----
-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.
----
+% A Dynamic Debugging System For Muddle
+% Joel Mayer Berez
+% January 1978
MIT Technical Memo 94
of the other ITS and DynaMod hackers who have built a system well
worth using.
-Introduction
-============
+I Introduction
+==============
-Background
-----------
+I.1 Background
+--------------
A time-consuming and frustrating aspect of computer programming is the
debugging of faulty programs. Current debugging techniques involve
ever progressed, and it was not in use as of the time of the proposal
for the current work.
-MEND
-----
+I.2 MEND
+--------
After the Muddle compiler became operational and many additional new
software features became available, it appeared that it would be
the ENVIRONMENT of the program. This means the user can examine
the state of the program or change it.
-Possible Additional Characteristics
------------------------------------
+I.3 Possible Additional Characteristics
+---------------------------------------
Certain other characteristics were seen as desirable for MEND but
possibly beyond the scope of this project. If time permitted these
actually reverse execution or a simulation displaying information
previously stored by the system.
-Design and Implementation
--------------------------
+I.4 Design and Implementation
+-----------------------------
MEND was designed with the intent of providing the application
programmer with many options so that debugging could proceed in the
should also serve as a good example of the type of debugging system
that can be built around an applicative type language.
-Terminal Display
-================
+II Terminal Display
+===================
-IMLAC Console Program
----------------------
+II.1 IMLAC Console Program
+--------------------------
One basic concern throughout the project was the display: how the
information made available by MEND would be presented to the user. To
written for it by Dave Lebling is MSC, a multiple-screen terminal
program. Up to four virtual screens (or pages) can be created that
will individually operate like the actual screen area of the standard
-terminal program (SSV\[\^13). Output may be directed to any one of the
+terminal program (SSV[^13]). Output may be directed to any one of the
screens and any screen may be visible or invisible at any instant of
time, at the programmer's option. Selection of screen is controllable
either by program from the host or locally by the user.
the success of many earlier projects, decided between the alternatives
and in this case led to the final decision to use SSV instead of MSC.
-Terminal Independence
----------------------
+II.2 Terminal Independence
+--------------------------
The MEND terminal handling capabilities are actually quite general and
MEND does not depend exclusively on SSV. For the purpose of dividing
large range of possible line and screen lengths. (The current version
of SSV provides four possible character sizes.)
-Control of Screen Sectioning
-----------------------------
+II.3 Control of Screen Sectioning
+---------------------------------
There still remained the question of how to handle the
multi-sectioning of the displayed information. Originally three
explained later, these items include all output automatically
displayed by MEND during execution of the application program.
+III MSTACK: MEND's Representation of the Control Stack
+======================================================
+
+III.1 Program Execution In Muddle
+---------------------------------
+
+MEND's primary role is to allow the applications programmer to
+visually monitor the execution of a program. In a language like
+Muddle, this is most easily accomplished by showing the programmer a
+picture of the control stack.
+
+A Muddle program consists of the evaluation of a single object. The
+object is usually structured in some manner and itself contains other
+objects. The most common object is the FORM. This is a list of objects
+in which the first is (or evaluates to) some function and the rest are
+arguments to that function. A FORM is evaluated by applying the
+function to its arguments, usually after the arguments are themselves
+evaluated. This evaluation actually is initiated by the Muddle
+interpreter by applying the function EVAL to the original object. EVAL
+takes an object as its argument and returns the value to which it
+evaluates.
+
+Figure 1 shows a (simplified) static representation of the evaluation
+of a Muddle object. Starting with the FORM (list of objects in
+angle-brackets) to be evaluated, the flow of control/evaluation may be
+described as a depth-first search through the tree pictured. The
+arrows represent values being returned to previous levels. At the end
+of this "search" the FORM returns the value shown at the top.
+
+ Figure 1:
+
+ 31
+ ↑
+ <+ .A .B .C 5>
+ ↑ ↑ ↑ ↑ ↑
+ + .A .B .C 5
+ ↑ ↑ ↑
+ 6 8 12
+
+Typically the control stack will start with one object on it, the FORM
+to be evaluated. This evaluation will first require that the objects
+in the FORM (possibly FORMs themselves) be added to the stack and
+evaluated. EVAL recursively calls itself for this purpose. The stack
+builds (downward, by convention) until some object is placed on it
+which ls known by EVAL to need no further evaluation. This object is
+returned as its own value to the previous level and values continue to
+be returned upward until a level is reached where another object must
+be evaluated. In this manner, the stack grows and shrinks until the
+topmost (initial) object returns a value.
