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VAX/VMS and Internals Data Structures LAWRENCE KENAH & SIMON E BATE J. VAX/VMS Internals and Data Structures VAX/VMS Internals and Data Structures LAWRENCE KENAH J. SIMON BATE F. Digital Press Copyright 1984 by Digital Equipment Corporation. All rights reserved. Reproduction of this book, in whole or in part, is prohibited. For information, write Digital Press, Educational Services, Digital Equipment Corporation, Bedford, Massachusetts. The painting reproduced on the front cover is "From Red to Violet" (1970, oil on can- vas) by Hannes Beckmann, courtesy of the DeCordova Museum Collection: Gift of Mr. Michael F. Lynch. DEC, DECnet, UNIBUS, VAX, and VMS are trademarks ofDigital Equipment Corpora- tion. Designed by David Ford. Automatically typeset utilizing a VAX-11/780 by York Graphic Services, Incorporated. Printed in U.S.A. by Halliday Lithograph. Order number EY-00014-DP. Library of Congress Cataloging in Publication Data Kenah, Lawrence 1946- J., VAX/VMS internals and data structures. Includes index. 1. VAX/VMS (Computer operating system) 2. VAX-11 (Computer)— Programming. 3. Data structures (Computer science] I. Bate, Simon. II. Title. III. Title: VA.X./VM.S. internals and data structures. QA76.6.K454 1984 001.64'2 83-26187 ISBN 0-932376-52-5 Preface This bookexplains howtheVAX/VMSexecutiveworks. It describes thedata structures maintained and manipulated by the VMS operating system, discusses the mechanisms that transfer control between userprocesses and the VMS operating system (and among the components of the operating system itself), and describes some of the features of the VAX hardware as they are used by the VMS operating system. It also describes the VMS executive, in- cludingall themajor componentsoftheexecutive, aswell as systeminitiali- zation and the operation of all system services. It does not include a general discussion of the I/O subsystem, because that subject is already described in the VAX/VMS Guideto WritingaDeviceDriver(DigitalEquipmentCorpora- tion, 1982). However, thedetailsofsomeVAX/VMSdevicedrivers, aswellas the operations of I/O-related system services are included in this book. Thisbookisintendedforsystemprogrammersandotherusers oftheVAX/ VMS operating system who wish to understand the internal workings of the executive. The detailed description of data structures should help system managers make better informed decisions when they configure systems for space-or time-critical applications. It will also help application designers to appreciate the effects (in speed or in memory consumption) of different design and implementation decisions. This book assumes that the reader is familiar with the VAX architecture and the VMS operating system, particu- larly with its use of system services and its techniques of memory management. In explaining the operation of a subsystem of the executive, thisbook em- phasizes thedata structures manipulated bythat component, rather than detailed flow diagrams of major routines. This book differs from the reference manuals that make up the VAX/VMS documentation set in that it describes internal operations and data structures. Whileitis unlikely thatany component describedinthis bookwill be drastically changed with any major release of the VAX/VMS operating system, there is no guarantee that a particular data structure or subroutine described here will remain the same from release to release. With each new version of the operating system, privileged applicationprograms that rely on details containedin thisbookmustbetestedbeforetheyareusedforproduc- tion work with a standard load of users. This book is divided into nine parts, each of which describes a different aspect of the operating system. • Part 1 presentsanoverviewoftheVAX/VMSoperatingsystemandreviews those concepts that are crucial to understandingthe workings of that system. 1 Preface • Part 2 describes the mechanisms used to pass control between user pro- grams and the operating system and within the VMS system itself. • Part 3 describes scheduling and timer support, concluding with a discussion of the internals of the VAX/VMS lock manager. • Part 4 discusses memory management. • Part 5 describes the I/O subsystem. • Part 6 describes the creation and deletion of a process and the activation and termination of an image in the context of a process. • Part 7 deals with system initialization and also includes a discussion on the VAX-11/782. • Part 8 discusses miscellaneous topics that are not conveniently classified in any conventional catalog of operating systems: — The implementation of logical names — —The functions of miscellaneous system services The use of listing and map files — The conventions used in naming symbols • Part 9 provides information on VMS data structures. Mostoftheoperations oftheVMS executivecanbeeasilyunderstoodonce the contents of the various data structures are known. Although selected structures are described throughout the book, Appendix B