memmgmt.docx

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memmgmt

MemoryManagement:

WhatEveryDriverWriterNeedstoKnow

February28,2005

Abstract

Thispaperprovidesinformationaboutallocatingandusingmemoryinkernel-modedriversfortheMicrosoft®Windows®familyofoperatingsystems.Itdescribesthetypesofmemorythatareavailablefordriveruse,theappropriatetechniquesforallocatingandusingeachtype,andthebestwaystotestformemory-relatedproblems.

Thisinformationappliesforthefollowingoperatingsystems:

MicrosoftWindowsVista™

MicrosoftWindowsServer™2003

MicrosoftWindowsXP

MicrosoftWindows2000

ThecurrentversionofthispaperismaintainedontheWebat:

Referencesandresourcesdiscussedherearelistedattheendofthispaper.

Contents

Introduction4

VirtualandPhysicalMemory4

VirtualAddressSpace5

VirtualAddressSpaceon32-bitHardware5

VirtualAddressSpaceon64-bitHardware7

Types,Constants,andMacrosUsedinAddressing7

MappingVirtualMemorytoPhysicalMemory8

PhysicalAddressSpace9

Processor-RelativePhysicalAddresses12

Device-Bus-RelativePhysicalAddresses12

MemoryDescriptorLists13

PoolMemory15

IRQLConsiderations15

LookasideLists16

Caching16

Alignment16

NaturalAlignment16

Cache-LineAlignment19

KernelStack19

MemoryAllocationTechniques20

AllocatingMemoryforLong-TermStorage21

CreatingandUsingLookasideLists22

AllocatingContiguousMemory24

AccessingDeviceRegisters25

AccessingMemoryforI/OBuffers28

MappingDeviceMemoryandRegistersintoUserSpace30

SharedMemoryandSynchronization31

TestingandTroubleshooting32

ToolsforDiscoveringMemory-RelatedErrors32

BestPracticesforProgrammingandTesting33

SummaryofGuidelines34

Resources34

Disclaimer

Thisisapreliminarydocumentandmaybechangedsubstantiallypriortofinalcommercialreleaseofthesoftwaredescribedherein.

TheinformationcontainedinthisdocumentrepresentsthecurrentviewofMicrosoftCorporationontheissuesdiscussedasofthedateofpublication.BecauseMicrosoftmustrespondtochangingmarketconditions,itshouldnotbeinterpretedtobeacommitmentonthepartofMicrosoft,andMicrosoftcannotguaranteetheaccuracyofanyinformationpresentedafterthedateofpublication.

ThisWhitePaperisforinformationalpurposesonly.MICROSOFTMAKESNOWARRANTIES,EXPRESS,IMPLIEDORSTATUTORY,ASTOTHEINFORMATIONINTHISDOCUMENT.

Complyingwithallapplicablecopyrightlawsistheresponsibilityoftheuser.Withoutlimitingtherightsundercopyright,nopartofthisdocumentmaybereproduced,storedinorintroducedintoaretrievalsystem,ortransmittedinanyformorbyanymeans（electronic,mechanical,photocopying,recording,orotherwise）,orforanypurpose,withouttheexpresswrittenpermissionofMicrosoftCorporation.

Microsoftmayhavepatents,patentapplications,trademarks,copyrights,orotherintellectualpropertyrightscoveringsubjectmatterinthisdocument.ExceptasexpresslyprovidedinanywrittenlicenseagreementfromMicrosoft,thefurnishingofthisdocumentdoesnotgiveyouanylicensetothesepatents,trademarks,copyrights,orotherintellectualproperty.

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©2005MicrosoftCorporation.Allrightsreserved.

Microsoft,Windows,andWindowsServerareeitherregisteredtrademarksortrademarksofMicrosoftCorporationintheUnitedStatesand/orothercountries.

Thenamesofactualcompaniesandproductsmentionedhereinmaybethetrademarksoftheirrespectiveowners.

Introduction

Kernel-modedriversallocatememoryforvariouspurposes,suchasstoringinternaldataorusingasI/Obuffers.Tohelpdriverwritersusememorycorrectly,thispaperexplainsthefollowing:

∙Fundamentalinformationaboutphysicalandvirtualmemoryandaddressspaces.

∙Typesofmemorythatareavailabletodriversandwhentouseeachtype.

∙Techniquesforallocatingmemorytosatisfyadriver’sdiverserequirements.

∙TechniquesforaccessingmemoryallocatedbyothercomponentsforI/Ooperations.

∙Techniquesforsharingmemorywithotherkernel-modeanduser-modecomponents.

∙Techniquesfortestingandtroubleshootingmemoryallocationandusageproblems.

Note:

Thispaperdoesnotcovermemoryallocationfordirectmemoryaccess（DMA）.YoucanfinddetailedinformationaboutallaspectsofDMAin“DMASupportinWindowsDrivers,”whichislistedintheResourcessectionattheendofthispaper.

1VirtualandPhysicalMemory

TheamountofvirtualandphysicalmemorythatissupportedonanycomputerthatrunstheMicrosoft®Windows®operatingsystemisdeterminedbythehardwareconfigurationandtheeditionofWindowsinuse.On32-bithardware,thevirtualaddressspaceis4GBandthemaximumamountofphysicalmemoryrangesfrom4to128GB.On64-bithardware,thevirtualaddressspaceis16terabytesandthemaximumamountofmemoryrangesfrom64GBto1terabyte.

