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(The Mach Exception Handling Facility)SH
10 /Helvetica AF
21124 10681 MT
(David L. Black, David B. Golub, Karl Hauth)SH
8 SS 
29571 11478 MT
(1)SH
10 SS 
22179 11829 MT
(Avadis Tevanian)SH
30294 XM
(, and Richard Sanzi)SH
23736 14115 MT
(Computer Science Department)SH
24626 15258 MT
(Carnegie Mellon University)SH
25875 16401 MT
(Pittsburgh, PA 15213)SH
12 /Helvetica-Bold AF
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(1. Introduction)SH
10 /Helvetica AF
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(Exceptions are caused by the occurrence of unusual conditions during program)
90 W( execution; raising an)89 W
7200 22442 MT
(exception invokes the operating system to)
47 W( manage recovery from the unusual condition.  Although many)48 W
7200 23585 MT
(exceptions can be handled and dismissed transparently)
171 W( to the user \050e.g. page faults\051, some must be)170 W
7200 24728 MT
(exported in some form \050invoke handler, core dump,)
17 W( etc.\051.  This paper concerns the design and implemen-)18 W
7200 25871 MT
(tation of exception handling facilities that)
99 W( perform this exporting for general purpose operating systems.)98 W
7200 27014 MT
(We discuss both the overall design principles for these facilities and the)
100 W( specific design and implemen-)101 W
7200 28157 MT
(tation of an exception handling facility for the Mach)
108 W( [1])
SH( operating system.)
108 W( We)
493 W( also discuss the support)107 W
7200 29300 MT
(provided to applications that use exceptions \050e.g. debuggers\051 by this facility and)
238 W( other Mach kernel)239 W
7200 30443 MT
(facilities.)SH
8312 32727 MT
(Mach supports debuggers via a)
121 W( combination of independent kernel facilities instead of concentrating)120 W
7200 33870 MT
(debugger support in a single kernel component \050e.g. ptrace\051.  This)
376 W( approach avoids duplicating)377 W
7200 35013 MT
(functionality within the kernel by designing debugger support facilities)
64 W( to allow use by other applications.)63 W
7200 36156 MT
(This results in increased)
115 W( flexibility and functionality which benefits all applications using these facilities,)116 W
7200 37299 MT
(including debuggers.  In addition the kernel is simplified by the)
119 W( corresponding reduction in special pur-)118 W
7200 38442 MT
(pose debugger support code.)SH
8312 40726 MT
(The independence and network transparency)
121 W( properties of Mach's communication facility \050Mach ipc\051)122 W
7200 41869 MT
(also provide)
138 W( additional debugging functionality.  Mach ipc is used to implement not only the exception)137 W
7200 43012 MT
(handling facility, but also the interfaces to all Mach kernel facilities.  The independence and network)189 W
7200 44155 MT
(transparency properties separate applications from the servers \050including the kernel\051 that)
243 W( implement)242 W
7200 45298 MT
(facilities. This)
286 W( separation supports parallel and)
4 W( distributed debugging by removing restrictions on relation-)5 W
7200 46441 MT
(ships between)
146 W( debuggers and other applications, and by allowing components of both debuggers and)145 W
7200 47584 MT
(applications to be distributed over a network without change.)SH
8312 49868 MT
(The remainder of this paper has the following structure.  We begin with an introduction to the Mach)121 W
7200 51011 MT
(system and a review of exceptions and their uses including previous work in this area.  We then)
28 W( describe)27 W
7200 52154 MT
(a general model for exception handling and use it to design)
127 W( the Mach exception handling facility.  Ad-)128 W
7200 53297 MT
(ditional sections discuss the implementation of applications that use)
9 W( exceptions with particular attention to)8 W
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(debuggers. We)
278 W( conclude by describing a debugger that has been implemented using this facility.)SH
12 /Helvetica-Bold AF
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(2. The Mach System)SH
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(Mach is)
162 W( a multiprocessor operating system under development at Carnegie-Mellon University.  It is)163 W
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(based on a simple communication-oriented kernel that)
108 W( exports a small number of abstractions to users)107 W
7200 61624 MT
(via an integrated interface; many traditional operating system functions are implemented)
352 W( by user)353 W
7200 62767 MT
(programs or servers outside the kernel.  The kernel)
98 W( itself exports exactly five abstractions to users, the)97 W
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(task)SH
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(,)SH
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(thread)SH
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(,)SH
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(port)SH
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(,)SH
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(message)SH
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(, and)189 W
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(memory object)189 W
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(. Of)
658 W( these the)190 W
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(memory object)190 W
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(is not relevant to the)190 W
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(subject matter of this paper.)SH
8312 67337 MT
(Mach splits the traditional notion of a process into)
21 W( the)20 W
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(task)SH
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(and)SH
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(thread)SH
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39596 XM
(abstractions. A)318 W
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(task)SH
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(is the basic)20 W
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(1)SH
8 SS 
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(Author's present address is NeXT, Inc., Palo Alto, CA.)SH
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(unit of resource allocation.  It includes a paged virtual address space and protected access to system)132 W
7200 9072 MT
(resources such as communication capabilities and virtual memory.  The)77 W
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39925 XM
(thread)SH
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(is the)
77 W( basic unit of com-)76 W
7200 10215 MT
(putation. It)
500 W( is a lightweight process executing withing a task, and consists solely of)
111 W( a processing state)112 W
7200 11358 MT
(\050i.e. program counter and registers\051.  All threads within a task share the address)
56 W( space and communica-)55 W
7200 12501 MT
(tion rights of that task.  A traditional process would consist of a task with a single)
136 W( thread in the Mach)137 W
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(system.)SH
8312 15928 MT
(Mach's communication facility is implemented by the)212 W
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(port)SH
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(and)SH
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(message)SH
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(abstractions. A)700 W
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(port)SH
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(is a)211 W
7200 17071 MT
(communication channel)92 W
/Symbol SF
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(-)SH
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(logically a queue for messages protected by the kernel.  Ports are the refer-)93 W
7200 18214 MT
(ence objects in)
38 W( Mach and are used in a similar fashion to object references in an object oriented system.)37 W
7200 19357 MT
(Send and Receive primitive operations implement communication by sending and receiving messages to)42 W
7200 20500 MT
(and from ports.  A)56 W
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(message)SH
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(is a typed collection of data objects)
56 W( used in communication; messages may)55 W
7200 21643 MT
(be of any size and can contain)
55 W( pointers and typed capabilities for ports.  This communication facility is a)56 W
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(basic building block on which more complicated objects may be constructed; it is)
145 W( the underlying com-)144 W
7200 23929 MT
(munication mechanism for the Mach exception handling facility.)SH
12 /Helvetica-Bold AF
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(3. Exceptions and their Uses)SH
10 /Helvetica AF
8312 28827 MT
(Exceptions are synchronous interruptions to the normal flow of program control caused by the program)20 W
7200 29970 MT
(itself. This)
690 W( excludes software)
206 W( interrupts and other actions caused by asynchronous external events.)205 W
7200 31113 MT
(Exceptions include illegal)
30 W( accesses \050bus errors, segmentation and protection violations\051, arithmetic errors)31 W
7200 32256 MT
(\050overflow, underflow,)
140 W( divide by zero\051, and hardware instructions intended to support emulation, debug-)139 W
7200 33399 MT
(ging, error detection, etc.  Hardware exceptions cause traps into the operating)
250 W( system; the system)251 W
7200 34542 MT
(handles certain exceptions transparently \050e.g.)
47 W( recoverable page faults\051, but the remaining exceptions are)46 W
7200 35685 MT
(exported to the user by the operating system's exception handling facility.)SH
8312 37969 MT
(There are four major classes of applications that use exceptions:)SH
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(\267)SH
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(Debugging. Debuggers)
712 W( rely on exceptions generated by hardware trace and breakpoint)217 W
9980 40584 MT
(facilities. Other)
842 W( exceptions that indicate errors must be reported to the debugger; the)282 W
9980 41727 MT
(presence of the debugger indicates the user's interest in any anomalous program behavior.)SH
/Symbol SF
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(\267)SH
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9980 XM
(Core dumps.  In)
37 W( the absence of a debugger, a fatal exception can cause the execution state)38 W
8 SS 
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(2)SH
10 SS 
9980 44690 MT
(of a program to be saved \050in a file\051 for later examination.  Unix)68 W
39055 XM
(systems refer to these files)68 W
9980 45833 MT
(as `core dumps' for historical reasons. [3])SH
/Symbol SF
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(\267)SH
/Helvetica SF
9980 XM
(Error handling.)
