenceofrestrictionsanditsgeneralitymakeitmoreconvenientandeffectiveformanytasksthansupposedlymorepowerfullanguages.

– KernighanandRitchie• Enabledfirstoperatingsystemnotwritteninassemblylanguage:UNIX- AportableOS!

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IntrotoC

• WhyC?:wecanwriteprogramsthatallowustoexploitunderlying featuresofthearchitecture– memorymanagement,specialinstructions,parallelism

• Candderivatives(C++/Obj-C/C#)stilloneofthemostpopularapplicationprogramminglanguagesafter>40years!

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TIOBEIndexofLanguagePopularity

24http://www.tiobe.comTheratingsarebasedon thenumber ofskilledengineersworld-wide,coursesandthirdpartyvendors.

TIOBEProgrammingCommunityIndex

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Disclaimer• YouwillnotlearnhowtofullycodeinCintheselectures!You’llstillneedyourCreference forthiscourse– K&Risamust-have

• Checkonlineformoresources– “JAVAinaNutshell,”O’Reilly

• Chapter2,“HowJavaDiffersfromC”• http://oreilly.com/catalog/javanut/excerpt/index.html

– BrianHarvey’shelpfultransitionnotes• OnCS61Cclasswebsite:pages3-19• http://inst.eecs.berkeley.edu/~cs61c/resources/HarveyNotesC1-3.pdf

• KeyCconcepts:Pointers,Arrays,ImplicationsforMemorymanagement

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Agenda

• EverythingisaNumber• ComputerOrganization• Compilevs.Interpret

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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<stdio.h>/*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