CS110ComputerArchitecture

Lecture5:MoreMIPS,MIPSFunctions

Instructor:SörenSchwertfeger

http://shtech.org/courses/ca/

School of Information Science and Technology SIST

ShanghaiTech University

1Slides based on UC Berkley's CS61C

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

ArchitectureImplementation

Anythingcanberepresentedasanumber,

i.e.,dataorinstructions

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LogicCircuitDescription(CircuitSchematicDiagrams)

Fromlastlecture…• Computer“words”and“vocabulary”arecalledinstructions andinstructionsetrespectively

• MIPSisexampleRISCinstructionsetusedinCS61C• Rigidformat:1operation,2sourceoperands,1destination– add,sub,mul,div,and,or,sll,srl,sra– lw,sw,lb,sb tomovedatato/fromregistersfrom/tomemory

– beq, bne, j, slt, slti fordecision/flowcontrol• Simplemappingsfromarithmeticexpressions,arrayaccess,inCtoMIPSinstructions

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Processor

Control

Datapath

Review:ComponentsofaComputer

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ProgramCounter

Registers

Arithmetic&LogicUnit(ALU)

MemoryInput

Output

Bytes

Enable?Read/Write

Address

WriteData

ReadData

Processor-Memory Interface I/O-MemoryInterfaces

Program

Data

HowProgramisStored

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Memory

Bytes

Program

Data

OneMIPSInstruction=32bits

AssemblertoMachineCode(morelaterincourse)

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foo.S bar.S

Assembler Assembler

foo.o bar.o

Linker lib.o

a.out

Assemblersourcefiles(text)

Machinecodeobjectfiles

Pre-builtobjectfilelibraries

Machinecodeexecutablefile

Assemblerconvertshuman-readableassemblycodetoinstructionbitpatterns

Processor

Control

Datapath

ExecutingaProgram

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PC

Registers

Arithmetic&LogicUnit(ALU)

Memory

BytesInstructionAddress

ReadInstructionBits

Program

Data

• ThePC(programcounter) isinternalregisterinsideprocessorholdingbyteaddressofnextinstruction tobeexecuted.

• Instructionisfetchedfrommemory, thencontrolunitexecutesinstructionusingdatapath andmemorysystem,andupdatesprogramcounter(defaultisadd+4bytestoPC,tomovetonextsequential instruction)

Reviewif-else Statement

• Assumingtranslationsbelow,compilef →$s0 g →$s1 h →$s2i →$s3 j →$s4

if (i == j) bne $s3,$s4,Else

f = g + h; add $s0,$s1,$s2

else j Exit

f = g – h; Else: sub $s0,$s1,$s2

Exit: 8

Control-flowGraphs:Avisualization

bne $s3,$s4,Else

add $s0,$s1,$s2 j Exit

Else:sub $s0,$s1,$s2Exit:

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bne $s3,$s4,Else

add$s0,$s1,$s2jExit

Else:sub$s0,$s1,$s2

Exit:…

Question!

Whatisthecodeabove?A: whileloopB: do…whileloopC: forloopD: AorCE: Notaloop

addi $s0,$zero,0Start: slt $t0,$s0,$s1

beq $t0,$zero,Exitsll $t1,$s0,2addu $t1,$t1,$s5lw $t1,0($t1) add $s4,$s4,$t1addi $s0,$s0,1j Start

Exit:

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MIPSboard

• Ifanybodyisinterested– Icanbuyoneortwoforexperimentation…

SixFundamentalStepsinCallingaFunction

1. Putparametersinaplacewherefunctioncanaccessthem

2. Transfercontroltofunction3. Acquire(local)storageresourcesneededfor

function4. Performdesiredtaskofthefunction5. Putresultvalueinaplacewherecallingcode

canaccessitandrestoreanyregistersyouused6. Returncontroltopointoforigin,sincea

functioncanbecalledfromseveralpointsinaprogram

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MIPSFunctionCallConventions

• Registersfasterthanmemory,sousethem• $a0–$a3:fourargumentregisterstopassparameters($4- $7)

• $v0,$v1:twovalueregisterstoreturnvalues($2,$3)

• $ra:onereturnaddressregistertoreturntothepointoforigin($31)

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InstructionSupportforFunctions(1/4)

... sum(a,b);... /* a,b:$s0,$s1 */}int sum(int x, int y) {return x+y;

}address (shown in decimal)1000 1004 1008 1012 1016 …2000 2004

C

MIPS

InMIPS,allinstructionsare4bytes,andstoredinmemoryjustlikedata.Sohereweshowtheaddressesofwheretheprogramsarestored.

