perm filename BAIL.DOC[DOC,AIL]6 blob sn#151450
filedate 1975-03-27 generic text, type C, neo UTF8
COMMENT ⊗ VALID 00006 PAGES
C REC PAGE DESCRIPTION
C00002 00002 BAIL -- A DEBUGGER for SAIL
C00004 00003 I. Compile-time action
C00011 00004 II. Run-time action
C00021 00005 III. Resources Used
C00023 00006 IV. Current Status
BAIL -- A DEBUGGER for SAIL
John F. Reiser
Computer Science Department
March 27, 1975
BAIL is a debugging aid for SAIL programs. At compile time,
BAIL generates a file which contains symbol table, program counter,
and text file information. At execution time BAIL uses this
information to interpret debugging requests. In many respects BAIL
is like DDT or RAID, except that BAIL is oriented towards SAIL and
knows about SAIL data types, primitive operations, procedure
implementation, and stack structure. In addition, BAIL can display
the text from the source or listing file which corresponds to the
program counter at a given point in the program.
I. Compile-time action
II. Run-time action
III. Use of resources
IV. Current status
I. Compile-time action
The principal result of activating BAIL at compile-time is
the generation of a file of information about the source program for
use by the run-time interpreter. This file has the same name as the
.REL file produced by the compilation, except that the extension is
.SM1. If requested, BAIL will also generate some additional code for
SIMPLE procedures to make them more palatable to the run-time
The action of BAIL at compile time is governed by the value
of the /B switch passed to the compiler. If the value of this switch
is zero (the default if no value is specified) then BAIL is
completely inactive. Otherwise, the low-order bits determine the
actions which BAIL performs. [The value of the /B switch is
interpreted as octal.]
1 If this bit is on, then the .SM1 file will contain
the program counter to source/listing text directory.
2 If this bit is on, then the .SM1 file will contain
symbol information for all SAIL symbols encountered
in the source. If this bit is off, then information
is kept only for procedures, parameters, blocks, and
internals; i.e., non- internal local variables are
4 If this bit is on, then SIMPLE procedures will get
procedure descriptors, and one additional instruction
(a JFCL 0, which is the fastest machine no-op
instruction) is inserted at the beginning of SIMPLE
procedures. Except for these two changes, all
properties of SIMPLE procedures remain the same as
before. The procedure descriptor is necessary if the
procedure is to be called interpretively or if the
procedure is to be TRACEd.
'10 If this bit is on, then BAIL will not be
automatically loaded and initialized, although all
other actions requested are performed. This is
primarily intended to make it easier to debug new
versions of BAIL without interfering with
'20 If this bit is on, then a request to load
SYS:BAIPDn.REL is generated. This file contains
procedure descriptors for most of the SAIL
predeclared runtime routines, making it possible to
call them from BAIL. The procedure descriptors and
their symbols occupy about 6K.
The B switch must occur on the binary term, not the listing or source
.R SAIL or .COM PROG(27B,)
The program counter to source/listing index is kept in terms
of coordinates. The coordinate counter is zeroed at the beginning of
the compilation and is incremented by one for each BEGIN, ELSE, and
semicolon seen by the parser, provided at least one word of code has
been compiled since the previous coordinate was defined. Note that
COMMENTs are seen only by the scanner, not the parser, and that
DEFINEs and many declarations merely define symbols and do not cause
instructions to be generated. For each coordinate the directory
contains the coordinate number, the value of the program counter, and
a file pointer to the appropriate place. The appropriate place is
the source file unless a listing file is being produced and the CREF
switch is off, in which case it is the listing file. [The listing
file produced for CREF is nearly unreadable.] On a non-CREF listing,
the program counter is replaced by the coordinate number if bit 1 of
the /B switch is on.
The symbol table information consists of the block structure
and the name, access information, and type for each symbol.
If a BEGIN-END pair has declarations (i.e., is a true block
and not just a compound statement) but does not have a name, then
BAIL will invent one. The name is of the form Bnnnn where nnnn is
the decimal value of the current coordinate.
II. Run-time action
The BAIL run-time interpreter is itself a SAIL program which
resides on the system disk area. It is loaded automatically whenever
any .REL file was compiled with the /B switch. The necessary
initialization of BAIL is also taken care of automatically.
The initialization generates a .BAI file of information
collected from the .SM1 files produced by separate compilations (if
any). The .SM1 files correspond to .REL files, and the .BAI file
corresponds to the .DMP or .SAV file. Like RPG or CCL, BAIL will try
to bypass much of the initialization and use an existing .BAI file if
appropriate. During initialization BAIL displays the names of the
.SM1 files it is processing. For each .SM1 file which contains
program counter/text index information, BAIL displays the names of
the text files and determines whether the text files are accessable.
The interpreter is activated by explicit call, previously
inserted breakpoints, or the SAIL error handler. For an explicit
call, say EXTERNAL PROCEDURE BAIL; ... BAIL;. From the error
handler, respond B. Breakpoints will be described later in this
Debugging Requests. When BAIL is entered it displays the
text of the current statement (if available), its block structure,
and a portion of the dynamic scope. BAIL then prints the debugging
recursion level followed by a colon, and awaits a debugging request.
A debugging request is essentially a SAIL expression followed by a
semicolon, except that IF-THEN-ELSE and CASE expressions are not
allowed and certain natural extensions to the expression syntax are
allowed. The expression is evaluated as if it had appeared in the
source file at the place indicated. The resulting value is then
displayed. For example, the request VAR1,3+VAR2; will print the
values of VAR1 and 3+VAR2. The expression !!GO (or __GO , since !
and _ are equivalent characters to both the compiler and the
debugger) causes an immediate exit from the current instantiation of
Breakpoints. Breakpoints are handled by evaluating a call to
one of four procedures defined inside BAIL.
TRACE("proc name") Prints the procedure name at each
entry and exit, the parameters at
entry, and the value RETURNed (if
any) at exit.
UNTRACE("proc name") Discontinues tracing of the procedure.
BREAK("location","condition", Plants a breakpoint (a call to BAIL)
"action",mpc) at location. The action at a
breakpoint is IF EVAL(condition) AND
(mpc←mpc-1) LEQ 0 THEN EVAL(action);
UNBREAK("location") Remove a breakpoint.
Specification of procedure and location names: Since the
procedure or location may not be in the scope of declaration at the
time a breakpoint is planted, the syntax of a location is
location := label | procedure | blockname delim location
where delim is any non-identifier character (e.g., altmode, period,
,virgule,space,...). The last blockname should be the block in which
the label or procedure is delared. The complete search algorithm is:
Search the blocks in memory-location order of BEGINs until a block is
found which has the first blockname as its name. If there is a
second blockname, continue the search of the blocks in
memory-location order, beginning with the block which immediately
follows the block which matched the first blockname. Continue until
all blocknames have been matched. This yields some block in the
program. Construct the block structure ancestry of that block. The
label or procedure must then be declared within the scope of one of
the blocks of that ancestry.
Examining variables not in the current lexical scope: The
procedure SETLEX(n) changes the lexical scope to the scope of the
n-th entry in the dynamic scope list. SETLEX(0) is the scope of the
breakpoint; SETLEX(1) is the scope of the most recent procedure call
in the dynamic scope; etc.
BAIL and DDT: If BAIL is initialized in a core image which
does not have DDT or RAID, then things will be set up so that the
monitor command DDT gets you into BAIL in the right way. That is,
BAIL will be your DDT. To enter BAIL from DDT, use
pushi P,<address which BAIL should take as the program counter>$X
For example, if .JBOPC contains the program counter,
Syntax extension: The TO and FOR substring and sublist
operators have been extended to operate as array subscript ranges,
FOR PRINT-OUT ONLY. Thus if FOO is an array, then FOO[3 TO 7]; will
act like FOO, FOO, FOO, FOO, FOO; but is easier to
type. This extension is for print-out only; no general APL syntax or
semantics are provided.
WARNING: Since BAIL is itself a SAIL procedure, entering BAIL from
the error handler or DDT after a push-down overflow or a string
garbage collection error will get you into trouble.
BAIL and SIMPLE procedures: SIMPLE procedures cause headaches
for BAIL because they do not keep a display pointer. [Indeed, the
compiler gets lost in the following example, and does not complain:
PROCEDURE A(INTEGER I); BEGIN "A"
SIMPLE PROCEDURE B; OUTSTR("THE VALUE OF I IS " & CVS(I));
PROCEDURE C(INTEGER J); B;
END "LOST"; ]
BAIL tries valiantly to do the right thing, but occasionally it also
gets lost. BAIL will try to warn you if it can. In general, looking
at value string parameters of SIMPLE procedures does not work.
III. Resources Used
1. One channel. This means that REQUIREd source files may only be
nested to a depth of about 9.
2. Memory. Upto 11*(maximum lexical nesting depth) more words of
memory may be required compared with previous compilations.
3. CPU time. Approximately 0.3 seconds per page of dense text.
1. Channels. Three during initialization, two thereafter. Channels
are obtained via GETCHAN.
2. BAIL uses 8 of the privileged breaktables, obtaining them by GETBREAK.
3. REQUIRE 64 STRING!PDL. Necessary if the debugging recursion
level will exceed 3 or 4.
4. Memory. (9K +((# of coordinates+127) DIV 128) + (2*# of blocks) +
(5*# of symbols)) words.
5. CPU time.
a. Initialization. Typically 4 seconds for a 30 page program.
b. Debugging requests. 0.07 seconds per simple request.
DDT response time.
C. Disk space
1. The .SM1 file for a /7B compilation is typically one-fourth the
size of the corresponding .REL file.
2. The .BAI file for a group of /7B compilations is typically
one-third the total size of the corresponding .REL files.
IV. Current Status
The state of the world is determined by the values of the accumulators
and the value of the SAIL varaible !SKIP!.
1. Records and references are not recognized.
2. Stepping by statement and ability to place breakpoints at arbitrary
3. PROPS is read-only.
As time permits
1. Functional arguments are not handled correctly.
2. The run-time interpreter recognizes only the first 15 characters of
identifier names; the rest are discarded without comment. The
characters which are legal in identifiers are
Notable for its absence: period.
3. Contexts are not recognized.
4. The run-time interpreter will not recognize macros.