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Lecture10–IntroductiontoMIPSDecisionsII

2008‐02‐13

Inwhatmayberegardedasthemostanticipatedgameinalongtime,EAwillbereleasingMaxis’SporeonPCs/Macs/NintendoDS™,andmobilephonesinthefall.Playerswillbeableto“createandevolvelife,establishtribes,buildcivilizations,sculptentireworldsandexploreothers’universes.”

LecturerSOEDanGarcia

www.spore.com

Obamasweeps8thstateinarow;it’sgettingtight!

CS61CL10IntroductiontoMIPS:DecisionsII(2) Garcia,Spring2008©UCB

Review

  Memoryisbyte‐addressable,butlwandswaccessonewordatatime.

  Apointer(usedbylwandsw)isjustamemoryaddress,sowecanaddtoitorsubtractfromit(usingoffset).

  ADecisionallowsustodecidewhattoexecuteatrun‐timeratherthancompile‐time.

  CDecisionsaremadeusingconditionalstatementswithinif,while,do while,for.

  MIPSDecisionmakinginstructionsaretheconditionalbranches:beqandbne.

  NewInstructions: lw, sw, beq, bne, j

CS61CL10IntroductiontoMIPS:DecisionsII(3) Garcia,Spring2008©UCB

Lasttime:Loading,Storingbytes1/2

  Inadditiontoworddatatransfers(lw,sw),MIPShasbytedatatransfers:  loadbyte:lb  storebyte:sb

  sameformataslw,sw   E.g.,lb $s0, 3($s1)

  contentsofmemorylocationwithaddress=sumof“3”+contentsofregisters1iscopiedtothelowbytepositionofregisters0.

CS61CL10IntroductiontoMIPS:DecisionsII(4) Garcia,Spring2008©UCB

x

Loading,Storingbytes2/2

 Whatdowithother24bitsinthe32bitregister?  lb:signextendstofillupper24bits

byteloaded…iscopiedto“sign‐extend”

Thisbit

xxxx xxxx xxxx xxxx xxxx xxxx zzz zzzz

 Normallydon’twanttosignextendchars

 MIPSinstructionthatdoesn’tsignextendwhenloadingbytes:

 loadbyteunsigned:lbu

CS61CL10IntroductiontoMIPS:DecisionsII(5) Garcia,Spring2008©UCB

OverflowinArithmetic(1/2)

  Reminder:Overflowoccurswhenthereisamistakeinarithmeticduetothelimitedprecisionincomputers.

  Example(4‐bitunsignednumbers): +15 1111 +3 0011 +18 10010  Butwedon’thaveroomfor5‐bitsolution,sothesolutionwouldbe0010,whichis+2,andwrong.

CS61CL10IntroductiontoMIPS:DecisionsII(6) Garcia,Spring2008©UCB

OverflowinArithmetic(2/2)

  Somelanguagesdetectoverflow(Ada),somedon’t(C)

 MIPSsolutionis2kindsofarithmeticinstructs:  Thesecauseoverflowtobedetected add(add) addimmediate(addi) subtract(sub)

  Thesedonotcauseoverflowdetection addunsigned(addu) addimmediateunsigned(addiu) subtractunsigned(subu)

  Compilerselectsappropriatearithmetic  MIPSCcompilersproduceaddu,addiu,subu

CS61CL10IntroductiontoMIPS:DecisionsII(7) Garcia,Spring2008©UCB

Two“Logic”Instructions

 Hereare2morenewinstructions ShiftLeft:sll $s1,$s2,2 #s1=s2<<2

  Storein$s1thevaluefrom$s2shifted2bitstotheleft,inserting0’sonright;<<inC

  Before:0000 0002hex0000 0000 0000 0000 0000 0000 0000 0010two

  After: 0000 0008hex0000 0000 0000 0000 0000 0000 0000 1000two

 Whatarithmeticeffectdoesshiftlefthave?