Table1liststheamountofvirtualmemoryandthemaximumamountofphysicalmemorythateacheditionofWindowssupports.

Table1.VirtualandPhysicalMemorySupportinWindows

Operatingsystemversion

Edition

Virtualmemory

Maximumphysicalmemory

MicrosoftWindowsServer™2003SP1

Standard

4GB

4GB

Web

4GB

2GB

Enterprise

4GB

64GB,ifhardwaresupportsPhysicalAddressExtension（PAE）

Enterprise（64-bit）

16terabytes

1terabyte

Datacenter

4GB

128GB,ifhardwaresupportsPAE

Datacenter（64-bit）

16terabytes

1terabyte

WindowsServer2003

Standard

4GB

4GB

Web

4GB

2GB

Enterprise

4GB

32GB,ifhardwaresupportsPAE

Enterprise（64-bit）

16terabytes

64GB

Datacenter

4GB

128GB,ifhardwaresupportsPAE

Datacenter（64-bit）

16terabytes

512GB

WindowsXP

Home

4GB

4GB

Professional

4GB

4GB

64-bitEditionVersion2003

16terabytes

128GB

Windows2000

Professional

4GB

4GB

Server

4GB

4GB

AdvancedServer

4GB

8GB

DatacenterServer

4GB

32GB,ifhardwaresupportsPAE

2VirtualAddressSpace

Avirtualaddressidentifiesalocationinvirtualmemory.Thevirtualaddressspaceisdividedintotworanges:

userspaceandsystemspace.

UserspaceistheportionofthevirtualaddressspaceintowhichWindowsmapsuser-modeprocesses,per-processdata,anduser-modedynamiclinklibraries（DLLs）.Eachprocesshasitsowncontext,thatis,thecodeanddatathattheprocessuses.Whiletheprocessisrunning,aportionofthecontext,calledtheworkingset,isresidentinmemory.

Systemspace,alsocalledkernelspace,istheportionoftheaddressspaceinwhichtheoperatingsystemandkernel-modedriversareresident.Itisaccessibleonlytokernel-modecode.

Thedistinctionbetweenuserspaceandsystemspaceisimportantinmaintainingsystemsecurity.Auser-modethreadcanaccessdataonlyinthecontextofitsownprocess.Itcannotaccessdatawithinotherprocesscontextsorwithinthesystemaddressspace.Thisrestrictionpreventsuser-modethreadsfromreadingandwritingdatathatbelongstootherprocessesortotheoperatingsystemanddrivers,whichcouldresultinsecurityvulnerabilitiesandpossiblyevenasystemcrash.

Kernel-modedriversaretrustedbytheoperatingsystemandconsequentlycanaccessbothuserspaceandsystemspace.Whenadriverroutineiscalledinthecontextofauserthread,allofthethread’sdataremainsintheuser-modeaddressspace.Thedrivercanaccesstheuser-spacedataforthethread—ifittakesthenecessarysecurityprecautions—inadditiontoaccessingsystem-spacedata.Theuser-modethread,however,doesnothaveaccesstothesystem-spacedataofthedriver.

Thesizesofthesystemanduseraddressspacesdependonwhetherthehardwareis32-bitor64-bitandontheoptionsappliedtoBoot.ini.

2.1VirtualAddressSpaceon32-bitHardware

On32-bitmachines,thelower2GBofthevirtualaddressspaceisuserspaceandtheupper2GBisreservedassystemspacebydefault.Ifuser-modeapplicationsrequireadditionalvirtualaddressspace,anadministratorcanreserve3GBforuserspace,thusleavingonly1 GBofsystemspace,byapplyingthe/3GBswitchtoBoot.iniandthenrestartingthemachine.Thisswitchisalsousefulduringtestingtoseehowadriverperformswithlimitedsystemaddressspace.TheswitchissupportedonWindowsServer2003（alleditions）,WindowsXP（allversions）,Windows2000AdvancedServer,andWindows2000DatacenterServer.

Figure1showshowthevirtualaddressspaceisorganizedon32-bitsystems.（Thefigureisnotdrawntoscale.）

Figure1.Layoutofvirtualaddressspaceon32-bitsystems

Asthefigureshows,theuseraddressspaceoccupiesthelowerrangeofthevirtualaddressspaceandthesystemaddressspaceoccupiestheupperrange.Bydefault,bothuserandsystemspaceare2GB.WhenWindowsisstartedwiththe/3GBswitch,thesizeoftheuserspaceincreasesto3GB.Theremaining1GBofvirtualaddressspacecontainsthesystemaddressspace.Theprocesspagetablesandhyperspaceremainatthesameaddresses,butothercomponentsdonot.Whenthesystemaddressspaceislimitedto1GB,themaximumsizesofthepagedandnonpagedpoolsarereducedandlessspaceisavailableforsystempagetableentries（PTEs）andthefilesystemcacheaswell.

Thefilesystemcache,whichisimplementedinsoftware,isnotthesameastheprocessorcache,whichisimplementedinhardware.Keepinmind,however,thatthis