50 W( Certain)
379 W( applications handle their own exceptions \050particularly arithmetic\051 un-)51 W
9980 48791 MT
(der some circumstances.)
138 W( For)
552 W( example, a handler could substitute zero for the result of a)137 W
9980 49934 MT
(floating underflow and continue execution.  Error handlers are often required)
134 W( by high-level)135 W
8 SS 
18930 50731 MT
(3)SH
10 SS 
9980 51082 MT
(languages \050e.g. Ada)SH
19653 XM
([2]\051.)SH
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(\267)SH
/Helvetica SF
9980 XM
(Emulation. Virtually)
400 W( all modern machines generate exceptions upon encountering operation)61 W
9980 54040 MT
(codes that cannot be executed by the hardware.  Emulators can)
192 W( be built to execute the)193 W
9980 55183 MT
(desired operation in software.  Such emulators)
187 W( serve to extend the instruction set of the)186 W
9980 56326 MT
(underlying machine by performing)
59 W( instructions that are not present in the hardware \050e.g. the)60 W
8 SS 
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(4)SH
10 SS 
9980 57474 MT
(Microvax II)SH
15537 XM
(relies on software "macrocode" to implement infrequently used instructions [5]\051)SH
8312 59758 MT
(Many computers regard system calls as another form of exception)
60 W( at the hardware level; system calls)59 W
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(5)SH
10 SS 
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(cause the hardware to suspend execution and trap into the operating system \050e.g.)
10 W( VAX)11 W
46519 XM
([4]\051. The)
300 W( operat-)11 W
7200 62049 MT
(ing system distinguishes system calls from other)
202 W( hardware exceptions; system calls are requests for)201 W
10800 50 7200 64568 UL
6 SS 
8090 65963 MT
(2)SH
8 SS 
8424 66276 MT
(Unix is a trademark of AT&T Bell Laboratories)SH
6 SS 
8090 67871 MT
(3)SH
8 SS 
8424 68184 MT
(Ada is a registered trademark of the U.S. Government, Ada Joint Program Office)SH
6 SS 
8090 69779 MT
(4)SH
8 SS 
8424 70092 MT
(Microvax and Microvax II are trademarks of Digital Equipment Corporation)SH
6 SS 
8090 71687 MT
(5)SH
8 SS 
8424 72000 MT
(VAX is a trademark of Digital Equipment Corporation)SH
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(services from the operating system,)
299 W( and are better viewed as extracodes or meta-instructions im-)300 W
7200 9072 MT
(plemented by the operating system)
167 W( [8].)
SH( This)
612 W( places system calls beyond the)
167 W( scope of any exception)166 W
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(handling facility.)SH
12 /Helvetica-Bold AF
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(4. Existing Approaches)SH
10 /Helvetica AF
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(Operating systems designers have)
14 W( approached the area of exceptions in a variety of ways. The Accent)15 W
7200 16256 MT
(operating system)
195 W( [7],)
SH( a predecessor of Mach, used a)
195 W( message-based facility for debugger invocation)194 W
7200 17399 MT
(upon exception occurrence.  This debugger was often the kernel debugger which had limited utility for)126 W
7200 18542 MT
(debugging user programs.)
75 W( In)
427 W( contrast, the Medusa operating system for Cm*)
74 W( [6])
SH( provided almost half a)74 W
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(dozen different options for handling exceptions.  Medusa introduced the idea of)
534 W( stopping a)535 W
7200 20828 MT
(thread/task/process when an exception)
73 W( occurred and invoking a separate "buddy" entity to deal with the)72 W
7200 21971 MT
(exception. Most)
404 W( other research systems have)
63 W( designed exception handling facilities on an ad-hoc basis)64 W
7200 23114 MT
(to provide the minimum functionality necessary for the)
108 W( debugger\050s\051 and/or error handler\050s\051 on that sys-)107 W
7200 24257 MT
(tem. Often)
450 W( this involved a)
86 W( simple mechanism to reflect exceptions back to a user program \050in a limited)87 W
7200 25400 MT
(fashion\051 or cause a core dump.  The major)
18 W( drawbacks of these approaches are limited functionality \050often)17 W
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(caused by design solely for use by debuggers\051, and lack of extensibility to a multi-threaded environment.)SH
8312 28827 MT
(Unix generalizes exception handling to the signal facility which handles all)
218 W( interruptions to normal)219 W
7200 29970 MT
(program flow.  Differing requirements of the various)
163 W( interruptions \050exceptions, timer expiration, control)162 W
7200 31113 MT
(character on terminal, etc.\051 entail semantics that vary from signal to signal;)
170 W( the default action can be)171 W
7200 32256 MT
(nothing, stop, continue from stop, or terminate \050with or without a core dump\051.  Users can)
95 W( change these)94 W
7200 33399 MT
(defaults or specify a handler to be invoked by a signal.  The interface to these handlers includes)
32 W( a partial)33 W
7200 34542 MT
(machine context; registers outside this)
108 W( context are not accessible.  Debugging support is centralized in)107 W
7200 35685 MT
(the ptrace\050\051 system call; it performs all data transfer and process control needed by debuggers,)
189 W( and)190 W
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(interacts with the signal facility to make signals)
86 W( visible to debuggers \050including signals that would other-)85 W
7200 37971 MT
(wise invoke error handlers)
32 W( or emulators\051.  The occurrence of a signal in a debugged process causes that)33 W
7200 39114 MT
(process to stop in a peculiar manner and notify the)
233 W( debugger that something has happened.  This)232 W
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(notification is implemented by special treatment of debugged processes in the)
37 W( wait\050\051 system call; this call)38 W
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(usually detects terminated \050zombie\051 processes, but also)
297 W( detects stopped processes that are being)296 W
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(debugged. One)
332 W( consequence of these features)
27 W( and their implementation is that debuggers are restricted)28 W
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(to debugging processes that are the immediate children of the debugger.)SH
8312 45970 MT
(There are)
100 W( a number of major problems with the Unix signal facility.  Two of the most important ones)99 W
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(are:)SH
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(\267)SH
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(Executing the signal handler in the same context as)
162 W( the exception makes many registers)163 W
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(inaccessible. These)
578 W( registers are often the very registers that an)
150 W( arithmetic error handler)149 W
9980 50871 MT
(needs to modify.  \050e.g. by substituting 0 for a floating underflow\051.)SH
/Symbol SF
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(\267)SH
/Helvetica SF
9980 XM
(The entire concept of signals is predicated on single-threaded)
18 W( applications.  Adapting signals)19 W
9980 53829 MT
(to multi-threaded applications is difficult and complicates the)
60 W( interface to them.  At least half)59 W
9980 54972 MT
(a dozen major changes to the Unix)
174 W( signal implementation in the Mach kernel have been)175 W
9980 56115 MT
(required for this reason.)SH
8312 58399 MT
(The typical)
112 W( use of signal handlers is to detect and respond to external events; for this they are ade-)111 W
7200 59542 MT
(quate, but as)
211 W( an exception handling facility they leave much to be desired. The original Unix signal)212 W
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(mechanism wasn't even adequate)
82 W( for detecting and responding to external events; signals were viewed)81 W
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(as almost always fatal [3].)SH
12 /Helvetica-Bold AF
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(5. A Model for Exception Handling)SH
10 /Helvetica AF
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(The Mach exception handling)
13 W( facility is based on a general model that describes the use of exceptions.)14 W
7200 67869 MT
(This model is derived from)
112 W( the requirements of applications that use exceptions.  The generality of the)111 W
7200 69012 MT
(resulting model is sufficient to describe)
207 W( virtually all uses of exceptions, including those made by the)208 W
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(applications discussed in Section 3.)SH
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0 SI
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(Applications that use exceptions can be divided into two major classes:)SH
9424 9330 MT
(1.)SH
10536 XM
(Error Handlers: These components perform)
122 W( recovery actions in response to an exception)121 W
10536 10473 MT
(and resume execution of the thread involved.  This class includes both error)
103 W( or exception)104 W
10536 11616 MT
(handlers and emulators.  Error Handlers typically execute in)
133 W( the same address space as)132 W
10536 12759 MT
(that thread for efficiency reasons \050access to state\051.)SH
9424 14574 MT
(2.)SH
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(Debuggers: These components examine the)
186 W( state of an entire application to investigate)187 W
10536 15717 MT
(why an exception occurred)
142 W( and/or why the program is misbehaving.  This class includes)141 W
10536 16860 MT
(interactive debuggers)
140 W( and servers that produce core dumps; the latter can be viewed as)141 W
10536 18003 MT
(front ends to debuggers that examine core dumps.  Debuggers usually execute in)
64 W( address)63 W
10536 19146 MT
(spaces distinct from the application for protection reasons.)SH
7200 20547 MT
(This paper uses)
86 W( the terms `error handler' and `debugger' to refer to the entire classes delineated above)87 W
7200 21690 MT
(\050e.g. a core dumper is a `debugger'\051.  In addition the term)
31 W( `handler' is used to refer to any application that)30 W
7200 22833 MT
(uses exceptions.)SH
8312 25117 MT
(Our model is derived by examining the requirements common to)
85 W( error handlers and debuggers.  The)86 W
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(occurrence of)
39 W( an exception requires suspension of the thread involved and notification of a handler.  The)38 W
7200 27403 MT
(handler receives)
243 W( the notification and performs some computation \050e.g. error handler fixes the error,)244 W
7200 28546 MT
(debugger decides what to do next\051, after which the thread is either resumed or terminated.  On)
31 W( this basis)30 W
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(we propose the following model to cover all uses of exceptions:  the occurrence of an)
6 W( exception invokes a)7 W
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(four step process involving the thread that caused the exception \050Victim\051 and the entity)
73 W( that handles the)72 W
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(exception \050Handler, may be the operating system\051:)SH
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(1.)SH
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(Victim:)SH
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(raise)SH
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(-- cause notification of an exception's occurrence.)SH
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(2.)SH
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(Victim:)SH
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(wait)SH
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(-- synchronize with completion of exception handling.)SH
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(3.)SH
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(Handler:)SH
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(catch)SH
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(-- receive)
78 W( notification.  This notification usually identifies the exception and)79 W
10536 38149 MT
(the victim.  Some of this identification may be implicit in where and how the)
94 W( notification is)93 W
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(received.)SH
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(4.)SH
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(Handler: take action.  There are two possible actions:)SH
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(\267)SH
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(clear)SH
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(-- clear exception causing victim to return from)SH
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(wait)SH
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(.)SH
/Symbol SF
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(\267)SH
/Helvetica-Bold SF
13316 XM
(terminate)SH
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(-- cause termination of victim thread.)SH
7200 45123 MT
(The boldface primitives in this model constitute the high-level model interface to exceptions and can be)89 W
7200 46266 MT
(viewed as operating on "exception objects".)
20 W( The)
317 W( handler will usually perform other functions between the)19 W
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(catch)SH
/Helvetica SF
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(and)SH
/Helvetica-Bold SF
12056 XM
(clear)SH
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(or)SH
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(terminate)SH
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(steps; these functions are particular to)
10 W( the handler application itself and are)11 W
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(not part of the exception model.)SH
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(Any exception handling facility must implement these primitives in some form;)
219 W( as an example we)218 W
7200 51979 MT
(consider signal handling in Unix.  An exception that invokes a signal handler uses the)
SH( following implemen-)1 W
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(tation of the model:)SH
9424 54523 MT
(1.)SH
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(raise)SH
/Helvetica SF
13153 XM
(- Internal kernel code that translates)
4 W( the hardware exception to a signal and sends the)3 W
10536 55666 MT
(signal to the process.  The kill\050\051 system call can be used to mimic this)
139 W( for user-detected)140 W
10536 56809 MT
(exceptions.)SH
9424 58624 MT
(2.)SH
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(wait)SH
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12879 XM
(- Implicit because the handler and victim execute in the same process; execution of)119 W
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(the victim cannot resume until the handler completes.)SH
9424 61582 MT
(3.)SH
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(catch)SH
/Helvetica SF
13445 XM
(- Internal kernel code that invokes the handler and sets)
19 W( up its stack.  Both the excep-)20 W
10536 62725 MT
(tion and the entity that caused it are implicit in the choice)
108 W( of the handler and the process)107 W
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(context that it executes in.)SH
9424 65683 MT
(4.)SH
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(clear)SH
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(- sigcleanup\050\051 or sigreturn\050\051 system call invoked when handler exits.  These calls)197 W
10536 66826 MT
(unwind the stack and clear the kernel state associated with a signal handler.)SH
9424 68641 MT
(5.)SH
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(terminate)SH
/Helvetica SF
15315 XM
(- exit\050\051 system call.)SH
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(The model)
1 W( also applies to Unix support for debuggers; this is of less interest because the support consists)SH
7200 71185 MT
(primarily of special-purpose code)
155 W( in the Unix kernel.  Much of this code duplicates functionality found)156 W
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(elsewhere in the kernel.)SH
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(This exception handling)
53 W( model also serves as a guide for future exception handling facilities.  Many of)52 W
7200 11356 MT
(the problems exhibited by other exception handling facilities are due to their implementation of a)
SH( restricted)1 W
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(version of our model \050e.g. Unix signal handler must execute in victim's context\051.)
43 W( These)
363 W( problems can be)42 W
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(avoided by a design that implements the model in full generality; such a design can)
43 W( be more flexible and)44 W
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(provide greater functionality without impacting the size of the kernel.  The next)
108 W( section provides an ex-)107 W
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(ample of such a design, the Mach exception handling facility.)SH
12 /Helvetica-Bold AF
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(6. Design of the Mach Exception Handling Facility)SH
10 /Helvetica AF
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(The Mach exception handling facility was designed from scratch as a general)
123 W( implementation of our)124 W
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(exception handling model.  The major design goals for this new facility were:)SH
/Symbol SF
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(\267)SH
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(A single facility with consistent semantics for all exceptions.)SH
/Symbol SF
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(\267)SH
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(Clean and simple interface.)SH
/Symbol SF
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(\267)SH
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9980 XM
(Full support for debuggers and error handlers.)SH
/Symbol SF
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(\267)SH
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9980 XM
(No duplication of functionality within kernel.)SH
/Symbol SF
9242 30701 MT
(\267)SH
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9980 XM
(Support user-defined exceptions.)SH
7200 32102 MT
(A consequence of these goals is that)
59 W( we reject the notion of a handler executing in the same context as)58 W
7200 33245 MT
(the exception it is handling.  We know of no clean and straightforward way to make)
111 W( a thread's context)112 W
7200 34388 MT
(available to the thread itself; this)
39 W( results in a single thread having multiple contexts \050a currently executing)38 W
7200 35531 MT
(context and one or more saved exception contexts\051. In turn this causes)
59 W( serious naming and functionality)60 W
7200 36674 MT
(problems for operations)
172 W( that access or manipulate thread contexts.  Because Mach supports multiple)171 W
7200 37817 MT
(threads within)
39 W( the same task, it is sufficient to stop the thread that caused the exception and execute the)40 W
7200 38960 MT
(handler as another thread in the same task.)SH
8312 41244 MT
(The Mach exception handling facility implements the)
65 W( exception handling model via Mach kernel primi-)64 W
7200 42387 MT
(tives to avoid duplication of kernel functionality.)
9 W( Because)
298 W( the handler never executes in the context of the)10 W
7200 43530 MT
(victim thread, the)91 W
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(raise)SH
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(,)SH
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(wait)SH
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(,)SH
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20939 XM
(notify)SH
/Helvetica SF
(, and)91 W
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(clear)SH
/Helvetica SF
29049 XM
(primitives constitute a)
91 W( remote procedure call \050rpc\051.  We)90 W
7200 44673 MT
(therefore implement them using a message-based rpc provided by Mach's communication facility.  The)103 W
7200 45816 MT
(remaining)SH
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(terminate)SH
/Helvetica SF
16831 XM
(primitive is exactly Mach's)92 W
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(thread_terminate)SH
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(or)SH
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(task_terminate)SH
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(primitive; no)
92 W( spe-)91 W
7200 46959 MT
(cial action is required to terminate the thread or task instead of completing the rpc.)SH
8312 49243 MT
(The exception rpc consists of two messages; an initial message to invoke)
3 W( the rpc, and a reply message)4 W
7200 50386 MT
(to complete the rpc.  The initial message contains the following items:)SH
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(\267)SH
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(Send and reply ports for the rpc.)SH
/Symbol SF
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(\267)SH
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(The identities of thread that caused the exception and the corresponding task.)SH
/Symbol SF
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(\267)SH
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(A machine independent exception class.  \050cf. appendix A\051)SH
/Symbol SF
9242 55287 MT
(\267)SH
/Helvetica SF
9980 XM
(Two machine-dependent fields that further identify the exception.)SH
7200 56688 MT
(If the rpc is completed, the reply message contains the two rpc ports and a)
50 W( return code from the handler)49 W
7200 57831 MT
(that handled the exception \050success in almost all cases\051.  MiG \050Mach)
11 W( interface Generator\051 generated stub)12 W
7200 58974 MT
(routines perform the generation and)
23 W( decoding of the messages; this allows users to avoid dealing directly)22 W
7200 60117 MT
(with the contents of the messages.)SH
8312 62401 MT
(An exception rpc corresponds to our exception model as follows:)SH
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(\267)SH
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(raise)SH
/Helvetica SF
12593 XM
(- send initial message.)SH
/Symbol SF
9242 65016 MT
(\267)SH
/Helvetica-Bold SF
9980 XM
(wait)SH
/Helvetica SF
12203 XM
(- wait for and receive reply message.)SH
/Symbol SF
9242 66159 MT
(\267)SH
/Helvetica-Bold SF
9980 XM
(catch)SH
/Helvetica SF
12870 XM
(- receive initial message.)SH
/Symbol SF
9242 67302 MT
(\267)SH
/Helvetica-Bold SF
9980 XM
(clear)SH
/Helvetica SF
12593 XM
(- send reply message.)SH
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(6.1. Exception Ports)SH
10 /Helvetica AF
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(The two messages that constitute the rpc are sent to and)
142 W( received from ports corresponding to the)143 W
7200 10288 MT
(handler \050initial message\051 and victim \050reply message\051.)
101 W( The)
478 W( handler's port is registered as the exception)100 W
7200 11431 MT
(port for either the victim's task or thread; the kernel consults this registration when an)
73 W( exception occurs.)74 W
7200 12574 MT
(The reply port is specified in the initial)
98 W( message; for hardware exceptions the kernel allocates the reply)97 W
7200 13717 MT
(port and caches it for reuse on a per-thread basis.  Mach kernel primitives are)
35 W( available to register a port)36 W
7200 14860 MT
(as an exception port for a task or thread,)
161 W( and to return the port currently registered; these and other)160 W
7200 16003 MT
(useful kernel primitives for implementing debuggers and error handlers are described in Appendix B.)SH
8312 18287 MT
(Registering exception ports for both tasks and threads)
59 W( effects a separation of concerns between error)60 W
7200 19430 MT
(handlers and debuggers.  Error handlers are)
199 W( supported by the thread exception ports because error)198 W
7200 20573 MT
(handlers usually affect only the victim thread; different threads)
196 W( within a task can have different error)197 W
7200 21716 MT
(handlers. The)
652 W( registered exception)
187 W( port for a thread defaults to the null port at thread creation; this)186 W
7200 22859 MT
(defaults the initial error)
118 W( handler to "no handler".  Debuggers are supported by the task exception ports)119 W
7200 24002 MT
(because debuggers operate on the application level; this includes at least all the)
94 W( threads in the victim's)93 W
7200 25145 MT
(task, so at most one debugger is ever associated)
55 W( with a single task.  The registered exception port for a)56 W
7200 26288 MT
(task is inherited from the parent task at task creation;)
56 W( this supports debuggers that handle trees of tasks)55 W
7200 27431 MT
(\050e.g. a multi-tasking parallel program\051 and inheritance of core-dump servers.)SH
8312 29715 MT
(The presence of both task and thread exception ports creates a potential)
144 W( conflict because both are)145 W
7200 30858 MT
(applicable to any exception.  This is)
97 W( resolved by examining the differences between error handlers and)96 W
7200 32001 MT
(debuggers. Error)
340 W( handlers use exceptions to implement portions of an application;)
31 W( an error handler is an)32 W
7200 33144 MT
(integral part of the application that generates its exceptions.  Exceptions handled by an)
14 W( error handler may)13 W
7200 34287 MT
(be unusual, but they do)
6 W( not indicate anomalous or erroneous behavior.  In contrast debuggers use excep-)7 W
7200 35430 MT
(tions to investigate anomalous or erroneous application behavior; as a result debuggers have little interest)SH
7200 36573 MT
(in exceptions successfully handled by error handlers.  This implies)
123 W( that exceptions should invoke error)124 W
7200 37716 MT
(handlers in)
142 W( preference to debuggers; this preference is implemented by having thread exception ports)141 W
7200 38859 MT
(take precedence over task exception ports in determining where to direct the rpc invoked by an exception.)SH
7200 40002 MT
(If neither an error handler nor a debugger can successfully handle an exception, the task is terminated.)SH
11 /Helvetica-Bold AF
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(6.2. User Extensibility)SH
10 /Helvetica AF
8312 44827 MT
(Mach's exception handling facility permits users to define)
72 W( and handle their own exceptions in addition)71 W
7200 45970 MT
(to those defined by)
101 W( the system.  The software class of exceptions \050cf. appendix A\051 contains a range of)102 W
7200 47113 MT
(codes reserved for user-defined exceptions; this allows the handling)
44 W( of these exceptions to be integrated)43 W
7200 48256 MT
(into the handling of all other \050i.e. system-defined\051 exceptions.  The same)
101 W( ports are used in both cases,)102 W
7200 49399 MT
(and the interface to handlers is identical.  An advantage of this approach is that)
59 W( user-defined exceptions)58 W
7200 50542 MT
(can immediately be recognized as such, even by debuggers that cannot decode the)
51 W( machine-dependent)52 W
7200 51685 MT
(fields which identify the exact exception.  Generation of user-defined exceptions is facilitated by a)
115 W( MiG)114 W
7200 52828 MT
(\050Mach interface Generator\051 stub)
321 W( routine that implements the exception rpc \050in turn this routine is)322 W
7200 53971 MT
(generated automatically from an interface description of the exception rpc\051.  User code that)
115 W( detects an)114 W
7200 55114 MT
(exception simply obtains the appropriate exception port from the kernel and calls this stub routine; the)126 W
7200 56257 MT
(stub routine handles the rpc and returns a return code from the)
22 W( handler.  Alternatively, users may use the)21 W
7200 57400 MT
(MiG exception interface with)
170 W( their own exceptions and exception ports; this approach may be advan-)171 W
7200 58543 MT
(tageous for applications that handle only user-defined exceptions.)SH
12 /Helvetica-Bold AF
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(7. Implementing Error Handlers)SH
10 /Helvetica AF
8312 63441 MT
(Error handlers are supported by thread exception ports and invoked)
183 W( by remote procedure calls on)182 W
7200 64584 MT
(those ports.  An error handler is associated with a thread by registering a port on which the)
38 W( error handler)39 W
7200 65727 MT
(receives exception rpcs as the thread's exception port.  This registration causes)
60 W( all exceptions occurring)59 W
7200 66870 MT
(in the thread to invoke rpcs to the error handler's port.  Since most error handlers cannot handle)
176 W( all)177 W
7200 68013 MT
(possible exceptions that could occur, they must check each exception and forward it)
15 W( to the corresponding)14 W
7200 69156 MT
(task exception port if it cannot be handled.  This forwarding can be performed by obtaining)
39 W( the exception)40 W
7200 70299 MT
(port for the task specified in the initial message and sending the initial message there.  Alternatively)
72 W( the)71 W
7200 71442 MT
(error handler can return a failure code in the reply)
15 W( message; this causes the sender of the initial message)16 W
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(to reinitiate the rpc using the task exception port.)SH
8312 10213 MT
(Implementation of error handlers requires additional functionality beyond)
209 W( completing the rpc.  This)208 W
7200 11356 MT
(functionality is supported by separate Mach kernel primitives that can also be used by)
37 W( other applications.)38 W
7200 12499 MT
(The most common actions and corresponding primitives are:)SH
/Symbol SF
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(\267)SH
/Helvetica SF
9980 XM
(Read/Write register state -)SH
/Helvetica-Bold SF
21928 XM
(thread_get_state)SH
/Helvetica SF
(,)SH
/Helvetica-Bold SF
30486 XM
(thread_set_state)SH
/Helvetica SF
(.)SH
/Symbol SF
9242 15786 MT
(\267)SH
/Helvetica SF
9980 XM
(Read/Write memory state - access memory directly within task, otherwise)412 W
/Helvetica-Bold SF
46829 XM
(vm_read)SH
/Helvetica SF
(,)SH
/Helvetica-Bold SF
9980 16929 MT
(vm_write)SH
/Helvetica SF
(.)SH
/Symbol SF
9242 18744 MT
(\267)SH
/Helvetica SF
9980 XM
(Terminate thread -)SH
/Helvetica-Bold SF
18483 XM
(thread_terminate)SH
/Helvetica SF
(.)SH
/Symbol SF
9242 20559 MT
(\267)SH
/Helvetica SF
9980 XM
(Resume thread - send reply message to complete rpc \050)SH
/Helvetica-Bold SF
(msg_send)SH
/Helvetica SF
(\051.)SH
8312 22843 MT
(Some applications may require that error handlers execute in the context of \050i.e. on the stack of\051)
98 W( the)99 W
7200 23986 MT
(thread that caused the exception \050e.g. emulation of Unix signal handlers\051.)
204 W( Although)
684 W( this appears to)203 W
7200 25129 MT
(conflict with our restriction)
8 W( of never executing an error handler in the context of the victim thread, it can be)9 W
7200 26272 MT
(implemented by using a system-invoked error handler)
23 W( to set up the application's handler.  Specifically the)22 W
7200 27415 MT
(error handler invoked by the exception rpc modifies the victim thread so that the application's handler)
73 W( is)74 W
7200 28558 MT
(executed when the)
38 W( thread is resumed.  Unwinding the stack when the application's error handler finishes)37 W
7200 29701 MT
(is the responsibility of the application implementor.)SH
12 /Helvetica-Bold AF
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(8. Implementing Debuggers)SH
10 /Helvetica AF
8312 34599 MT
(Debuggers are supported by the task exception ports; exceptions invoke debuggers via remote proce-)47 W
7200 35742 MT
(dure calls on those ports.  A debugger is associated with a task by registering a port)
202 W( on which the)201 W
7200 36885 MT
(debugger receives exception rpcs as the task's exception port.  An exception rpc only)
118 W( stops the victim)119 W
7200 38028 MT
(thread pending rpc completion; other threads in the task continue running.  This has two consequences:)SH
/Symbol SF
9242 39500 MT
(\267)SH
/Helvetica SF
9980 XM
(If the debugger wishes to stop)
208 W( the entire task, a)207 W
/Helvetica-Bold SF
33345 XM
(task_suspend)SH
/Helvetica SF
40499 XM
(must be performed.  A)207 W
9980 40643 MT
(straightforward way to)
51 W( accomplish this is to do it inside the exception rpc and then complete)52 W
9980 41786 MT
(the rpc; the victim thread cannot resume execution upon rpc completion because)
16 W( its task has)15 W
9980 42929 MT
(been suspended.)SH
/Symbol SF
9242 44744 MT
(\267)SH
/Helvetica SF
9980 XM
(Multiple exceptions from a)
153 W( multi-threaded task may be outstanding for the debugger on a)154 W
9980 45887 MT
(single debugger)
103 W( invocation.  If the debugger does not handle these pending exceptions for)102 W
9980 47030 MT
(the task, some may appear to occur at impossible times \050e.g. breakpoint)
115 W( occurs after user)116 W
9980 48173 MT
(removes it\051.)SH
8312 50457 MT
(The Mach exception handling facility is one small component of the kernel that can be used)
36 W( by debug-)35 W
7200 51600 MT
(gers. The)
334 W( various actions required to support debuggers are implemented via general purpose primitives)28 W
7200 52743 MT
(that also)
112 W( support other applications.  Some of the more important debugger actions and corresponding)111 W
7200 53886 MT
(kernel primitives are:)SH
/Symbol SF
9242 55358 MT
(\267)SH
/Helvetica SF
9980 XM
(Detect event)
110 W( -)111 W
/Helvetica-Bold SF
16815 XM
(msg_receive)SH
/Helvetica SF
23263 XM
(- System components that generate or detect external events)111 W
9980 56501 MT
(\050e.g. interrupt character on a terminal\051 signal the events by sending messages.)SH
/Symbol SF
9242 58316 MT
(\267)SH
/Helvetica SF
9980 XM
(Read and write application memory \050includes setting breakpoints\051 -)SH
/Helvetica-Bold SF
39824 XM
(vm_read)SH
/Helvetica SF
(,)SH
/Helvetica-Bold SF
44493 XM
(vm_write)SH
/Helvetica SF
(.)SH
/Symbol SF
9242 60131 MT
(\267)SH
/Helvetica SF
9980 XM
(Read and write application registers \050includes setting single-step mode if available\051 -)364 W
/Helvetica-Bold SF
9980 61274 MT
(thread_get_state)SH
/Helvetica SF
(,)SH
/Helvetica-Bold SF
18538 XM
(thread_set_state)SH
/Helvetica SF
(.)SH
/Symbol SF
9242 63089 MT
(\267)SH
/Helvetica SF
9980 XM
(Continue application - Task and thread control primitives.)SH
/Symbol SF
9242 64904 MT
(\267)SH
/Helvetica SF
9980 XM
(End debugging session -)SH
/Helvetica-Bold SF
21264 XM
(task_terminate)SH
/Helvetica SF
(.)SH
7200 66305 MT
(More detail on these and other useful kernel primitives can be found in appendix B.)SH
8312 68589 MT
(Exceptions that invoke error handlers via thread exception ports are not visible to debuggers.  A)226 W
7200 69732 MT
(debugger that wishes to detect error)
98 W( handler invocation can insert one or more breakpoints in the error)97 W
7200 70875 MT
(handler itself; exceptions caused by these breakpoints will be reported to the debugger.)SH
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(9. Communication Properties and Extended Functionality)SH
10 /Helvetica AF
8312 9218 MT
(The Mach communication facility has two important properties that)
111 W( facilitate extended functionality of)112 W
7200 10361 MT
(systems based on it:)SH
/Symbol SF
9242 11833 MT
(\267)SH
/Helvetica SF
9980 XM
(Independence - A port is an independent)
56 W( entity from the tasks which use it to communicate.)55 W
9980 12976 MT
(Port rights can be exchanged in messages, and are tracked)
18 W( by the kernel to maintain protec-)19 W
9980 14119 MT
(tion.)SH
/Symbol SF
9242 15934 MT
(\267)SH
/Helvetica SF
9980 XM
(Network Transparency - User-mode network message servers transparently extend)
297 W( the)296 W
9980 17077 MT
(Mach communication facility over the network.  The server on the remote machine)
107 W( creates)108 W
9980 18220 MT
(proxy ports to represent the ports exported to it over the network.  A message)
36 W( destined for a)35 W
9980 19363 MT
(remote port is sent to the local proxy port;)
133 W( the local network message server receives the)134 W
9980 20506 MT
(message and forwards it over the net)
88 W( to the remote remote network message server which)87 W
9980 21649 MT
(sends it on to the actual destination port.  This forwarding is invisible to)
156 W( the senders and)157 W
9980 22792 MT
(receivers of messages.)SH
8312 25076 MT
(Extended functionality can be obtained from the Mach exception)
56 W( handling facility and kernel primitives)55 W
7200 26219 MT
(because the Mach communication facility is used as an implementation base.  In addition to the use)
98 W( of)99 W
7200 27362 MT
(messages in the)
140 W( exception handling facility, the kernel interface itself is message based; most system)139 W
7200 28505 MT
(calls are actually invoked by a message rpc to the kernel.)SH
11 /Helvetica-Bold AF
7200 32187 MT
(9.1. Debugger Attachment)SH
10 /Helvetica AF
8312 33330 MT
(The independence property described above allows Mach to)
4 W( support debugger attachment and detach-)5 W
7200 34473 MT
(ment without change to the kernel.  Traditional Unix systems require that the)
94 W( debugged process be the)93 W
7200 35616 MT
(child of the debugger; this makes it impossible to debug a process that was not started by)
43 W( the debugger.)44 W
7200 36759 MT
(Subsequent implementors have expended considerable effort to implement an attach primitive that allows)14 W
7200 37902 MT
(a debugger to attach to a previously started process and debug it; this)
12 W( allows analysis of failures that may)13 W
7200 39045 MT
(not be repeatable.  Similarly these systems allow a debugger to detach from)
69 W( a running process and exit)68 W
7200 40188 MT
(without affecting the process.  No change is required in our design)
215 W( to support this functionality; the)216 W
7200 41331 MT
(debugger need only obtain the port representing the task to be debugged,)
105 W( and may then use all of the)104 W
7200 42474 MT
(primitives previously discussed to debug that task.  A debugger)
78 W( can detach from a task by resetting the)79 W
7200 43617 MT
(task's exception port to its former)
143 W( value; there is no other connection between the debugger and task)142 W
7200 44760 MT
(being debugged.)SH
12 /Helvetica-Bold AF
7200 48515 MT
(10. Parallel and Distributed Debugging)SH
10 /Helvetica AF
8312 49658 MT
(Our design also supports parallel and distributed)
51 W( debugging without change.  There are several cases)52 W
7200 50801 MT
(to be considered based on the structure)
96 W( of the debugger and the application being debugged.  In all of)95 W
7200 51944 MT
(these cases the debugger itself)
174 W( may be a parallel and/or distributed application consisting of multiple)175 W
7200 53087 MT
(tasks and/or threads.)SH
8312 55371 MT
(For parallel applications composed of multiple threads within)
207 W( a single task, a debugger need only)206 W
7200 56514 MT
(register its exception rpc port as that task's exception port.)
211 W( Multiple)
702 W( concurrent exceptions result in)212 W
7200 57657 MT
(multiple rpc invocations being queued to that port; each invocation identifies the thread involved.  Mach's)37 W
7200 58800 MT
(communication facility allows the debugger to accept all of these rpcs before responding to any of)
59 W( them,)60 W
7200 59943 MT
(and to respond to them in)
6 W( any order.  \050Of course the debugger must keep track of the rpcs and make sure)5 W
7200 61086 MT
(they are all responded to when)
303 W( continuing the application.\051 A straightforward implementation is to)304 W
7200 62229 MT
(suspend the task in response to the first)
43 W( rpc, and then complete all pending exception rpcs recording the)42 W
7200 63372 MT
(threads and exceptions involved.  The exceptions can then be reported to the user en masse.)SH
8312 65656 MT
(For parallel applications composed of multiple tasks within a single machine,)
11 W( only minor changes to the)12 W
7200 66799 MT
(above debugger logic are required.  The)
102 W( debugger must now register its exception rpc port as the task)101 W
7200 67942 MT
(exception port)
65 W( for each task, and may choose to identify components of the parallel application by tasks)66 W
7200 69085 MT
(instead of threads.  Suspending)
134 W( or resuming the entire application now requires an operation on each)133 W
7200 70228 MT
(task. If)
349 W( the application dynamically creates tasks, an additional interface to report these new tasks to the)36 W
7200 71371 MT
(debugger may be required so that the new tasks can be suspended and resumed by the debugger.)SH
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(Network transparency allows the components of a)
244 W( debugger and the debugged application to be)243 W
7200 9072 MT
(spread throughout a network; all)
47 W( required operations extend transparently across the network.  This sup-)48 W
7200 10215 MT
(ports a number of possible debugging scenarios:)SH
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(\267)SH
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(Application and debugger on separate hosts.)SH
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(\267)SH
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(Debugging of a distributed application.  The debugger does not require)
23 W( modifications beyond)22 W
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(those needed to deal with applications composed of multiple tasks.)SH
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9242 16460 MT
(\267)SH
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9980 XM
(The debugger itself can be distributed over the network.)SH
7200 17861 MT
(The last scenario is useful for)
92 W( implementing fast exception response in a debugger for applications that)93 W
7200 19004 MT
(runs in parallel on several distributed hosts; if the exception rpc)
92 W( stays within the host, suspending of all)91 W
7200 20147 MT
(application components on that host can be done faster.)SH
12 /Helvetica-Bold AF
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(11. Gdb Enhancements)SH
10 /Helvetica AF
8312 25045 MT
(As part of this work we have used the Mach exception handling facility and)
98 W( other Mach kernel primi-)99 W
7200 26188 MT
(tives to enhance Gdb \050the Gnu Debugger\051)
16 W( [9])
SH( for debugging multi-threaded tasks.)
16 W( This)
309 W( enhanced version)15 W
7200 27331 MT
(of gdb operates at the task level \050i.e. any exception causes it to suspend the entire task\051. A notion of)
47 W( the)48 W
7200 28474 MT
("current thread" has been added to the user interface; this thread is used)
133 W( by any thread-specific com-)132 W
7200 29617 MT
(mand that does not specify a thread.  New commands are provided to list the)
26 W( threads in the task, change)27 W
7200 30760 MT
(the "current thread", and examine or control individual threads.)
181 W( Thread-specific)
639 W( breakpoints are sup-)180 W
7200 31903 MT
(ported by logic that transparently continues the application from the breakpoint until the desired thread)119 W
7200 33046 MT
(hits it.  Implementation of attachment to)
81 W( running tasks as described in section 9.1 is in progress, as are)80 W
7200 34189 MT
(changes to deal with multiple concurrent breakpoints.)SH
8312 36473 MT
(The existence of multiple threads within)
29 W( a debugged task complicates gdb's execution control logic.  In)30 W
7200 37616 MT
(addition to the)98 W
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(task_suspend)SH
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21098 XM
(required upon exception detection, resuming from a breakpoint becomes)97 W
7200 38759 MT
(somewhat intricate.  Vanilla gdb removes the breakpoint, single)
70 W( steps the process, puts back the break-)71 W
7200 39902 MT
(point and continues.  The enhanced version)
49 W( must ensure that only the thread at the breakpoint executes)48 W
7200 41045 MT
(while performing the single step; this)
62 W( requires switching from task suspension to suspension of all of the)63 W
7200 42188 MT
(threads except one and then back again before resuming the application.)SH
8312 44472 MT
(The Mach exception handling facility is an important implementation base for)
167 W( the enhancements to)166 W
7200 45615 MT
(gdb. Identification)
480 W( of the victim thread in the initial message avoids)
101 W( confusion over which thread in the)102 W
7200 46758 MT
(process is being manipulated by ptrace\050\051; without)
23 W( this identification it is necessary to compare the context)22 W
7200 47901 MT
(accessed by ptrace\050\051 to all other thread contexts in the task to determine this.  This)
134 W( identification also)135 W
7200 49044 MT
(make it possible to)
74 W( handle multiple concurrent exceptions; all of the Unix primitives are restricted to one)73 W
7200 50187 MT
(current signal per task, and hence preclude handling of multiple concurrent exceptions.  Finally the)
126 W( in-)127 W
7200 51330 MT
(dependence of the debugger from the debugged application makes it possible to)
141 W( implement debugger)140 W
7200 52473 MT
(attachment without kernel modifications;)
111 W( Unix requires extensive kernel modifications to achieve similar)112 W
7200 53616 MT
(functionality [10])SH
12 /Helvetica-Bold AF
7200 57371 MT
(12. Conclusion)SH
10 /Helvetica AF
8312 58514 MT
(This paper)
142 W( has discussed the design and implementation of exception handling facilities for general)141 W
7200 59657 MT
(purpose operating systems.  In the past these facilities have often been designed as afterthoughts)
159 W( to)160 W
7200 60800 MT
(support a few specific applications.  In contrast to this approach we)
15 W( have set forth a general model for the)14 W
7200 61943 MT
(use of exceptions and designed an exception handling facility that implements the model in full)
11 W( generality.)12 W
7200 63086 MT
(The resulting facility is both simpler and more powerful than its predecessors; among)
234 W( its major ad-)233 W
7200 64229 MT
(vantages is a clean separation between debuggers and error handlers.)SH
8312 66513 MT
(We have also shown that significant benefits can be derived from the properties)
48 W( of the communication)49 W
7200 67656 MT
(facility used for exception handling.  The independence)
207 W( of the Mach communication facility from the)206 W
7200 68799 MT
(applications using it to communicate provides direct support for debugger)
81 W( attachment without the kernel)82 W
7200 69942 MT
(modifications required by previous implementations.  The combination of this independence property with)28 W
7200 71085 MT
(network transparency enables Mach to support parallel)
72 W( and distributed debugging without change to the)73 W
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(operating system kernel.)SH
12 /Helvetica-Bold AF
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(Appendix A. Exception Classification)SH
10 /Helvetica AF
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(The Mach)
183 W( exception handling facility employs a new hardware-independent classification of excep-)182 W
7200 13970 MT
(tions. This)
618 W( is in contrast to previous systems \050e.g. Unix\051, whose exception classifications are closely)171 W
7200 15113 MT
(wedded to the hardware on which they were originally developed.  Our new classification)
193 W( divides all)192 W
7200 16256 MT
(exceptions into six classes based on the causes and uses of)
233 W( the exceptions; further hardware and)234 W
7200 17399 MT
(software specific distinctions can be made within these classes as needed.  The six classes are:)SH
9424 18800 MT
(1.)SH
10536 XM
(Bad Access - A user access to memory failed for some reason and)
104 W( the operating system)103 W
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(was unable to recover.  \050e.g. invalid memory, protection violation\051)SH
9424 21758 MT
(2.)SH
10536 XM
(Bad Instruction - A user executed an illegitimate instruction.)
171 W( \050e.g.)
621 W( undefined instruction,)172 W
10536 22901 MT
(reserved operand, privileged instruction\051)SH
9424 24716 MT
(3.)SH
10536 XM
(Arithmetic - A user arithmetic instruction)
189 W( failed for an arithmetic reason.  \050e.g. overflow,)188 W
10536 25859 MT
(underflow, divide by zero\051.)SH
9424 27674 MT
(4.)SH
10536 XM
(Emulation - A user executed an instruction requiring software emulation.)SH
9424 29489 MT
(5.)SH
10536 XM
(Software - A broad class including all exceptions intended to support)
83 W( software.  These fall)84 W
10536 30632 MT
(into three subclasses)SH
12760 32033 MT
(a.)SH
13872 XM
(Hardware - Hardware instructions to support error detection.  \050e.g.)
42 W( trap on overflow,)41 W
13872 33176 MT
(trap on subscript out of range\051)SH
12760 34991 MT
(b.)SH
13872 XM
(Operating System - Exceptions detected by)
194 W( operating system during system call)195 W
13872 36134 MT
(execution. \050e.g.)
296 W( no receiver on)
9 W( pipe\051 These are for operating system emulation \050e.g.)8 W
13872 37277 MT
(Unix\051. Mach)
278 W( does not use exceptions for system call errors.)SH
12816 39092 MT
(c.)SH
13872 XM
(User - Exceptions defined and caused by user software for its own purposes.)SH
9424 40907 MT
(6.)SH
10536 XM
(Debugger - Hardware)
188 W( Exceptions to support debuggers. \050e.g. breakpoint instruction and)189 W
10536 42050 MT
(trace trap\051.)SH
7200 43451 MT
(In cases of potential confusion \050e.g. is a bad opcode a bad instruction or an instruction)
72 W( requiring emula-)71 W
7200 44594 MT
(tion?\051 the)
34 W( correct classification is always clear from the intended uses of the instruction as determined by)35 W
7200 45737 MT
(the hardware and system designers.)SH
8312 48021 MT
(Two machine dependent fields are used to identify the precise exception within)
56 W( a class for flexibility in)55 W
7200 49164 MT
(encoding exception numbers.  Two fields are needed for emulation instructions containing a single)
135 W( ar-)136 W
7200 50307 MT
(gument \050one for the)
40 W( instruction, one for the argument\051, but we have also found them useful for construct-)39 W
7200 51450 MT
(ing machine-dependent exception classifications \050e.g. use one field to hold the)
7 W( trap number or vector, and)8 W
7200 52593 MT
(the other to distinguish this trap from the others that use this number or vector\051.  Cases in which)
3 W( two fields)2 W
7200 53736 MT
(do not suffice require a separate interface to extract the additional machine-dependent status.)SH
12 /Helvetica-Bold AF
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(Appendix B. Kernel Interface)SH
10 /Helvetica AF
8312 58634 MT
(This appendix lists the Mach system calls that relate directly to the exception handling facility and other)14 W
7200 59777 MT
(calls of particular interest to users of the facility.  This)
37 W( is a partial listing of system calls with brief descrip-)36 W
7200 60920 MT
(tions; a complete listing with full documentation can be found in the Mach Kernel Interface Manual.)SH
8312 63204 MT
(The following four operations are used to read and modify exception ports:)SH
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9242 64676 MT
(\267)SH
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(task_get_exception_port)SH
/Helvetica SF
(\050)SH
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(task)SH
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(,)SH
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24484 XM
(exception_port)SH
/Helvetica SF
(\051 - obtain task's exception port.)SH
/Symbol SF
9242 66491 MT
(\267)SH
/Helvetica-Bold SF
9980 XM
(task_set_exception_port)SH
/Helvetica SF
(\050)SH
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(task)SH
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(,)SH
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24429 XM
(exception_port)SH
/Helvetica SF
(\051 - set task's exception port.)SH
/Symbol SF
9242 68306 MT
(\267)SH
/Helvetica-Bold SF
9980 XM
(thread_get_exception_port)SH
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(\050)SH
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(thread)SH
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(,)SH
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(exception_port)SH
/Helvetica SF
(\051 - obtain thread's exception port.)SH
/Symbol SF
9242 70121 MT
(\267)SH
/Helvetica-Bold SF
9980 XM
(thread_set_exception_port)SH
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(\050)SH
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(thread)SH
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(,)SH
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(exception_port)SH
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(\051 - set thread's exception port.)SH
7200 71522 MT
(The above operations are)
86 W( implemented as macros that expand to the exception port version of the cor-)87 W
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(responding set_special_port or get_special_port calls.)SH
8312 10213 MT
(The following additional kernel)
9 W( operations may be of interest to programmers writing error handlers and)8 W
7200 11356 MT
(debuggers:)SH
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(Communication Operations)SH
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(\267)SH
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(msg_send)SH
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(\050)SH
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(msg_header)SH
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(,)SH
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24153 XM
(option)SH
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(,)SH
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(timeout)SH
/Helvetica SF
(\051 - send a message to a port.)SH
7 /Symbol AF
12160 15894 MT
(\267)SH
10 /Helvetica-Bold AF
12760 16044 MT
(msg_receive)SH
/Helvetica SF
(\050)SH
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(msg_header)SH
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(,)SH
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25266 XM
(option)SH
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(,)SH
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(timeout)SH
/Helvetica SF
(\051 - receive a message from a port.)SH
7 /Symbol AF
12160 17709 MT
(\267)SH
10 /Helvetica-Bold AF
12760 17859 MT
(msg_rpc)SH
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(\050)SH
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(msg_header)SH
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(,)SH
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(option)SH
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(reply_size)SH
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31795 XM
(send_timeout)SH
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(,)SH
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(reply_timeout)SH
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(\051 - send a mes-)28 W
12760 19002 MT
(sage to a port and receive the reply.)SH
7 /Symbol AF
12160 20667 MT
(\267)SH
10 /Helvetica-Bold AF
12760 20817 MT
(port_allocate)SH
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(\050)SH
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(task)SH
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(,)SH
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(port)SH
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(\051 - allocate new port in task.)SH
7 /Symbol AF
12160 22482 MT
(\267)SH
10 /Helvetica-Bold AF
12760 22632 MT
(port_deallocate)SH
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(\050)SH
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(task)SH
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(,)SH
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(port)SH
/Helvetica SF
(\051 - deallocate port from task.)SH
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9242 24447 MT
(\267)SH
/Helvetica SF
9980 XM
(Task Operations)SH
7 /Symbol AF
12160 25698 MT
(\267)SH
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12760 25848 MT
(task_create)SH
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24614 XM
(inherit_memory)SH
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(,)SH
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(child_task)SH
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(\051 - Create)
237 W( new child task from)236 W
12760 26991 MT
(parent task.)SH
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12160 28656 MT
(\267)SH
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12760 28806 MT
(task_terminate)SH
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(\050)SH
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(task)SH
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(\051 - Terminate execution of the task.)SH
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12160 30471 MT
(\267)SH
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12760 30621 MT
(task_suspend)SH
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(\050)SH
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(task)SH
/Helvetica SF
(\051 - Suspend execution of the entire task.)SH
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12160 32286 MT
(\267)SH
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12760 32436 MT
(task_resume)SH
/Helvetica SF
(\050)SH
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(task)SH
/Helvetica SF
(\051 - Resume execution of the entire task.)SH
7 /Symbol AF
12160 34101 MT
(\267)SH
10 /Helvetica-Bold AF
12760 34251 MT
(task_threads)SH
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(task)SH
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(,)SH
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21652 XM
(thread_list)SH
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(,)SH
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26821 XM
(thread_count)SH
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(\051 - Obtain a list of all threads in the task.)SH
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9242 36066 MT
(\267)SH
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9980 XM
(Thread Operations)SH
7 /Symbol AF
12160 37317 MT
(\267)SH
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12760 37467 MT
(thread_create)SH
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(task)SH
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(,)SH
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22220 XM
(thread)SH
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(\051 - Create a new thread in)
179 W( the specified task.  The new)180 W
12760 38610 MT
(thread is created in a suspended state.)SH
7 /Symbol AF
12160 40275 MT
(\267)SH
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12760 40425 MT
(thread_terminate)SH
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(\050)SH
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(thread)SH
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(\051 - Terminate execution of the thread.)SH
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12160 42090 MT
(\267)SH
10 /Helvetica-Bold AF
12760 42240 MT
(thread_suspend)SH
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(\050)SH
/Helvetica-Oblique SF
(thread)SH
/Helvetica SF
(\051 - Suspend execution of the thread.)SH
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12160 43905 MT
(\267)SH
10 /Helvetica-Bold AF
12760 44055 MT
(thread_resume)SH
/Helvetica SF
(\050)SH
/Helvetica-Oblique SF
(thread)SH
/Helvetica SF
(\051 - Resume execution of the thread.)SH
7 /Symbol AF
12160 45720 MT
(\267)SH
10 /Helvetica-Bold AF
12760 45870 MT
(thread_get_state)SH
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(\050)SH
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(thread)SH
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(,)SH
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24727 XM
(flavor)SH
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(,)SH
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27969 XM
(state)SH
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(,)SH
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30934 XM
(count)SH
/Helvetica SF
(\051 - Read registers in thread's)
241 W( context.)240 W
12760 47013 MT
(Flavor distinguishes)
88 W( between multiple register sets \050e.g. general and floating-point\051 in)89 W
12760 48156 MT
(hardware.)SH
7 /Symbol AF
12160 49821 MT
(\267)SH
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12760 49971 MT
(thread_set_state)SH
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(\050)SH
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(thread)SH
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(,)SH
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24431 XM
(flavor)SH
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(,)SH
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27432 XM
(state)SH
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(,)SH
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30156 XM
(count)SH
/Helvetica SF
(\051 - Write registers in thread's context.)SH
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9242 51786 MT
(\267)SH
/Helvetica SF
9980 XM
(Virtual Memory Operations)SH
7 /Symbol AF
12160 53037 MT
(\267)SH
10 /Helvetica-Bold AF
12760 53187 MT
(vm_read)SH
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(\050)SH
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(task)SH
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(,)SH
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19596 XM
(address)SH
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(,)SH
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23709 XM
(size)SH
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(,)SH
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26043 XM
(data)SH
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(,)SH
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28545 XM
(count)SH
/Helvetica SF
(\051 - Read another task's \050virtual\051 memory.)SH
7 /Symbol AF
12160 54852 MT
(\267)SH
10 /Helvetica-Bold AF
12760 55002 MT
(vm_write)SH
/Helvetica SF
(\050)SH
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(task)SH
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(,)SH
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19818 XM
(address)SH
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(,)SH
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23931 XM
(data)SH
/Helvetica SF
(,)SH
/Helvetica-Oblique SF
26433 XM
(count)SH
/Helvetica SF
(\051 - Write another task's \050virtual\051 memory.)SH
12 /Helvetica-Bold AF
7200 58757 MT
(References)SH
10 /Helvetica AF
7200 60629 MT
([1])SH
10536 XM
(Accetta, M.J., Baron, R.V., Bolosky, W., Golub, D.B., Rashid, R.F., Tevanian, A., and Young,)SH
10536 61772 MT
(M.W.)SH
10536 62915 MT
(Mach: A New Kernel Foundation for UNIX Development.)SH
10536 64058 MT
(In)SH
/Helvetica-Oblique SF
11648 XM
(Proceedings of Summer Usenix)SH
/Helvetica SF
(. July,)
278 W( 1986.)SH
7200 65930 MT
([2])SH
/Helvetica-Oblique SF
10536 XM
(American National Standard Reference Manual for the Ada Programming Language)SH
/Helvetica SF
10536 67073 MT
(ANSI, New York, NY, 1983.)SH
10536 68216 MT
(ANSI/MIL-STD 1815A-1983.)SH
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7200 7929 MT
([3])SH
10536 XM
(Bach, M. J.)SH
/Helvetica-Oblique SF
10536 9072 MT
(The Design of the Unix Operating System.)SH
/Helvetica SF
10536 10215 MT
(Prentice-Hall, Inc., Englewood Cliffs, NJ, 1986.)SH
7200 12087 MT
([4])SH
/Helvetica-Oblique SF
10536 XM
(VAX Architecture Handbook)SH
/Helvetica SF
10536 13230 MT
(DEC, Marlboro, MA, 1981.)SH
7200 15102 MT
([5])SH
10536 XM
(Dobberpuhl, D.W., Supnik, R.M., and Witek, R.T.)SH
10536 16245 MT
(The MicroVAX 78032 Chip, A 32-Bit Microprocessor.)SH
/Helvetica-Oblique SF
10536 17388 MT
(Digital Technical Journal)SH
/Helvetica SF
21706 XM
(\0502\051:12-23, March, 1986.)SH
7200 19260 MT
([6])SH
10536 XM
(Ousterhout, J.K.)SH
/Helvetica-Oblique SF
10536 20403 MT
(Partitioning and cooperation in a Distributed Multiprocessor Operating System: Medusa)SH
/Helvetica SF
(.)SH
10536 21546 MT
(PhD thesis, Carnegie Mellon University, 1980.)SH
7200 23418 MT
([7])SH
10536 XM
(Rashid, R.F., and Robertson, G.)SH
10536 24561 MT
(Accent: A Communication Oriented Network Operating System Kernel.)SH
10536 25704 MT
(In)SH
/Helvetica-Oblique SF
11648 XM
(Proceedings of the Eighth Symposium on Operating System Principles)SH
/Helvetica SF
(, pages 64-75.  Decem-)SH
12204 26847 MT
(ber, 1981.)SH
7200 28719 MT
([8])SH
10536 XM
(Siewiorek, D.P., Bell, C. G., and Newell, A.)SH
/Helvetica-Oblique SF
10536 29862 MT
(Computer Structures: Principles and Examples.)SH
/Helvetica SF
10536 31005 MT
(McGraw-Hill Book Company, New York, NY, 1982.)SH
7200 32877 MT
([9])SH
10536 XM
(Richard M. Stallman.)SH
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(GDB Manual: The GNU source-Level Debugger)SH
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(First edition, Free Software Foundation, 1987.)SH
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(Sun Operating System Version 3.2.)SH
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(Operating System Kernel Source Code.)SH
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(i)SH
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(Table of Contents)SH
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(1. Introduction)SH
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(2. The Mach System)SH
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(3. Exceptions and their Uses)SH
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(4. Existing Approaches)SH
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(5. A Model for Exception Handling)SH
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(6. Design of the Mach Exception Handling Facility)SH
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(6.1. Exception Ports)SH
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(6.2. User Extensibility)SH
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(7. Implementing Error Handlers)SH
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(8. Implementing Debuggers)SH
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(9. Communication Properties and Extended Functionality)SH
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(9.1. Debugger Attachment)SH
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(10. Parallel and Distributed Debugging)SH
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(11. Gdb Enhancements)SH
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(12. Conclusion)SH
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(Appendix A. Exception Classification)SH
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(Appendix B. Kernel Interface)SH
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(References)SH
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