MemoryUsageAsoftwareconventionstacksegment149holdsprocesslocalstoragemanagedinLIFOfashion149automaticallyallocatedbytheoperatingsystem149stackgrowsdowntowardloweraddressesasdataisputontoi ID: 837189
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1 Whyweusesubroutines•moremodularprog
Whyweusesubroutines•moremodularprogram(smallroutines,outsidedatapassedin)•morereadable,easiertodebug•codereusei.e.smallercodespace MemoryUsageAsoftwareconventionstacksegment:•holdsprocess-localstoragemanagedinLIFOfashion•automaticallyallocatedbytheoperatingsystem•stackgrowsdown(towardloweraddresses)asdataisputontoitstackpointerisautomaticallyloadedtopointtothelastallocatedlocationonthestack 0x00000000 0x04000000 0x10
2 000000 0x7fffffff Reserved staticdata dy
000000 0x7fffffff Reserved staticdata dynamic stack UsingtheStackStackpointer($spor$29)•pointstolastallocatedlocationonthestackStackoperations:•eachrequires2instructions•onetoadjustthestackpointer•onetotransferthedataonto/offthestacksubu$sp,$sp,4#allocateanewwordonthestacksw$8,0($sp)#pushthecontentsof$8lw$8,0($sp)#popthecontentsinto$8addu$sp,$sp,4#adjustthestackpointer•theassemblerrecognizesastackpointeroperation&generate
3 s•moreefficienttoallocateallthespac
s•moreefficienttoallocateallthespaceyou’llneedatonce•oftenallocate&deallocatethesameamount UsingtheStackExamplesavingvaluesonthestacksubu$sp,$sp,12sw$t1,8($sp)sw$t0,4($sp)sw$s0,0($sp)restoringvaluesfromthestacklw$t1,8($sp)lw$t0,4($sp)lw$s0,0($sp)addu$sp,$sp,12•allocateallthestackspaceyouwillneedforagroupofdata•thestacklocationsare“above”wherethestackpointerispointing,sothedisplacementsfrom$spwillbepositive $sp P
4 rocedureCallingConventionProtocolbetween
rocedureCallingConventionProtocolbetweenthecaller&thecallee:•whosaveswhichregisters•howparametersarepassed:inregisters?onthestack?•wherethereturnaddressislocatedWhyhaveacallingconvention?•caller&calleecaninteractcorrectly&efficiently•canhavesubroutinesthatarewrittenindifferentlanguages&compiledwithdifferentcompilers•easierforastandarddebuggerAcontractbetweenthehardware&software•hardware:•performssimpleinstr
5 uctions•hasdedicatedregisters•
uctions•hasdedicatedregisters•software:•controlsthesequenceofinstructionsforcalling&•determineswhichregisterpointstothestack•therulesoftheprotocolEacharchitecturehasitsownprotocol(s) WhyHaveTwoClassesofRegisters?Toreduceregisterspilling(storingregistersinmemorybecauseyouneedtousethemforsomethingelse)Callerknowswhatregistersitneedsaftertheprocedurecall.Calleeknowswhatregistersitisgoingtouse.Thereforedividetheresponsibiliti
6 es.Caller-savedregisters(t’s):notpr
es.Caller-savedregisters(t’s):notpreservedacrossprocedurecalls•callersavesthemifitwantstousethemaftertheprocedure•ifitdoesn’tneedthemlater,itdoesn’thavetosavethem•soacallerusesthemforshort-livedvalues•calleeknowsitcanusetheseregisterswithoutsavingthemsavings:caller-savedregistersonlyspilledifthecallerusesthemaftertheprocedurecallCallee-savedregisters(s’s):preservedacrossprocedurecalls•callerdoesn’
7 tsavethemeveniftheyareusedafterthe•
tsavethemeveniftheyareusedafterthe•acallerusesthemforlong-livedvalues•ifthecalleeneedstousethem,itmustsavethemfirstsavingscallee-savedregistersonlyspilledifthecalleeusesthem MIPSProcedureCallingConvention•savesthe“a”registersonthestack•savesthe“t”registersonthestack:$t0-$t9•passestheargumentsin$a0-$a3•savesadditionalpassedvaluesonthestack•executeswhichputsthereturnPCin$ra•determinesthesi
8 zeofthestackframe&changes$sptopointtothe
zeofthestackframe&changes$sptopointtotheendoftheframestackframe:aregionofthestackthatholdsallthedataforasingleprocedure(alsocalledprocedurecallframe,activationrecord)subu$sp,$sp,frameSize•putsitsreturnaddress&framepointer($fp)onthestack•savesthe“s”registersonthestack:$s0-$s7•sets$fpto$sp+stackframesize-4•putsitslocalvariablesonthestack•ifthecalleecallsaprocedure,itbecomesacallerNeithermodifiestheother’sport
9 ionofthestack StackFrameframepointer($fp
ionofthestack StackFrameframepointer($fpor$30):•pointstothefirstwordofastackframe•valueneverchanges:stableoffsetbaseforlocationswithinthestackframe•notalwaysused ExampleCallSequenceAssume:•argumentsarein$t0&$t1•wanttosavecaller-savedregisters$t6&$t7move$a0,$t0#firstargumentmove$a1,$t3#secondargumentsubu$sp,$sp,8#adjustthestackpointersw$t6,4($sp)#save$t6onthestacksw$t7,0($sp)#save$t7onthestackjaltargetThefirstthingthecalle
10 ewilldo(assumingitdoesnothavetosaveany$s
ewilldo(assumingitdoesnothavetosaveany$sregisters)subu$sp,$sp,4#adjustthestackpointersw$ra,0($sp)#savethereturnaddress MIPSProcedureReturnConventionReturnconvention:•putthereturnvaluesin$v0,$v1•restorethe“s”(caller-saved)registers•restores$fp&$ra•poptheactivationrecordbyaddingitssizeto$sp•returntothecallerbyjr$ra ExampleReturnSequenceBeforereturning,thecalleewillputresultsin$v0&$v1ifneeded&lw$ra,0($sp)#loadthere
11 turnaddressinto$raaddu$sp,$sp,4#adjustth
turnaddressinto$raaddu$sp,$sp,4#adjustthestackpointerjr$ra#returnThecallerwillrestore$t6&$t7andadjustthestacklw$t6,4($sp)#restore$t6lw$t7,0($sp)#restore$t7addu$sp,$sp,8#adjustthestackpointer UseofRegisters&theStackRegistersareusedfor:•passingasmallnumberofarguments(upto4,$a0-$a3)•passingthereturnaddresstothecallee($ra)•locatingthebeginningofthestackframe($fp)•keepingtrackofthetopofthestack($sp)•returningfunctionvalues($v0
12 ,$v1)Stackisusedfor:•passingparamet
,$v1)Stackisusedfor:•passingparametersifmorethan4•savingthecaller’sregisterparameters(a’s)(mightbeusedbythecallee)•savingthecaller’sregistersthataregoingtobeusedbythecalleraftertheprocedurereturns($t’s)•localdataforthecallee•thereturnaddressofthecaller(incasethecalleecallsaprocedure)•theframepointer(samereason)•anycallee-savedregistersthecalleeisgoingtouse($s’s)•stackframe•nota