 ShiftRight:srlisoppositeshift;>>

CS61CL10IntroductiontoMIPS:DecisionsII(8) Garcia,Spring2008©UCB

LoopsinC/Assembly(1/3)

  SimpleloopinC;A[]isanarrayofints do { g = g + A[i]; i = i + j; } while (i != h);

  Rewritethisas: Loop: g = g + A[i]; i = i + j; if (i != h) goto Loop;

 Usethismapping: g, h, i, j, base of A $s1, $s2, $s3, $s4, $s5

CS61CL10IntroductiontoMIPS:DecisionsII(9) Garcia,Spring2008©UCB

LoopsinC/Assembly(2/3)

  FinalcompiledMIPScode: Loop: sll $t1,$s3,2 # $t1= 4*I addu $t1,$t1,$s5 # $t1=addr A+4i lw $t1,0($t1) # $t1=A[i] addu $s1,$s1,$t1 # g=g+A[i] addu $s3,$s3,$s4 # i=i+j bne $s3,$s2,Loop # goto Loop # if i!=h

 Originalcode: Loop: g = g + A[i]; i = i + j; if (i != h) goto Loop;

CS61CL10IntroductiontoMIPS:DecisionsII(10) Garcia,Spring2008©UCB

LoopsinC/Assembly(3/3)

  TherearethreetypesofloopsinC:  while   do… while   for

  Eachcanberewrittenaseitheroftheothertwo,sothemethodusedinthepreviousexamplecanbeappliedtotheseloopsaswell.

  KeyConcept:ThoughtherearemultiplewaysofwritingaloopinMIPS,thekeytodecision‐makingisconditionalbranch

CS61CL10IntroductiontoMIPS:DecisionsII(11) Garcia,Spring2008©UCB

Administrivia

  Project1dueFriday!  (ok,Saturday,buttellyourbrainit’sFriday!)

 HowusefulwasFauxExam1? Otheradministrivia?

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InequalitiesinMIPS(1/4)

 Untilnow,we’veonlytestedequalities(== and!=inC).Generalprogramsneedtotest<and>aswell.

  IntroduceMIPSInequalityInstruction:  “SetonLessThan”  Syntax:slt reg1,reg2,reg3   Meaning: if (reg2 < reg3) reg1 = 1; else reg1 = 0;

“set”means“changeto1”,“reset”means“changeto0”.

reg1 = (reg2 < reg3);

Samething…

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InequalitiesinMIPS(2/4)

  Howdoweusethis?Compilebyhand:if (g < h) goto Less; #g:$s0,h:$s1

  Answer:compiledMIPScode… slt $t0,$s0,$s1 # $t0 = 1 if g<h bne $t0,$0,Less # goto Less # if $t0!=0 # (if (g<h)) Less:

  Register$0alwayscontainsthevalue0,sobneandbeqoftenuseitforcomparisonafteransltinstruction.

  Asltbnepairmeansif(… < …)goto…

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InequalitiesinMIPS(3/4)

 Nowwecanimplement<,buthowdoweimplement>,≤and≥?

 Wecouldadd3moreinstructions,but:  MIPSgoal:SimplerisBetter

  Canweimplement≤inoneormoreinstructionsusingjustsltandbranches? Whatabout>? Whatabout≥?

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InequalitiesinMIPS(4/4)

# a:$s0,b:$s1 slt $t0,$s0,$s1 # $t0 = 1 if a<b beq $t0,$0,skip # skip if a >= b <stuff> # do if a<b skip:

Twoindependentvariationspossible:Useslt $t0,$s1,$s0 insteadofslt $t0,$s0,$s1

Usebneinsteadofbeq

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ImmediatesinInequalities

  Thereisalsoanimmediateversionofslttotestagainstconstants:slti   Helpfulinforloops

if (g >= 1) goto Loop Loop: . . .

slti $t0,$s0,1 # $t0 = 1 if # $s0<1 (g<1) beq $t0,$0,Loop # goto Loop # if $t0==0 # (if (g>=1))

C

MIPS

An slt beq pair means if(… ≥ …)goto…

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Whataboutunsignednumbers?

