From: Jason Self Date: Sun, 4 Jun 2017 22:48:16 +0000 (-0700) Subject: Add initial work on converting debugging documentation X-Git-Url: https://jxself.org/git/?a=commitdiff_plain;h=2658a5a2e3a6900e7217cd55c0b7414c0750ddb2;p=mudman.git Add initial work on converting debugging documentation --- diff --git a/md/debugging.md b/md/debugging.md new file mode 100644 index 0000000..9e19ab9 --- /dev/null +++ b/md/debugging.md @@ -0,0 +1,200 @@ +% A Dynamic Debugging System For Muddle +% Joel Mayer Berez +% January 1978 + +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]: + +# 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 +of Bachelor of Science. + +# 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 +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 +Research under Contract No. N00014-75-C-0661. + +# Introduction + +## 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 +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 +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 +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 +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 +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 +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 +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 +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 +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 +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