+
+III.2 Monitoring Program Execution
+----------------------------------
+
+The manner in which the stack builds, the objects are evaluated, and
+the values are returned illustrate most of what the program is doing.
+Other factors, including side effects and complied code, will be
+discussed at the end of this chapter. MEND's main display therefore
+shows a representation of the stack being continuously updated as
+execution/evaluation proceeds.
+
+There are essentially two ways that MEND could follow the flow of
+control of the application program. The most direct way, as attempted
+by MUMBLE[^14] and discussed in the last chapter, would be for MEND to
+execute the application program by simulating the operation of Muddle.
+This type of simulation has been shown to require a complex end all
+too often fragile structure. The debugging program would need to be
+constantly updated to match changes and additions to the Muddle
+language , but more importantly it would fail to properly handle
+programmer-defined primitives, several varieties of which are provided
+for by Muddle.
+
+A far more satisfactory method is to allow Muddle to execute the
+application program in more or less its normal fashion but to stand
+beck somewhere and watch. Fortunately Muddle contains a mechanism
+ideal for this, called multiprocessing. Basically another control
+stack , or process, may be created (independent of the first) that may
+be used to execute a different function with its own set of variable
+bindings. One such process, in this case MEND, may place another, the
+application program, into a single step mode where the latter will be
+stopped before each call to EVAL and again as each call returns. The
+MEND process will at these points be restarted and given information
+about what the application process is doing. MEND stores this
+information in a multi-level structure it creates called an MSTACK.
+
+Each level of an MSTACK corresponds to a level of the control stack
+being monitored. Each level contains the original object (actually, a
+pointer to it) being evaluated at that level and a new object, called
+the "displayed object", that will or does contain the results of
+evaluating each of the arguments or elements of the original. The
+displayed object is initially the same as the original (in a real
+sense, it is the original) but is systematically rebuilt as each
+element is evaluated and replaced by what it returns. Thus MEND can
+keep track of the relationship between the changing display and the
+unchanging program. (Figure 2 shows the various stages that the
+displayed object corresponding to the FORM being evaluated in Figure 1
+goes through. Each stage would in fact be painted over the previous
+one so that all of these stages represent only one line of the screen.
+The down-arrow shown in some of the stages is a place-holder that
+represents an object that is actually expanded on the next line of the
+screen.) A pointer is also kept showing which element is currently
+above this on the stack to be evaluated unless, of course, this is the
+initial element of the stack.
+
+ Figure 2:
+
+ <+ .A .B .C 5>
+ <+ ↓ .B .C 5>
+ <+ 6 .B .C 5>
+ <+ 6 ↓ .C 5>
+ <+ 6 8 .C 5>
+ <+ 6 8 ↓ 5>
+ <+ 6 8 12 5>
+
+ 31
+
+III.3 A Displayed Representation of Program Execution
+-----------------------------------------------------
+
+The printed representation of the stack occupies the top section of
+the screen and is the most prominent and important characteristic of
+MEND. Most often, in fact, it is only necessary to watch this display
+as the application program is executed to ascertain where a bug is
+located or to observe exactly how the program operates. Therefore
+considerable time has been spent making the operation of the MSTACK
+and associated display as natural and informative as possible.
+
+Using the collected information described in the previous section, a
+representation of the stack is displayed as a number of lines, each
+corresponding to one level. Each line shows a level number and the
+displayed object printed in "&-printing", named for the printing
+function &[^15] created by Greg Pfister. Although strictly speaking
+the stack builds upward (towards higher memory locations), it seams
+more natural to display and speak of the stack as building from the
+top downward, in the direction that printing normally occurs. As each
+element of the bottom level (the bottom line on this section of the
+screen) is placed on the stack for evaluation, a new line is added
+beneath the previous bottom-level line showing that element, and the
+element is replaced in the previous line by a pointer ("↓") marking
+its location. When the element finally returns a value, the returned
+value will replace the pointer in the displayed object.
+
+To avoid visual distraction, a minimum of updating is done on the
+screen. In most cases random access is used to replace only those
+lines that have changed. In general the complete stack will not fit in
+the display area allocated for it (whose size is user adjustable) so a
+scrolling procedure has been devised. The complete display area is
+rewritten whenever an attempt is made to write past the bottom or
+erase upward past a certain level while some lines are invisible
+because they have been scrolled off. The scrolling parameters have
+been selected to optimize the number of lines visible on the average
+vs. the frequency of scrolling. Level numbers allow the user to see
+how much is hidden "above the screen" and how deep the evaluation is
+nested.
+
+III.4 MEND Program Steps Vs. Muddle Steps
+-----------------------------------------
+
+To make it easier for the programmer to follow what the application
+program is doing, the speed of execution of the program must be
+controllable. MEND does this by inserting a constant, user adjustable
+delay between program "steps." One MEND step is not precisely the same
+as one Muddle step. Remember that a Muddle step is one call to or
+return from EVAL. Each MSTACK level, and therefore each displayed
+line, will have two Muddle steps associated with it. At the first
+step, MEND will create the level and add one line to the screen. At
+the second step, MEND will erase that line and put the returned value
+back into the previous line. For clarity MEND adds a third step
+between these two which actually occurs at the second Muddle stop.
+Before substituting into the previous line, the current one is first
+replaced by the returned value, the normal delay occurs, and then the
+"second" step occurs as described.
+
+Some types of Muddle objects are self-evaluating; EVAL will simply
+return the object it was given. Although nothing interesting has
+happened, two steps have occurred. In this case MEND will avoid
+clutter by pretending that no steps have occurred. (MEND only does
+this with built-in types that MEND recognizes, and the programmer
+should recognize, as being self-evaluating. Programmer-defined types
+that are self-evaluating will generate the usual number of MEND
+steps.)
+
+Another case of disparity between Muddle and MEND stops is more
+complex. First some further explanations about Muddle objects are
+needed Generally the interesting objects, the ones that generate MEND
+steps, are linear (usually list) structures containing a number of
+other Muddle objects, as are the FORMs described earlier. Normally
+during evaluation of such an object the elements will be evaluated one
+at a time from first to last. However this sequence is not always
+followed by Muddle. MEND cannot directly determine which element is
+about to be evaluated at each call to EVAL. It is only given the
+object to be evaluated itself and not its position in the parent
+structure. It is normally sufficient to do a comparison of this object
+with the elements of the parent, starting with the first element
+believed to not yet have been evaluated. Naturally, if the elements
+are evaluated out of order, this procedure may fail to find the
+desired match, because a Muddle object may contain the same element in
+two or more positions. Thus it is possible to match the
+about-to-be-evaluated object to the wrong occurrence of it. Further
+complications arise because some functions can sometimes back up and
+re-evaluate their arguments.
+
+A strong attempt was made to make MEND dependent only upon the general
+characteristics of Muddle functions and not upon specific exceptions
+and idiosyncrasies. It was felt to be desirable to make MEND
+independent of both future changes to the language and
+programmer-defined "primitives" that would not be known to MEND.
+Besides, without actually simulating Muddle it is not possible to
+always get the information MEND needs. It was felt that the "general
+rule" approach would take care of a sufficiently large number of cases
+without falling into the simulator problem.
+
+It was determined after some experimentation, however, that MEND could
+not be made to work properly without some specific knowledge about
+several important cases in Muddle. Two functions, PROG and REPEAT,
+allow for branching the flow of control. They are normally
+first-to-last functions as described above but at times control may
+jump backwards to re-evaluate some elements. MEND was implemented so
+that if it cannot locate an element in its normal search path, it will
+start looking again from the beginning of the structure. If it is then
+found, the displayed object will be reinitialized to be as if
+evaluation had not yet proceeded past that point. Evaluation will then
+continue normally.
+
+Another phenomenon of Muddle that we must discuss is what this author
+labels the "clause" behavior. A clause is a list of objects given to a
+function as a single argument. The list is not itself evaluated, but
+some or all of its elements are evaluated. The most common function
+illustrating this behavior is COND, a general purpose conditional
+function. COND's arguments are all lists of objects. It sequences
+through these lists, evaluating the first object in each, until an
+evaluated object returns something considered "true." Then the rest of
+the objects in that list are evaluated and COND returns what the last
+object in the list returns. MEND's normal search path only looks at
+top-level elements and would therefore never find the ones actually
+being evaluated.