describes (or provides pointers to) all the structures used by the operating system. The structures related todevice drivers andthefile systemarenotdescribed. The data structures related to device drivers are described in the VAX/VMS Guide to Writing a Device Driver. Data structures specific to the file system have yet to be documented. Several documents in theVAX/VMS document set supplyimportant back- ground information for the topics discussed in this book. The followingpro- videanespeciallyvaluablefoundation: VAX/VMS System ServicesReference Manual, the VAX-11 software installation guides, and the chapter in the VAX-11 Run-Time Library Reference Manual that describes condition han- dling. The concepts underlying the operating system are discussed in the VAX/ VMS SummaryDescription andGlossary, and the VAXSoftwareHandbook. Thefollowingdocumentsarealsohelpfulreferences: the VAX/VMS Guide to Writing a Device Driver, the VAX-11 Architecture Reference Manual, and the VAX Hardware Handbook. AnexcellentdescriptionoftheVAXarchitecture, aswellasadiscussionof some of the design decisions madeforits firstimplementation, the VAX-11/ 780, can befoundin ComputerProgrammingandArchitecture: The VAX-1 by Henry M. Levy and Richard H. Eckhouse, Jr. (Digital Press, 1980). This VI Preface bookalso contains abibliographyofsomeoftheliteraturedealingwithoper- ating system design. The reader should be aware of several conventions used throughout this book.Inalldiagramsofmemory, thelowestvirtualaddressappearsatthe top of the page and addresses increase toward the bottom of the page. This con- vention means that the direction of stack growth is toward the top of the page. In diagrams that display more detail, such as bytes within longwords, addresses also increase from right to left. That is, the lowest addressed byte (orbit) inalongwordis ontherighthandside ofafigureandthemostsignifi- cant byte (or bit) is on the lefthand side. Thewords "system" or "VMS system" areusedtodescribe theentire software package that is a part of a VAX-11 system, including privileged proc- esses, utilities, and other support software as well as the executive itself. The word "executive" refers to those parts of the VMS operating system that reside in system virtual address space. The executive includes the contents of the file SYS.EXE, device drivers, and other code and data structures loaded at initialization time, including RMS and the system message file. When either "process controlblock" or "PCB" is usedwithout amodifier, it refers to the software structure used by the scheduler. The data structure that contains copies of the general registers (that the hardware locates through the PR$_PCBB register) is always called the "hardware PCB." When referring to access modes, the term "inner access modes" means those access modes with more privilege. The term "outer access modes" means those access modes with less privilege. Thus, the innermost access mode is kernel and the outermost access mode is user. The term "SYSBOOT parameter" is used to describe any of the adjustable parameters that are used by the secondary bootstrap program SYSBOOT to configure the system. The adjustable parameters include both the dynamic parametersthatcanbechangedontherunningsystemandthestaticparame- ters that require a reboot in order for their values to change. These parameters are referred to by their parameter names rather than by the global locations where their values are stored. Appendix A relates the SYSBOOT parameter names to their corresponding global locations. The terms "byte index," "wordindex," "longword index," and so on, refer toamethodofaccessthatusestheVAX-11 contextindexingaddressingcapa- bility. That is, the index value will be multiplied by one, two, four, or eight (depending on whether a byte, word, longword, or quadword is being referenced)aspartofoperandevaluationinordertocalculatetheeffectiveaddress of the operand. In general, the component called INIT refers to a module of that name in the executive and not the volume initialization utility. When that utility program is being referenced, it will be clearly specified. vn Preface Three conventions are observed for lists. Inlists suchas this one, where thereisno orderorhierarchy, list elements are indicated by l—eading bullets (•). Sublists without hierarchy are indicated by dashes ( ). Lists thatindicateanorderedset ofoperations arenumbered. Sublists that indicate an ordered set of operations are lettered. Numbered lists with the numbers enclosed in circles indicate a corre- spondence between individual list elements and numbered items in a figure. ACKNOWLEDGMENTS Our first thanks must go to Joe Carchidi, for suggesting that this book be written, and to Dick Hustvedt, for his help and enlightening conversations. Wewouldlike to thankJohnLucasforputtingtogethertheinitialversions of Chapters 7, 10, 11, and 30 and Vik Muiznieks for writing the initial ver- sions of Chapters 5, 18, and 19. Appreciationgoes to all thosewho reviewed the