  Alsounsignedinequalityinstructions: sltu,sltiu…whichsetsresultto1or0dependingonunsignedcomparisons

 Whatisvalueof$t0,$t1?($s0 = FFFF FFFAhex,$s1 = 0000 FFFAhex)

slt $t0, $s0, $s1 sltu $t1, $s0, $s1

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MIPSSignedvs.Unsigned–diffmeanings!

 MIPStermsSigned/Unsigned“overloaded”:  Do/Don'tsignextend (lb, lbu)

  Do/Don'toverflow (add, addi, sub, mult, div)  (addu, addiu, subu, multu, divu)

  Dosigned/unsignedcompare (slt, slti/sltu, sltiu)

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WhatCcodeproperlyfillsintheblankinloopbelow?

PeerInstruction

do {i--;} while(__);

Loop:addi $s0,$s0,-1 # i = i - 1 slti $t0,$s1,2 # $t0 = (j < 2) beq $t0,$0 ,Loop # goto Loop if $t0 == 0 slt $t0,$s1,$s0 # $t0 = (j < i) bne $t0,$0 ,Loop # goto Loop if $t0 != 0

0: j < 2 && j < i 1: j ≥ 2 && j < i 2: j < 2 && j ≥ i 3: j ≥ 2 && j ≥ i 4: j > 2 && j < i 5: j < 2 || j < i 6: j ≥ 2 || j < i 7: j < 2 || j ≥ i 8: j ≥ 2 || j ≥ i 9: j > 2 || j < i

($s0=i, $s1=j)

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“Andinconclusion…”

  Tohelptheconditionalbranchesmakedecisionsconcerninginequalities,weintroduce:“SetonLessThan”calledslt,slti,sltu,sltiu

 Onecanstoreandload(signedandunsigned)bytesaswellaswordswithlb,lbu

 Unsignedadd/subdon’tcauseoverflow NewMIPSInstructions: sll, srl, lb, lbu slt, slti, sltu, sltiu addu, addiu, subu

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BonusSlides

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Example:TheCSwitchStatement(1/3)

  Chooseamongfouralternativesdependingonwhetherkhasthevalue0,1,2or3.CompilethisCcode:

switch (k) { case 0: f=i+j; break; /* k=0 */ case 1: f=g+h; break; /* k=1 */ case 2: f=g–h; break; /* k=2 */ case 3: f=i–j; break; /* k=3 */ }

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Example:TheCSwitchStatement(2/3)

  Thisiscomplicated,sosimplify.  Rewriteitasachainofif‐elsestatements,whichwealreadyknowhowtocompile:if(k==0) f=i+j; else if(k==1) f=g+h; else if(k==2) f=g–h; else if(k==3) f=i–j;

 Usethismapping: f:$s0, g:$s1, h:$s2, i:$s3, j:$s4, k:$s5

CS61CL10IntroductiontoMIPS:DecisionsII(24) Garcia,Spring2008©UCB

Example:TheCSwitchStatement(3/3)

  FinalcompiledMIPScode:bne $s5,$0,L1 # branch k!=0 add $s0,$s3,$s4 #k==0 so f=i+j j Exit # end of case so Exit L1: addi $t0,$s5,-1 # $t0=k-1 bne $t0,$0,L2 # branch k!=1 add $s0,$s1,$s2 #k==1 so f=g+h j Exit # end of case so Exit L2: addi $t0,$s5,-2 # $t0=k-2 bne $t0,$0,L3 # branch k!=2 sub $s0,$s1,$s2 #k==2 so f=g-h j Exit # end of case so Exit L3: addi $t0,$s5,-3 # $t0=k-3 bne $t0,$0,Exit # branch k!=3 sub $s0,$s3,$s4 #k==3 so f=i-j Exit: