-% A Dynamic Debugging System For Muddle
-% Joel Mayer Berez
-% January 1978
+---
+title: 'A Dynamic Debugging System For Muddle'
+author: Joel Mayer Berez
+date: January 1978
+abstract: Program debugging is a time consuming process. Conventional
+ debugging techniques and aids typically give the user a narrow view
+ of the program's operation, making debugging difficult. A debugging
+ system that would present a clear overall picture of a program's
+ behavior and would be both flexible and simple to operate would be a
+ valuable tool. Such a system was designed and implemented in and for
+ Muddle, a high-level applicative programming language. This report
+ discusses the design alternatives considered during the debugging
+ system's design and implementation phases, the reasons for the
+ resulting design choices, and the system attributes. A major
+ attribute of the system (MEND) is that it does not simulate the
+ program being debugged but instead monitors it from another
+ process. This attribute results in a robust and viable debugging
+ system, because MEND need not be modified in order to handle each
+ new extension to Muddle and/or each new user-defined primitive.
+---
MIT Technical Memo 94
-Laboratory for Computer Science
-Massachusetts Institute of Technology
-545 Technology Square
-Cambridge, Massachusetts 02139
-
-# Copyright
-
-This report was originally published in the United States in January
-1978 without a copyright notice and without subsequent registration
-with the U.S. Copyright Office within 5 years. Doing at least one of
-those was a requirement of United States copyright law at that time
-[^1]. This report is therefore in the public domain in the United
-States for failure to comply with the required formalities. This means
-you're free to download, modify and redistribute this report. People
-outside of the United States must check the copyright laws of their
-country before downloading or redistributing.
-
-[^1]: <http://copyright.cornell.edu/resources/publicdomain.cfm>
-
-# Abstract
-
-Program debugging is a time consuming process. Conventional debugging
-techniques and aids typically give the user a narrow view of the
-program's operation, making debugging difficult. A debugging system
-that would present a clear overall picture of a program's behavior and
-would be both flexible and simple to operate would be a valuable tool.
-Such a system was designed and implemented in for Muddle, a high-level
-applicative programming language. This report discusses: the design
-alternatives considered during the debugging system's design and
-implementation phases, the reasons for the resulting design choices,
-and the system attributes. A major attribute of the system (MEND) is
-that it does not simulate the program being debugged but instead
-monitors it from another process. This attribute results in a robust
-and viable debugging system, because MEND not be modified in order to
-handle each new extension to Muddle and/or each new user-defined
+Laboratory for Computer Science
+Massachusetts Institute of Technology
+545 Technology Square Cambridge, Massachusetts 02139
+
+Copyright
+=========
+
+This research was supported by the Advanced Research Projects Agency
+of the Department of Defense and was monitored by the Office of Naval
+Research under Contract No. N00014-75-C-0661.
+
+This report was originally published in the United States in January
+1978 without a copyright notice and without subsequent registration
+with the U.S. Copyright Office within 5 years. Doing at least one of
+those was a requirement of United States copyright law at that time.
+This report is therefore in the public domain in the United States for
+failure to comply with the required formalities. This means you're
+free to download, modify and redistribute this report. People outside
+of the United States must check the copyright laws of their country
+before downloading or redistributing.
+
+Abstract
+========
+
+Program debugging is a time consuming process. Conventional debugging
+techniques and aids typically give the user a narrow view of the
+program's operation, making debugging difficult. A debugging system
+that would present a clear overall picture of a program's behavior and
+would be both flexible and simple to operate would be a valuable tool.
+Such a system was designed and implemented in for Muddle, a high-level
+applicative programming language. This report discusses: the design
+alternatives considered during the debugging system's design and
+implementation phases, the reasons for the resulting design choices,
+and the system attributes. A major attribute of the system (MEND) is
+that it does not simulate the program being debugged but instead
+monitors it from another process. This attribute results in a robust
+and viable debugging system, because MEND not be modified in order to
+handle each new extension to Muddle and/or each new user-defined
primitive.
-This report reproduces a thesis Of the same title submitted to the
-Department of Electrical Engineering, Massachusetts Institute of
-Technology, in partial fulfillment of the requirements for the Degree
+This report reproduces a thesis of the same title submitted to the
+Department of Electrical Engineering, Massachusetts Institute of
+Technology, in partial fulfillment of the requirements for the Degree
of Bachelor of Science.
-# Acknowledgements
+Acknowledgements
+================
-I wish to thank Al Vezza, for supervising this work and guiding me
-along the road to winnage; Stu Galley, for the original idea; Bruce
-Daniels and Gerald Farrell, for laying some of the ground work I have
-built upon; Brian Berkowitz and Chris Reeve, for patiently repairing
-my ailing Muddles; Marc Blank and Tak To, for support work and for
-providing me with company during all-night console sessions; and all
-of the other ITS and DynaMod hackers who have built a system well
+I wish to thank Al Vezza, for supervising this work and guiding me
+along the road to winnage; Stu Galley, for the original idea; Bruce
+Daniels and Gerald Farrell, for laying some of the ground work I have
+built upon; Brian Berkowitz and Chris Reeve, for patiently repairing
+my ailing Muddles; Marc Blank and Tak To, for support work and for
+providing me with company during all-night console sessions; and all
+of the other ITS and DynaMod hackers who have built a system well
worth using.
-This research was supported by the Advanced Research Projects Agency
-of the Department of Defense and was monitored by the Office of Naval
+This research was supported by the Advanced Research Projects Agency
+of the Department of Defense and was monitored by the Office of Naval
Research under Contract No. N00014-75-C-0661.
-# Introduction
+Introduction
+============
-## Background
+Background
+----------
-A time-consuming and frustrating aspect of computer programming is the
-debugging of faulty programs. Current debugging techniques involve
-tracing through the present operation of the program and mentally
-comparing its action with one's concept of what should be happening.
-With few exceptions, an understanding of where the program fails to
-conform to "correct" operation must be made before the cause of the
-failure can be determined and corrective action taken. This is where
+A time-consuming and frustrating aspect of computer programming is the
+debugging of faulty programs. Current debugging techniques involve
+tracing through the present operation of the program and mentally
+comparing its action with one's concept of what should be happening.
+With few exceptions, an understanding of where the program fails to
+conform to "correct" operation must be made before the cause of the
+failure can be determined and corrective action taken. This is where
much of the difficulty occurs.
-In conventional debugging, it is rare for the programmer to have
-available any more than the most basic aids. One usually has to
-extrapolate from a bare minimum of information (such as machine
-generated error messages) or one may be buried under a large excess of
-information, most irrelevant (such as a core dump). Even with the more
-advanced aids, the programmer typically gets but a small window in the
-operation of the program through which, sooner or later, she or he
-will locate the problem. A well-localized fault will be relatively
-easy to spot compared to a global problem that the programmer may only
+In conventional debugging, it is rare for the programmer to have
+available any more than the most basic aids. One usually has to
+extrapolate from a bare minimum of information (such as machine
+generated error messages) or one may be buried under a large excess of
+information, most irrelevant (such as a core dump). Even with the more
+advanced aids, the programmer typically gets but a small window in the
+operation of the program through which, sooner or later, she or he
+will locate the problem. A well-localized fault will be relatively
+easy to spot compared to a global problem that the programmer may only
catch glimpses of through the debugging window.
-In a compiler language, the programmer's best hope is to insert
-statements to print intermediate results or to try to separate the
-program into easier-to-handle modules. There are a few more advanced
-aids available, but their use is limited. One problem is that the
-program is, in effect, first translated into a lower-level language
-(generally "machine" language) and then executed interpretively in
-that language. The original symbols and syntax of the source program
-are lost, or saved only with great difficulty, making analysis and
+In a compiler language, the programmer's best hope is to insert
+statements to print intermediate results or to try to separate the
+program into easier-to-handle modules. There are a few more advanced
+aids available[^1], but their use is limited. One problem is that the
+program is, in effect, first translated into a lower-level language
+(generally "machine" language) and then executed interpretively in
+that language. The original symbols and syntax of the source program
+are lost, or saved only with great difficulty, making analysis and
manipulation of the executing program a very painful process.
-If the programmer is using an interpretive language with facilities
-for interaction, things are considerably easier. The common technique
-is to stop execution at strategic points and examine the state of the
-environment. Since this is done interactively, with the source program
-still available in more or less its original form, the cause of the
-problem can often be found in less time than it could otherwise. one
-of the best example of this approach is the use of DDT (Dynamic
-Debugging Tool/Technique).
-
-DDT basically works with machine-language programs. However, by freely
-translating between numbers and symbols through the use of a table
-generated by the assembler, DDT makes programs look to the programmer
-like symbolic assembly-language programs. The user of DDT can operate
-in free-run mode or in n-step (statement) mode, switching between them
-at will. In either case one can set a breakpoint at any statement,
-which will cause execution to stop just before the statement is
-executed. Whenever stopped in DDT, the user can examine or change the
-contents of any location. This can be done in several data modes
-(e.g., unsigned octal, full ASCII, sixbit, etc.) including the use of
-symbols to represent addresses. Arbitrary numeric expressions can also
+If the programmer is using an interpretive language with facilities
+for interaction, things are considerably easier. The common technique
+is to stop execution at strategic points and examine the state of the
+environment. Since this is done interactively, with the source program
+still available in more or less its original form, the cause of the
+problem can often be found in less time than it could otherwise. one
+of the best example of this approach is the use of DDT (Dynamic
+Debugging Tool/Technique)[^2][^3].
+
+DDT basically works with machine-language programs. However, by freely
+translating between numbers and symbols through the use of a table
+generated by the assembler, DDT makes programs look to the programmer
+like symbolic assembly-language programs. The user of DDT can operate
+in free-run mode or in n-step (statement) mode, switching between them
+at will. In either case one can set a breakpoint at any statement,
+which will cause execution to stop just before the statement is
+executed. Whenever stopped in DDT, the user can examine or change the
+contents of any location. This can be done in several data modes
+(e.g., unsigned octal, full ASCII, sixbit, etc.) including the use of
+symbols to represent addresses. Arbitrary numeric expressions can also
be evaluated without affecting the program.
-One main advantage of DDT is that the debugging environment is very
-similar to the language environment the programmer used to write the
-program. One has to learn only the DDT commands rather than an
-entirely new language. Another advantage is the user's ability for
-viewing the results of the changes. In addition, one can quickly see
+One main advantage of DDT is that the debugging environment is very
+similar to the language environment the programmer used to write the
+program. One has to learn only the DDT commands rather than an
+entirely new language. Another advantage is the user's ability for
+viewing the results of the changes. In addition, one can quickly see
the results of the changes and act accordingly.
-The main deficiency of DDT is that, although is names include the word
-"dynamic", its operation is really static. The application program can
-run freely, but when the programmer wants to see what is taking place,
-the program must be stopped. Although the real interest may be about
-changes on a gross scale, perhaps thousands of program statements, if
-one does not know exactly where the program is misbehaving, one may be
-required to suspend execution of it every few instructions to examine
-variable in order to obtain a true picture of the program's behavior.
-This the programmers see *not what is taking place*, but *what has
-taken place*, and through small windows at that. This is inefficient,
-and the programmer can become bogged down in detail that hinders the
-discovery of the true problem. The situation can be improved with the
-use of breakpoints that allow the program is execute freely until a
-breakpoint is reached, at which point the program halts. DDT is a
+The main deficiency of DDT is that, although is names include the word
+"dynamic", its operation is really static. The application program can
+run freely, but when the programmer wants to see what is taking place,
+the program must be stopped. Although the real interest may be about
+changes on a gross scale, perhaps thousands of program statements, if
+one does not know exactly where the program is misbehaving, one may be
+required to suspend execution of it every few instructions to examine
+variable in order to obtain a true picture of the program's behavior.
+This the programmers see *not what is taking place*, but *what has
+taken place*, and through small windows at that. This is inefficient,
+and the programmer can become bogged down in detail that hinders the
+discovery of the true problem. The situation can be improved with the
+use of breakpoints that allow the program is execute freely until a
+breakpoint is reached, at which point the program halts. DDT is a
powerful tool but still leaves much to be desired in a debugging tool.
-ESP (Execution Simulator and Presenter) is one solution to the static
-problem. It really is dynamic is that large amounts of data are
-constantly being displayed for the programmer while the program is
-being executed (actually, simulated). The information is presented in
-graphic form to improve readability and reduce confusion. A user of
-ESP may watch areas of the display where data of particular interest
-are being presented. One also has many options including control over
-the speed of execution, the type of quantity of data displayed, and
-special (more flexible than just a breakpoint) conditions for halting
-execution. In this way one can structure and control the picture
-presented to more easily understand what the simulated program is
+ESP[^4][^5] (Execution Simulator and Presenter) is one solution to the
+static problem. It really is dynamic is that large amounts of data are
+constantly being displayed for the programmer while the program is
+being executed (actually, simulated). The information is presented in
+graphic form to improve readability and reduce confusion. A user of
+ESP may watch areas of the display where data of particular interest
+are being presented. One also has many options including control over
+the speed of execution, the type of quantity of data displayed, and
+special (more flexible than just a breakpoint) conditions for halting
+execution. In this way one can structure and control the picture
+presented to more easily understand what the simulated program is
doing. And that is one key step in the process of debugging.
-Like DDT, ESP has deficiencies also. These are mainly in the area of
-editing and DDT-like examination of a faulty program. DDT is a
-sophisticated language that ESP does not attempt to entirely replace.
-The flaw in ESP is that it is not compatible with DDT. Ideally both
-should be simultaneously available to the programmer, who can use
+Like DDT, ESP has deficiencies also. These are mainly in the area of
+editing and DDT-like examination of a faulty program. DDT is a
+sophisticated language that ESP does not attempt to entirely replace.
+The flaw in ESP is that it is not compatible with DDT. Ideally both
+should be simultaneously available to the programmer, who can use
features of each as the need dictates.
-DDT and ESP work with a low-level language whose operation can be
-shown fairly simply. For example, ESP often shows flow of control by
-just displaying the actual section of program being executed and
-pointing to the current statement. It also draws lines to show where
-branching has occurred and in some cases even indicates looping. The
-display philosophy can be readily extended to higher level languages
-that are line oriented, like BASIC, but it fails with applicative
-ones, like LISP or Muddle. The latter type does not use a linear
-control flow, but uses a complex depth-first tree structure.
-Furthermore, quite complicated data structures can be built (or
-themselves executed) that bear little relation to the appearance of
+DDT and ESP work with a low-level language whose operation can be
+shown fairly simply. For example, ESP often shows flow of control by
+just displaying the actual section of program being executed and
+pointing to the current statement[^6]. It also draws lines to show
+where branching has occurred and in some cases even indicates looping.
+The display philosophy can be readily extended to higher level
+languages that are line oriented, like BASIC, but it fails with
+applicative ones, like LISP or Muddle. The latter type does not use a
+linear control flow, but uses a complex depth-first tree structure.
+Furthermore, quite complicated data structures can be built (or
+themselves executed) that bear little relation to the appearance of
the program.
-A good basic display for Muddle was used in MUMBLE (Gerald Farrell's
-monitor and debugging aid). The code being executed is shown in stack
-form. Each line shows a piece of code being evaluated. As each object
-in the bottom line is evaluated, it is replaced by a single
-downward-arrow symbol in this line and then printed on a new line. In
-this way the evaluation can be followed from the top level down to the
-current object being evaluated. Furthermore, after the bottom is
-reached, the value returned by each line replaces its symbol in the
-previous line. With this mechanism, the programmer can follow
+A good basic display for Muddle was used in MUMBLE[^7] (Gerald
+Farrell's monitor and debugging aid). The code being executed is shown
+in stack form. Each line shows a piece of code being evaluated. As
+each object in the bottom line is evaluated, it is replaced by a
+single downward-arrow symbol in this line and then printed on a new
+line. In this way the evaluation can be followed from the top level
+down to the current object being evaluated. Furthermore, after the
+bottom is reached, the value returned by each line replaces its symbol
+in the previous line. With this mechanism, the programmer can follow
execution in a natural and reasonably clear representation.
-MUMBLE had some difficulties arising from the fact that it simulated
-execution rather than just watching it and letting it proceed
-naturally. This caused it to run slowly and to be complex yet fragile.
-At the time MUMBLE was written, the Muddle compiler was not yet
-perfected and the language itself lacked some of the multiprocessing
-features that would have made simulation unnecessary. Later Farrell
-replaced MUMBLE with a debugger utilizing new software related to
-single-stepping a process, which eliminated the simulation but also
-eliminated the feature reflecting results of an evaluation back into
-the original code. Also, a mode was added that allowed the programmer
-to attach conditions to parts of the program which would stop
-execution when and if a condition was false. This is as far as MUMBLE
-ever progressed, and it was not in use as of the time of the proposal
-for the current work.
\ No newline at end of file
+MUMBLE had some difficulties arising from the fact that it simulated
+execution rather than just watching it and letting it proceed
+naturally. This caused it to run slowly and to be complex yet fragile.
+At the time MUMBLE was written, the Muddle compiler was not yet
+perfected and the language itself lacked some of the multiprocessing
+features that would have made simulation unnecessary. Later Farrell
+replaced MUMBLE with a debugger utilizing new software related to
+single-stepping a process, which eliminated the simulation but also
+eliminated the feature reflecting results of an evaluation back into
+the original code. Also, a mode was added that allowed the programmer
+to attach conditions to parts of the program which would stop
+execution when and if a condition was false. This is as far as MUMBLE
+ever progressed, and it was not in use as of the time of the proposal
+for the current work.
+
+MEND
+----
+
+After the Muddle compiler became operational and many additional new
+software features became available, it appeared that it would be
+possible to design and implement a debugging system that would be
+comprehensive, easy to use, and reasonably fast. It was therefore
+proposed that a debugging system for Muddle called MEND (Muddle
+Executor , aNalyzer, and Debugger) be designed and implemented. It has
+the following characteristics. (A glossary of special terms, those in
+all capital letters, used here and throughout the rest of this memo
+may be found in Appendix B.)
+
+1. MEND possesses a display similar to that of MUMBLE, including the
+ replacement of arguments in a FORM by their values as they are
+ evaluated.
+2. Execution is monitored from another process (as opposed to being
+ simulated as in ESP) using 1STEP and related features[^8].
+3. Execution speed is variable by the user including a static single
+ or multi-step mode where desired.
+4. MEND allows execution to run freely below a certain depth of
+ evaluation or between certain points in the program and to run
+ controlled elsewhere.
+5. Unconditional and conditional breakpoints are available that can
+ be attached to any object to halt execution before evaluation of
+ that object.
+6. The system is capable of keeping track of programmer specified
+ conditions and of changing modes or giving some visible indication
+ when the conditions are met (or not met).
+7. Information, such as the local value of programmer specified ATOMs
+ and the values of programer specified FORMs, is constantly
+ displayed beneath the main display.
+8. Each line in the main display area is &-printed (abbreviated
+ printing, see glossary) and can be viewed in full at any time.
+9. At any time, with execution stopped, the user can EVAL objects in
+ the ENVIRONMENT of the program. This means the user can examine
+ the state of the program or change it.
+
+Possible Additional Characteristics
+-----------------------------------
+
+Certain other characteristics were seen as desirable for MEND but
+possibly beyond the scope of this project. If time permitted these
+features were to be included in the system:
+
+1. The IMLAC multi-screen capability would be used to allow the user
+ to rapidly switch between the debugger display and the program's
+ own output. Other system output could also be put on additional
+ pages.
+2. The editor IMED (an editor for Muddle objects analogous to
+ IMEDIT[^9]) would be tied into the system to allow easy editing.
+ PRINTTYPE and READ-TABLEs would be used to allow breakpoints to be
+ easily set and removed in IMED as single symbols. Other control
+ codes and statements could also be inserted using this editor.
+3. At the applications programmer's option and within certain limits,
+ execution could be reversed either so that something different
+ could be tried or for purposes of reexamining the process for
+ something that may have been missed the first time. This feature
+ could come in two possible forms, the UNDO package[^10] to
+ actually reverse execution or a simulation displaying information
+ previously stored by the system.
+
+Design and Implementation
+-------------------------
+
+MEND was designed with the intent of providing the application
+programmer with many options so that debugging could proceed in the
+most suitable manner for each situation. In the normal running state,
+MEND displays several kinds of information on the screen. Most
+important is an area showing the execution of the application program
+being debugged in stack form. The only other area that is always
+present is a line or two of status information about the current
+operation of MEND showing its current speed of execution (user
+adjustable) and the state of each modifiable mode.
+
+It was intended that the output of the application program be saved by
+MEND for later reference. The user of MEND could then elect to have
+the most recent output constantly displayed in a window on the main
+screen (see section on future work). If multi-screening were
+available, the output could be kept on another virtual screen. That
+screen could be displayed or made invisible at the user's option
+without stopping MEND.
+
+Information such as programmer specified values of ATOMs, structured
+objects, and, in general, the value of any Muddle expression may be
+constantly displayed. MEND is also capable of displaying such
+information on an exception basis according to some predescribed
+condition. Such information is &-printed but is viewable in full when
+desired.
+
+It is important for a debugging system like MEND to be compatible with
+and to take advantage of available software in related areas. One such
+area is editing. There were two Muddle editors in use when the
+proposal for this work was made, EDIT[^11] and IMED. The main
+difference between them is that IMED uses the local editing features
+of the IMLAC while EDIT does not. EDIT, however, has the advantage of
+being the only one that possesses breakpoint capabilities. Whichever
+proved to be most compatible with MEND (possibly both) would be
+slightly modified to allow the setting and removing of certain MEND
+codes including, in the case of IMED, breakpoints.
+
+Muddle itself has many features that greatly aid the debugging
+process. One of these is FRAMES. This function can be used to print
+the stack of functional evaluations and applications when execution is
+halted at any depth below the top level. At this point it is also
+possible to get the values of objects in the current ENVIRONMENT and
+to change them. One can even restart execution at a higher level after
+making such changes. Because the Muddle debugging features are quite
+powerful, MEND was designed to allow the user to stop execution (of
+the application program) and to use these aids or any others built in
+to Muddle with the MEND system itself transparent. Evaluation would
+take place in the ENVIRONMENT of the application program.
+
+MEND now includes the main features of all the debuggers that have
+been mentioned and enough other features that it should prove to be
+quit useful for the analysis and debugging of Muddle programs. It
+should also serve as a good example of the type of debugging system
+that can be built around an applicative type language.
+
+[^1]: G.A. Mann, "A Survey of Debug Systems", Honeywell Computer
+ Journal 1973, volume 7, number 3, pages 182-198
+
+[^2]: Digital Equipment Corporation, "DDT-10 Programmer's Reference
+ Manual", Assembly Language Handbook, DEC-10-NRZA-D
+
+[^3]: B. Bressler, DDT: A Debugging Aid, SYS.06.01, Programming
+ Technology Division Document, Laboratory for Computer Science,
+ M.I.T., November, 1971
+
+[^4]: S.W. Galley, Debugging with ESP -- Execution Simulator and
+ Presenter, SYS.09.01, Programming Technology Division Document,
+ Laboratory for Computer Science, M.I.T., November, 1971
+
+[^5]: S.W. Galley and R.P. Goldberg, "Software Debugging: The Virtual
+ Machine Approach", Proceedings of the Association for Computing
+ Machinery Annual Conference, November, 1974, volume 2, pages
+ 395-401
+
+[^6]: M.H. Liu, DETAIL: A Graphical Debugging Tool, S.B. Thesis,
+ Department of Electrical Engineering, M.I.T., February, 1972
+
+[^7]: G.J. Farrell, A System for Muddle Programming, M.S. Thesis,
+ Department of Electrical Engineering, M.I.T., August, 1973
+
+[^8]: S.W. Galley and G. Pfister, Muddle Programming Language Primer
+ and Manual, Laboratory for Computer Science, M.I.T., May, 1977
+
+[^9]: J. Haverty, IMEDIT Editor Program for Use with the Imlac
+ Terminals, SYS.08.01.02, Programming Technology Division Document,
+ Laboratory for Computer Science, M.I.T., August, 1972
+
+[^10]: B. Berkowitz, UNDO, Undergraduate Research Report, Programming
+ Technology Division, Laboratory for Computer Science, M.I.T.,
+ December, 1974
+
+[^11]: N. Ryan, EDIT: The Muddle Editor, SYS.11.14, Programming
+ Technology Division Document, Laboratory for Computer Science,
+ M.I.T., August, 1974
projects / mudman.git / blobdiff