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InstructionSupportforFunctions(2/4)

... sum(a,b);... /* a,b:$s0,$s1 */}int sum(int x, int y) {return x+y;

}address (shown in decimal)1000 add $a0,$s0,$zero # x = a1004 add $a1,$s1,$zero # y = b1008 addi $ra,$zero,1016 # $ra=10161012 j sum # jump to sum1016 … # next instruction…2000 sum: add $v0,$a0,$a12004 jr $ra # new instr. “jump register”

C

MIPS

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InstructionSupportforFunctions(3/4)

... sum(a,b);... /* a,b:$s0,$s1 */}int sum(int x, int y) {return x+y;

}

2000 sum: add $v0,$a0,$a12004 jr $ra # new instr. “jump register”

• Question:Whyuse jr here?Whynot usej?

• Answer:summightbecalledbymanyplaces,sowecan’treturntoafixedplace.Thecallingproctosummustbeabletosay“returnhere”somehow.

C

MIPS

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InstructionSupportforFunctions(4/4)• Singleinstructiontojumpandsavereturnaddress:jumpandlink(jal)

• Before:1008 addi $ra,$zero,1016 # $ra=10161012 j sum # goto sum

• After:1008 jal sum # $ra=1012,goto sum

• Whyhaveajal?– Makethecommoncasefast:functioncalls verycommon.– Don’thavetoknowwhere codeis inmemorywithjal!

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MIPSFunctionCallInstructions• Invokefunction:jumpandlinkinstruction(jal)

(reallyshouldbelaj “linkandjump”)– “link”meansformanaddressorlinkthatpointstocallingsitetoallowfunctiontoreturntoproperaddress

– Jumpstoaddressandsimultaneouslysavestheaddressofthefollowing instructioninregister$ra ($31)

jal FunctionLabel

• Returnfromfunction:jumpregisterinstruction(jr)– Unconditionaljumptoaddressspecifiedinregisterjr $ra

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NotesonFunctions• Callingprogram(caller)putsparametersintoregisters$a0-$a3 andusesjal X toinvoke(callee)ataddresslabeledX

• Musthaveregisterincomputerwithaddressofcurrentlyexecutinginstruction– InsteadofInstructionAddressRegister (bettername),historicallycalledProgramCounter (PC)

– It’saprogram’scounter;itdoesn’tcountprograms!

• Whatvaluedoesjal X placeinto$ra?????• jr $ra putsaddressinside$ra backintoPC

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WhereAreOldRegisterValuesSavedtoRestoreThemAfterFunctionCall?• Needaplacetosaveoldvaluesbeforecallfunction,restorethemwhenreturn,anddelete

• Idealisstack:last-in-first-outqueue(e.g.,stackofplates)– Push:placingdataontostack– Pop:removingdatafromstack

• Stackinmemory,soneedregistertopointtoit• $sp isthestackpointerinMIPS($29)• Conventionisgrowfromhightolowaddresses– Push decrements$sp,Pop increments$sp

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Exampleint Leaf(int g, int h, int i, int j)

{int f;f = (g + h) – (i + j);return f;

}• Parametervariablesg,h,i,andj inargumentregisters$a0,$a1,$a2,and$a3,andf in$s0

• Assumeneedonetemporaryregister$t0

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StackBefore,During,AfterFunction

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• Needtosaveoldvaluesof$s0 and$t0

Contentsof$s0Contentsof$t0

MIPSCodeforLeaf()

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Leaf: addi $sp,$sp,-8 # adjuststackfor2itemssw $t0, 4($sp) # save$t0foruseafterwardssw $s0, 0($sp) # save$s0foruseafterwards

add $s0,$a0,$a1 # f=g+hadd $t0,$a2,$a3 # t0=i +jsub $v0,$s0,$t0 # returnvalue(g+h)– (i +j)

lw $s0, 0($sp) # restoreregister$s0forcallerlw $t0, 4($sp) # restoreregister$t0forcalleraddi $sp,$sp,8 # adjuststacktodelete2itemsjr $ra # jumpbacktocallingroutine

Administrivia

• HW1wasdueyesterday– don’tpushafterthedeadlinetoavoidautomaticuseofslipdays(unlessyouhaveto)

• HW2hasbeenposted– duenextMonday• Useofslipdays…– Projectteamnumberofslipdays=min(#a,#b)

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WhatIfaFunctionCallsaFunction?RecursiveFunctionCalls?

• Wouldclobbervaluesin$a0 to$a3 and$ra• Whatisthesolution?

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NestedProcedures(1/2)

int sumSquare(int x, int y) {return mult(x,x)+ y;

}• SomethingcalledsumSquare,nowsumSquare iscallingmult

• Sothere’savaluein$ra thatsumSquarewantstojumpbackto,butthiswillbeoverwrittenbythecalltomult

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NeedtosavesumSquare returnaddressbeforecalltomult

NestedProcedures(2/2)

• Ingeneral,mayneedtosavesomeotherinfoinadditionto$ra.

• WhenaCprogramisrun,thereare3importantmemoryareasallocated:– Static:Variablesdeclaredonceperprogram,ceasetoexistonlyafterexecutioncompletes- e.g.,Cglobals

– Heap:Variablesdeclareddynamicallyviamalloc– Stack:Spacetobeusedbyprocedureduringexecution;thisiswherewecansaveregistervalues

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OptimizedFunctionConventionToreduceexpensiveloadsandstoresfromspillingandrestoringregisters,MIPSdividesregistersintotwocategories:

1. Preservedacrossfunctioncall– Callercanrelyonvaluesbeingunchanged– $sp,$gp,$fp,“savedregisters”$s0- $s7

2. Notpreservedacrossfunctioncall– Callercannotrelyonvaluesbeingunchanged– Returnvalueregisters$v0,$v1,Argumentregisters

$a0-$a3,“temporaryregisters”$t0-$t9,$ra28

Question

• WhichstatementisFALSE?

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B: jal savesPC+1in$ra

C: Thecallee canusetemporaryregisters($ti)withoutsavingandrestoringthem

D: Thecallercanrelyonsaveregisters($si)withoutfearofcallee changingthem

A:MIPSusesjal toinvokeafunctionandjr toreturnfromafunction

AllocatingSpaceonStack• Chastwostorageclasses:automaticandstatic– Automatic variablesarelocaltofunctionanddiscardedwhenfunctionexits

– Staticvariablesexistacrossexitsfromandentriestoprocedures

• Usestackforautomatic(local)variablesthatdon’tfitinregisters

• Procedure frameor activationrecord:segmentofstackwithsavedregistersandlocalvariables

• SomeMIPScompilersuseaframepointer($fp)topointtofirstwordofframe

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StackBefore,During,AfterCall

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UsingtheStack(1/2)

• Sowehavearegister$sp whichalwayspointstothelastusedspaceinthestack.

• Tousestack,wedecrementthispointerbytheamountofspaceweneedandthenfillitwithinfo.

• So,howdowecompilethis?int sumSquare(int x, int y) {

return mult(x,x)+ y;}

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UsingtheStack(2/2)

• Hand-compilesumSquare:

addi $sp,$sp,-8 # space on stacksw $ra, 4($sp) # save ret addrsw $a1, 0($sp) # save yadd $a1,$a0,$zero # mult(x,x)jal mult # call multlw $a1, 0($sp) # restore yadd $v0,$v0,$a1 # mult()+ylw $ra, 4($sp) # get ret addraddi $sp,$sp,8 # restore stackjr $ra

mult: ...

int sumSquare(int x, int y) {return mult(x,x)+ y; }

“push”

“pop”

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BasicStructureofaFunction

entry_label: addi $sp,$sp, -framesizesw $ra, framesize-4($sp) # save $rasave other regs if need be

...

restore other regs if need belw $ra, framesize-4($sp) # restore $raaddi $sp,$sp, framesizejr $ra

Epilogue

Prologue

Body (call other functions…)

ra

memory

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WhereistheStackinMemory?• MIPSconvention• Stackstartsinhighmemoryandgrowsdown– Hexadecimal(base16):7ffffffchex

• MIPSprograms(textsegment)inlowend– 00400000hex

• staticdatasegment(constantsandotherstaticvariables)abovetextforstaticvariables– MIPSconventionglobalpointer($gp)pointstostatic

• Heapabovestaticfordatastructuresthatgrowandshrink;growsuptohighaddresses

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MIPSMemoryAllocation

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RegisterAllocationandNumbering

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AndinConclusion…

• Functionscalledwithjal,returnwithjr $ra.• Thestackisyourfriend:Useittosaveanythingyouneed.Just leaveitthewayyoufoundit!

• Instructionsweknowsofar…Arithmetic:add, addi, sub, addu, addiu, subuMemory: lw, sw, lb, sb

Decision: beq, bne, slt, slti, sltu, sltiuUnconditionalBranches(Jumps):j, jal, jr

• Registersweknowsofar– Allofthem!– $a0-$a3forfunctionarguments,$v0-$v1forreturnvalues– $sp,stackpointer,$fp framepointer,$ra returnaddress

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