draftsfor both editions of the book (VAX/VMS Version 2.2 and 3.3). We would particularity like to thank Kathy Morse for reviewing the first edition in its entirety and Wayne Cardoza for reviewing the entire second edition. Our special thanks go to Ruth Goldenberg for reviewing both editions in their entirety, and for her many corrections, comments, and suggestions. We owe a lot of thanks to our editing staff, especially to Jonathan Ostrowskyforhis labors inpreparingthefirstedition, andBetty Steinfeldfor herhelp and suggestions. Many thanks go to Jonathan Parsons forreviewing andeditingthesecondedition, andforallhishelp, patience, and suggestions. We would like to thank the Graphic Services department at Spitbrook, particularily Pat Walker for her help in paging and production of the first edition, and Paul King for his help in transforming innumerable slides and rough sketches into figures. Thanks go to Kathy Greenleaf and Jackie Markow for converting the files to our generic markup language. Thanks go to Larry Bohn, Sue Gault, Bill Heffner, Kathleen Jensen, and Judy Jurgens for their support and interest in this project. Finally, we would like to thank all those who originally designed and im- plemented the VAX/VMS operating system, and all those who have contrib- uted to later releases. Lawrence Kenah J. Simon F. Bate August 1983 Vlll 21 1 Contents PARTI/Introduction 2.1.2 OtherIPLLevelsUsedfor Synchronization 32 1 SystemOverview 3 2.1.3 IPL$_QUEUEAST 33 1. Process,Job, andImage 3 2.1.4 IPL2 34 1.1.1 Process 3 2.2 SerializedAccess 35 1.1.2 Image 5 2.2.1 ForkProcessing 35 1.1.3 Job 6 2.2.2 I/OPostprocessing 36 1. FunctionalityProvidedBy 2.3 MutualExclusionSemaphores VAX/VMS 6 (Mutexes) 36 1.2.1 OperatingSystemKernel 6 2.3.1 LockingaMutexforRead Access 37 1.2.3 UserInterface 9 1.2.4 InterfaceamongKernel 2.3.2 LockingaMutexforWrite Access 38 Subsystems 1 1.3 HardwareImplementation 2.3.3 MutexWaitState 39 oftheOperatingSystem 2.3.4 UnlockingaMutex 39 Kernel 13 2.3.5 ResourceWaitState 40 1.3.1 VAXArchitectureFeatures 2.4 VAX/VMSLockManagement ExploitedbyVMS 13 SystemServices 40 1.3.2 VAX-11 InstructionSet 14 1.3.3 ImplementationofVMSKernel Routines 15 DynamicMemory 1.3.4 MemoryManagementand Allocation 42 AccessModes 19 1.3.5 Exceptions, Interrupts,and 3.1 AllocationStrategyand REI 20 Implementation 42 1.3.6 ProcessStructure 21 3.1.1 AllocationofDynamic Memory 43 1.4 OtherSystemConcepts 22 1.4.1 ResourceControl 22 3.1.2 ExampleofAllocationof DynamicMemory 44 1.4.2 OtherSystemPrimitives 23 3.1.3 DeallocationofDynamic 1.5 LayoutofVirtualAddress Memory 45 Space 24 3.1.4 ExampleofDeallocationof 1.5.1 SystemVirtualAddress DynamicMemory 45 Space 24 1.5.2 TheControlRegion(PI 3.1.5 Synchronization 47 Space) 26 3.1.6 GranularityofAllocation 49 1.5.3 TheProgramRegion P0 3.2 PreallocatedRequest ( Packets 50 Space) 26 3.2.1 AllocationfromOneofthe LookasideLists 50 - 2 Synchronization 3.2.2 DeallocationtotheLookaside Techniques 30 List 51 2.1 ElevatedIPL 30 3.3 UseofDynamicMemory 53 2.1.1 UseofIPL$_SYNCH 31 3.3.1 ProcessAllocationRegion 53 IX 1 11 1 111 11 7 Contents 3.3.2 PagedDynamicMemory 53 5 HardwareInterrupts 98 3.3.3 NonpagedDynamic Memory 56 5. HardwareInterrupt Dispatching 98 5.1.1 InterruptDispatching 99 5.1.2 SystemControlBlock 100 PART II/Control Mechanisms 5.2 VAX/VMSInterruptService Routines 104 4 ConditionHandling 61 5.2. RestrictionsImposedon InterruptService 4. OverviewoftheCondition Routines 104 HandlingFacility 61 5.2.2 ServicingUNIBUS 4.1.1 GoalsoftheVAX-11 Condition Interrupts 105 HandlingFacility 61 5.2.3 MASSBUSInterruptService 4.1.2 FeaturesoftheVAX-1 Routines 109 ConditionHandling 5.2.4 DR32InterruptService Facility 62 Routine 112 4.2 GenerationofExceptions 63 5.2.5 MA780Interrupt 4.2. ExceptionsThatOriginatein Dispatching 112 theHardware 63 5.2.6 MA780Interruptsonthe 4.2.2 ExceptionsDetectedby VAX-11/782 114 Software 74 5.3 Connect-to-Interrupt 4.3 UniformException Mechanism 115 Dispatching 75 4.3. EstablishingaCondition Handler 77 6 SoftwareInterrupts 117 4.3.2 TheSearchforaCondition 6. TheSoftwareInterrupt 11 Handler 78 6.1.1 HardwareMechanismof 4.3.3 MultiplyActiveSignals 81 SoftwareInterrupts 117 4.4 ConditionHandlerAction 83 6.1.2 SoftwareInterruptService 4.4.1 ContinueorResignal 84 Routines 119 4.4.2 UnwindingtheCallStack 84 6.2 SoftwareInterruptLevelsin 4.4.3 ExampleofUnwindingtheCall VAX/VMS 119 Stack 85 6.2. MountVerification 4.4.4 PotentialInfiniteLoop 88 Cancellation 120 4.4.5 UnwindingMultiplyActive 6.2.2 ForkProcessing 121 Signals 88 6.2.3 SoftwareTimer 123 4.4.6 CorrectUseofDefaultDepth 6.2.4 I/OPostprocessing 123 inSYS$UNWIND 89 6.2.5 ReschedulingInterrupt 124 4.4.7 UnwindingAST's 92 6.2.6 ASTDeliveryInterrupt 125 4.5 Default(VMS-Supplied) ConditionHandlers 95 7 ASTDelivery 126 4.5.1 TracebackHandlerEstablished byImageStartup 95 7. HardwareAssistancetoAST 4.5.2 Catch-AilCondition Delivery 126 Handler 95 7.1.1 REIInstruction 126 4.5.3 HandlersUsedbyOtherAccess 7.1.2 ASTLVLProcessorRegister Modes 96 (PR$_ASTLVL) 127