+
+This phenomenon seemed to be more widespread than the PROG/REPEAT one
+and could not be easily attributed to certain functions. The solution
+chosen here was to in all cases do a nested search in elements that
+looked as if they might be clauses. (The search actually goes one
+extra level deep to allow for certain special cases.) When a match is
+found in a clause, MEND will for clarity generate extra steps to make
+it appear to the user as if first the clause and then its appropriate
+element was put on the stack for evaluation. The clause will stay on
+the stack until some element that is not above it in the evaluation
+tree is evaluated. At that time the clause will be removed in an
+orderly manner, and the new element or clause plus element will be put
+onto the stack. To do this smoothly, up to six MEND steps may have to
+be generated for the one Muddle step. (See the example in Appendix A.)
+
+III.5 What the User Does Not See
+--------------------------------
+
+MEND, in its display of the MSTACK, attempts to show the user
+everything of importance that is happening as the program is executed.
+However certain features of Muddle cannot be captured in this sort of
+representation.
+
+One such feature is the existence of compiled code. Although Muddle
+was originally intended to be a high-level interpretive language, an
+assembler was written[^16], producing machine code that executes in
+the Muddle environment, to allow programmers to create "primitives"
+that perform functions not otherwise available in Muddle. Once the
+assembler was written, it was natural that a compiler[^17] followed.
+It translates normal Muddle code into Muddle assembly code which is
+then assembled. Typically Muddle programs are tested interpretively
+and, when fully debugged, complied in order to obtain a major increase
+in speed of execution.
+
+A call to a compiled (or assembled) function usually looks to the
+programmer and to MEND like a call to a Muddle primitive. The
+operation of the function is not seen except when it calls uncompiled
+functions. This does not present a major problem to MEND since
+generally only uncompiled functions are being debugged, and the
+compiled ones encountered are hopefully performing known functions
+properly.
+
+One feature of Muddle that MEND is unable to cope with is the
+interrupt system. The programmer may enable a large class of
+interrupts and assign handling functions wherever desired. Examples
+Include interrupts for characters being typed to a certain input
+channel and notification that the system is about to be brought down.
+(MEND uses interrupts to catch single character commands and to catch
+errors.)
+
+Recall that MEND monitors application program execution by placing its
+process into a single-stepping mode. When Muddle passes control to an
+interrupt handier, it temporarily causes the process to leave
+single-stepping mode. This is necessary partially because such a
+handler may be specified to run in a process other than the current
+one at the time of the interrupt. It unfortunately makes the handler
+invisible to MEND. it is therefore not currently possible to use MEND
+to debug interrupt handlers.
+
+There is a whole series of side-effects, such as printing by the
+application program, that are not directly seen in the MSTACK
+representation but are made visible by various other features of MEND.
+These will be discussed where appropriate in later sections.
+
[^1]: G.A. Mann, "A Survey of Debug Systems", Honeywell Computer
Journal 1973, volume 7, number 3, pages 182-198
[^12]: D. Eastlake, R. Greenblatt, J. Holloway, T. Knight, and S.
Nelson, ITS 1.5 Reference Manual, Memo No. 161A, Artificial
Intelligence Laboratory, M.I.T., July 1969
+
+[^13]: P.D. Lebling, SSV User's Manual, SYS.52.07, Programming
+ Technology Division Document, Laboratory for Computer Science,
+ M.I.T., (in preparation)
+
+[^14]: G.J. Farrell, A System for Muddle Programming, M.S. Thesis,
+ Department of Electrical Engineering, M.I.T., August, 1973
+
+[^15]: S.W. Galley, Pre-loaded Pure MDL RSUBRs, SYS.11.28, Programming
+ Technology Division Document, Laboratory for Computer Science,
+ M.I.T., November 1975
+
+[^16]: B. Daniels, The MDL Assembler, SYS.11.07, Programming
+ Technology Division Document, Laboratory for Computer Science,
+ M.I.T., (in preparation)
+
+[^17]: C. Reeve, The MDL Compiler, SYS.11.25, Programming Technology
+ Division Document, Laboratory for Computer Science, M.I.T., (in
+ preparation)
projects / mudman.git / commitdiff