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3.6.3CachesandWriteBuffer
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Co-Processor15(CP15)
TheARM926EJ-Sprocessorincludes:
•AnInstructioncache(Icache)
•ADatacache(Dcache)
•Awritebuffer
Thesizeofthedatacacheis8KB,instructioncacheis16KB,andwritebufferis17bytes.
Thecacheshavethefollowingfeatures:
•Virtualindex,virtualtag,addressedusingtheModifiedVirtualAddress(MVA)
•Four-waysetassociative,withacachelinelengthofeightwordsperline(32bytesperline),andtwo
dirtybitsintheDcache
•Dcachesupportswrite-throughandwrite-back(orcopyback)cacheoperation,selectedbymemory
regionusingtheCandBbitsintheMMUtranslationtables
•Performcritical-wordfirstcacherefilling
•Cachelockdownregistersenablecontroloverwhichcachewaysareusedforallocationonalinefill,
providingamechanismforbothlockdownandcontrollingcachepollution.
•DcachestoresthePhysicalAddressTAG(PATAG)correspondingtoeachDcacheentryinthe
TAGRAMforuseduringthecachelinewrite-backs,inadditiontotheVirtualAddressTAGstoredinthe
TAGRAM.ThismeansthattheMMUisnotinvolvedinDcachewrite-backoperations,removingthe
possibilityofTLBmissesrelatedtothewrite-backaddress.
•Cachemaintenanceoperationstoprovideefficientinvalidationofthefollowing:
–TheentireDcacheorIcache
–RegionsoftheDcacheorIcache
–TheentireDcache
–Regionsofvirtualmemory
•Theyalsoprovideoperationsforefficientcleaningandinvalidationofthefollowing:
–TheentireDcache
–RegionsoftheDcache
–Regionsofvirtualmemory
Thewritebufferisusedforallwritestoanon-cachablebufferableregion,write-throughregion,andwrite
missestoawrite-backregion.AseparatebufferisincorporatedintheDcacheforholdingwrite-backfor
cachelineevictionsorcleaningofdirtycachelines.
Themainwritebufferhasa16-worddatabufferandafour-addressbuffer.
TheDcachewrite-backhaseightdatawordentriesandasingleaddressentry.
TheMCRdrainwritebufferenablesbothwritebufferstobedrainedundersoftwarecontrol.
TheMCRwaitforinterruptcausesbothwritebufferstobedrainedandtheARM926EJ-Sprocessortobe
putintoalowpowerstateuntilaninterruptoccurs.
Note:SeeChapter4oftheCachesandWriteBufferoftheARM926EJ-STRM,downloadablefrom
http://www.arm.com/arm/TRMsformoredetailedinformation.
SPRUEP9A–May2008ARMCore25
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