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A Turbo Pascal Debugger.
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A Turbo Pascal Debugger.
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Contents of the TDEBUG.DOC file









December 31, 1985

TDEBUG.DOC

OVERVIEW

TDEBUG is a source code debugger for use in debugging Turbo
Pascal (tm) Programs on the IBM-PC (tm). When used in
conjunction with TURBO.COM, it allows you to:

1. View your source code while debugging.
2. Trace the operation of your program statement by
statement.
3. Insert breakpoints in your program by procedure/function
name or by statement number.
4. Examine (and change) global or local variables in your
program, including strings, array variables, and
records.


SYSTEM REQUIREMENTS

1. One of the following versions of Turbo Pascal for the
IBM-PC:
TURBO.COM v3.00b
TURBO.COM v3.01a
TURBO-87.COM v3.01a
2. Approximately 128k more memory than would normally be
required to run your program in the Turbo memory mode.
3. A hard disk is desirable and may be a necessity when
debugging large programs with the optional local
symbols.


OPERATION

To use TDEBUG, include the file TDEBUG.COM on the disk (or in the
directory) with TURBO.COM (or TURBO-87.COM), TURBO.MSG, and your
program file. At the DOS prompt, call for TDEBUG. After a brief
sign on message, TDEBUG will load TURBO.COM and transfer control
to it. (The message 'Incorrect TURBO.COM version!' indicates
that TDEBUG and your version of TURBO.COM are incompatible.)

When calling for TDEBUG, the following optional parameters may be
used:

G
Use global symbols only. Users with limited disk space may
not be able to accommodate the local symbol disk file.

T=
The T option allows the name (and path) of the Turbo compiler


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to be specified. '.COM' should be a part of this
specification. The default is TURBO.COM.

L=
The L option allows specification of the local symbol file
name. This option has been included mostly to allow a path to
be specified. If enough space is available, a ramdisk drive is
ideal with a hard disk being a second choice. The default
filename is '@@LOCAL.SYM'.

B=n
For this option, n specifies the symbol table buffer size
(decimal). This is important mainly when using local symbols
because the local symbol file size will be approximately n
times the number of procedures and functions in the program
under test. Users with limited disk space will probably want
to make n as small as possible. Unfortunately, the actual size
required is not known until the program under test is compiled
so some experimenting may be necessary. The default value is
6000 bytes. n may not be made less than 1000 bytes.

Examples:

TDEBUG T=TURBO-87.COM L=D:LOCAL B=5000
TDEBUG G


Once TDEBUG has loaded and begun execution of TURBO.COM, all of
the commands and features of Turbo Pascal should operate as they
always have with the exception of the Run command. The only
special requirement for programs to be debugged is that the {$U+}
compiler directive not be used. When the Run command is
executed, TDEBUG will regain control and the debugging screen
will appear.

The debugging screen is divided into two parts. The lower
section is the command and data section. The '*' is a prompt for
one of the commands described later. The top part of the screen
displays the program source code text which at this point will be
centered at the first executable statement of your program.

Within the text part of the display, the current statement (the
next one to be executed) will be highlighted. The left margin
contains the statement number of the first statement occurring on
that particular line. Statement numbers are consecutive numbers
and are used to refer to individual statements in the commands
described later. Since there may be more than one statement on a
line, the statement number in the margin is that of the first
statement on the line. The first character of each statement is
highlighted. Note also that there may be no statements on a
given line so those lines will have no numbers.



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Dividing the two portions of the debugging screen is a line which
at this point will probably display 'Memory'. This indicates
that the text file being observed is in memory. Later, when
viewing a portion of the program source code which is in an
'include' file, the name of the 'include' file will appear here.

When the Run command is issued after a fresh compilation, TDEBUG
will issue a report giving the symbol table buffer size actually
required, the number of statements and include files found and
any disk errors which might have occurred during compilation. In
the event of any errors, it will not be possible to continue with
the debugging session until the errors are corrected. Hit any
key to return to TURBO to make the corrections. The most likely
error is a symbol table buffer overflow which will require a
change in the 'B' option at load time.


SPECIAL KEYS FOR VIEWING TEXT

The following keys may be used to change the text being
displayed:

Pg Up
Pg Dn Move the display up or down one page.

^Pg Up
^Pg Dn Move to the start or end of the current file.

Up arrow
Down arrow Scroll up or down one line.

F10 key Toggle between the debug screen and the screen
of the program under test.

In addition, the text being displayed may be changed using the
View command described later.


COMMAND EDITOR

When entering commands, the following keys may used to edit the
command: (They perform the same functions as they do in the
Turbo Pascal editor.)

^A,^S,^D,^F,^G,^T,^H,LFARW,RTARW,^LFARW,^RTARW,
Ins, Del, Backspace

The Insert mode (default) is indicated by a slightly fatter
cursor while the cursor is normal in the Overwrite mode.

In addition, the F3 function key causes the last command entered
(having more than three characters) to be redisplayed. This may


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be used to make minor changes to a previous command or to correct
a syntax error.

Note that commands are not acted upon until the Enter key is hit.
(The cursor does not have to be at the line end to enter the
command.)


COMMAND PARAMETERS

Commands consist of one or two letters which may be followed by
one or more parameters. A non alphabetic character must follow
the command to delimit it from a following parameter. The
parameters which may be used are:


A Pascal source code symbol representing a
procedure/function name, variable, or constant as
appropriate. The special symbol 'RTN' may be used to refer
to the return point of a procedure/function. The usage of
global or local symbols is explained more fully in a later
section.


A statement number as displayed in the left margin of the
text display.


COMMANDS

G (Go)

Format:
G [ [
The Go command starts execution of the program under test.
Execution will continue until a breakpoint is reached, the
program terminates, or the key is hit.

One or more breakpoints may be entered with the Go command.
These breakpoints are called temporary breakpoints as they are
in effect only until the first stopping point is reached. Any
temporary breakpoint would then have to reentered with the next
Go command if desired.

The program under test may be interrupted using the Break> key. Note that does not perform the same
function but rather performs its normal Turbo Pascal function
(as defined by the {$C } compiler directive.





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Examples:

G Start execution with no temporary breakpoints. (There
might be some permanent breakpoints, however.)

G 345 PROC1 RTN
Start execution with temporary breakpoints at statement
345, at the start of procedure PROC1, and at the return
point of the current procedure/function. In addition,
there might be other permanent breakpoints in effect.

T (Trace)

Format:
T

Execute the current statement. Only one statement is executed.


N (Next)

Format:
N

Execute the current statement in its entirety.

The difference between the Trace and the Next command is that
if the current statement is a procedure or function call, Next
will completely execute the procedure/function whereas Trace
will Trace through the procedure/function.


P (Permanent breakpoint)

Format:
[-]P []

The P command is used to specify permanent breakpoints. Unlike
temporary breakpoints, permanent breakpoints remain in effect
until removed (using a '-' preceding the P command).

If no parameter is entered, the breakpoints in effect are
listed.

Examples:

P List all permanent breakpoints.
-P Delete all permanent breakpoints.
P 345 Install a breakpoints at statement 345.
-P PROC1 Remove a breakpoint at procedure PROC1.




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V (View text)

Format:
V []

The View command may be used to change the text being observed
in the text window. If no parameter is entered, the text will
be centered around the current statement (the one to be
executed next).

Examples:

V InitScreens View text at procedure InitScreens.
V 545 View text around statement 545.
V Rtn Find out where this procedure/function was
called from.


E (Examine/change variable or constant)

Format:
E

The Examine/change command allows the the value of global
and local variables and constants to be displayed. In
addition, for variables, a new value may be entered. Note that
Turbo Pascal 'typed constants' are actually initialized
variables and these may also be changed.

After a variable value has been output in response to the
Examine/change command, a new value may be entered followed by
the Enter key. For byte and integer types, a hex value may be
specified by preceding it with a '$'. Hitting the Enter key
only retains the old variable value.

8 bit variables (byte, char, boolean) are displayed in decimal,
hex, and character (if printable). In the case of boolean, 0
is false, and 1 is true.

Integer variables are displayed in both decimal and hex.

Real variables are displayed in 'E' format and also decimal
format if within a reasonable range.

Pointer variables are displayed with the phrase 'Points to:'
followed by an address and symbolic name if appropriate. If
the pointer variable is followed by '^', the contents of the
pointed to variable are displayed. To change a pointer
variable, use either a symbolic name or a segment:offset
numerical address with a colon separator such as $1234:$5678.




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String variables are displayed as a string within quotes. When
specifying a change to a string variable, the new string should
be enclosed in quotes. A null string is specified by two
quotes enclosing nothing. Individual characters of a string
may be displayed and changed by treating the string as an
array.

Non numerical scaler variables are displayed numerically. For
instance, a variable defined as

Var Color :(red,green,blue,orange);

would be be given a value of 2 if it contained blue. However,
note that:

E BLUE

could be used to verify that blue was represented by 2.

Sets cannot be examined.

Examples:

E RealArray[4,3]
E InString
E PersonRecord[12].ID.Lastname
E PT^.Index1
E START^.NEXT^.NEXT^.NEXT^.VALUE[10][3]


Q (Quit)

Format:
Q

The Quit command when entered in TDEBUG returns control to
Turbo Pascal without running the program under test to
completion.


X (Translate)

Format:
X

The translate command translates symbols and statements into
segment-offset addresses. This is useful mainly when using an
external assembly language debugger. Unlike the E (Examine)
command, a variable need not be completely specified to get an
address.




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Examples:

X PersonRecord ;yields the first address of
PersonRecord
X PersonRecord[12] ;yields the first address of
PersonRecord[12]
X PersonRecord[12].ID ;yields the first address of
PersonRecord[12].ID


DB (DeBug)

Format:
DB

This command is used to enter an optional external assembly
language debugger. Its use is explained in a later section.


SYMBOLS

TDEBUG makes use of two sets of symbols, the global symbols and
(optionally) the local symbols. Global symbols are those which
would be available in the main block of the Pascal program and
include all non nested procedure/function names as well as
variables and constants defined outside of procedure/functions.

Local symbols are those defined within a procedure/function and
are valid and only accessible within the block of that
procedure/function. Local symbols include the formal parameters
defined in the procedure heading and also function results. Note
that the block boundaries are delimited by the BEGIN and END for
any particular procedure. In particular, this means that local
symbols cannot be accessed when the current instruction is a
procedure/function heading. This should be kept in mind since
breakpoints are frequently placed at procedure/function headings.

Error messages such as 'Not a GLOBAL symbol' or 'Not a GLOBAL
Variable' are meant to be a reminder that only the GLOBAL symbols
are being searched. From a procedure/function heading it is only
necessary to do one 'T' (Trace) command to be within procedure
block where the local symbols can be accessed.

TDEBUG searches local symbols before global symbols. This is of
significance in cases where duplicate symbol names are used.


HINTS

It is possible to insert breakpoints in the source code using
INLINE($CC). These breakpoints cannot be removed by TDEBUG
but may be useful for conditional breaks. For instance:


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IF I=49 THEN INLINE($CC);


USING AN EXTERNAL ASSEMBLY LANGUAGE DEBUGGER

It is possible to use an assembly language debugger in
conjunction with TDEBUG. It is often desirable to examine the
compiled code of a program to determine what the problems really
are and this provides a convenient way to do this. The following
instructions are for using DOS DEBUG although other debuggers may
be used in a similar manner.

Load DEBUG by:

DEBUG TDEBUG.COM

where the are the optional TDEBUG parameters. Once
the load is completed and you are in DEBUG, issue the 'G' (Go)
command. Operation of TDEBUG and TURBO will proceed in a normal
fashion. At the appropriate time in the debugging session, use
the X (Translate) command to find the machine addresses to be
inspected. Then issue the DB (DeBug) command. You will find
yourself back in DEBUG.

At this point, the DEBUG U (Unassemble) or D (Display) commands
can be used to inspect code or data.

To reenter TDEBUG, it is necessary to jump over the INT 3
instruction which caused the break to DEBUG. Do this by using
the G command:

G=xxxx

where xxxx is the contents of the IP register plus 1. No
breakpoints should be installed with DEBUG as TDEBUG takes over
the breakpoint vector when it is reentered.


RESTRICTIONS, PROBLEMS, BUGS

The following are current limits.

Permanent and temporary breakpoints are limited to 6 each.
Maximum number of include files = 20.
Maximum include file size = 32k.
Maximum number of statements = 4000.
Maximum symbol length = 28.
Maximum number of local symbol blocks = 250.


Presently symbolic constants cannot be used as subscripts in the
Examine/change command. For instance, if


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Var Opcode :ARRAY[MOV..CMP] OF BYTE;

it is presently necessary to specify OPCODE[0] rather than
OPCODE[MOV] when using the Examine/change command.

The program interrupt does not perform properly
when the program under test is waiting for keyboard input.

A simple screen switch is currently used. This limits debugging
to text only programs.

Function results should be accessible only within the block of
the function whereas they can now be accessed from wherever the
function may be called. Any results displayed outside of the
function block are not valid. Use care not to alter a function
result outside of its block as you will be altering an odd memory
location.

Variables defined as 'absolute' are sometimes stored as pointers.
TDEBUG presently does not resolve these correctly.

There is a problem when compiling to a disk COM file. Turbo
often takes a short cut and transfers directly from memory to
disk without recompiling. However, the memory file has been
altered by TDEBUG and the resulting COM file is corrupted. It
is best at this stage to compile to disk only without the
debugger.





IBM-PC is a trademark of International Business Machines
Corporation.
Turbo Pascal is a trademark of Borland International Inc.


















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