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5.1CPURegisterTypes
Read−only Scratch Preserve
Init
Global
Local
Read−write
CPURegisterTypes
DSPalgorithmsareoftenwritteninassemblylanguageand,asaresult,theywilltakefulladvantageof
theinstructionset.Unfortunatelyforthesystemintegrator,thisoftenmeansthatmultiplealgorithms
cannotbeintegratedintoasinglesystembecauseofincompatibleassumptionsabouttheuseofspecific
featuresoftheDSP(e.g.,useofoverflowmode,useofdedicatedregisters,etc.).Thischaptercovers
thoseguidelinesthatarespecifictoaparticularDSPinstructionset.Theseguidelinesaredesignedto
maximizetheflexibilityofthealgorithmimplementers,whileatthesametimeensurethatmultiple
algorithmscanbeintegratedintoasinglesystem.
Forthepurposeoftheguidelinesbelow,wedefineseveralcategoriesofregistertypes.
•Scratchregister-theseregisterscanbefreelyusedbyanalgorithm,cannotbeassumedtocontain
anyparticularvalueuponentrytoanalgorithmfunction,andcanbeleftinanystateafterexitinga
function.
•Preserveregisters-theseregistersmaybeusedbyanalgorithm,cannotbeassumedtocontainany
particularvalueuponentrytoanalgorithmfunction,butmustberestoreduponexitfromanalgorithm
tothevalueithadatentry.
•Initializedregister-theseregistersmaybeusedbyanalgorithm,containaspecifiedinitialvalueupon
entrytoanalgorithmfunction(asstatednexttotheregister),andmustberestoreduponexitfromthe
algorithm.
•Read-onlyregister-theseregistersmaybereadbutmustnotbemodifiedbyanalgorithm.
Inadditiontothecategoriesdefinedabove,allregisterscanbefurtherclassifiedasbeingeitherlocalor
global.Localregistersarethreadspecific;i.e.,everythreadmaintainsitsowncopyofthisregisteranditis
activewheneverthisthreadisrunning.Globalregisters,ontheotherhand,aresharedbyallthreadsin
thesystem;ifonethreadchangesaglobalregisterthenallthreadswillseethechange.
Figure5-1belowdepictstherelationshipamongthevariousregistertypesdefinedabove.
Figure5-1.RegisterTypes
Inpreemptivesystems,globalregisterscanchangeatanypointthatpreemptionmayoccur.Local
registers,ontheotherhand,canonlybemodifiedbythecurrentexecutingthread.Thus,applicationcode
thatdependsexclusivelyonlocalregisterswillbeunaffectedbyotherpreemptingthreads.Conversely,
applicationcodethatdependsonglobalregistersmustpreventpreemptionaroundthosesectionsthat
havethisdependence.
Guideline10
Algorithmsshouldavoidtheuseofglobalregisters.
DSP-SpecificGuidelines 46SPRU352G–June2005–RevisedFebruary2007
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