The Debug Mode

The term debug refers to the process of finding programming errors and correcting them. BASIC09's debugging features include symbolic debugging capabilities that let you examine variable values and test and manipulate procedures.

With Debug, you can:
Examine and change variables.

Trace procedure execution. Debug lets you execute procedures and watch them run in slow motion.

Pause procedure execution.
Resume procedure execution.

· Set procedure breakpoints that automatically switch to the debug mode.

· Select the use of degrees or radians for trigonometric functions.

Perform calculations.

e Call OS-9 system commands.

Entering the Debug Mode

You enter Debug:

9 Automatically, whenever an error occurs during the execution of a procedure (unless you have included an ON ERROR GOTO statement to handle the error).

Automatically, when a procedure executes a PAUSE statement.

When you press cTRyc during the execution of a procedure.

You can tell when BASIC09 enters the Debug mode by the appearance of the D : prompt . When you see D:, followed by the cursor, Debug is waiting for your command.

The following is a reference of all the Debug commands and what they accomplish:

BASIC09 Reference

Command Function

$ Calls OS-9's command shell interpreter to run

a program or an OS-9 command. From the

Debug prompt, type $ , followed by the name of

the program or command you want to execute.

Example: s 1 i s t procedure-one ENTER

BREAK Sets a breakpoint immediately before the spec
ified procedure. Use this command to re-enter
Debug when one procedure calls another.
If you have three procedures that call each
other Procl, Proc2, and Proc3 and Proc3
does not seem to pass the correct values to
Proc2 when it returns, set a breakpoint at
Proc2. This causes BASIC09 to enter Debug
before re-entering Proc2. You can then check
your variable values.
You can use one breakpoint for each active pro
cedure. Debug removes breakpoints immedi
ately after encountering them.
A procedure must run before you can set a
breakpoint in it. Use BREAK to stop execution
when a called procedure returns to a proce
dure previously executed.
Example: BREAK p r o c 2 ENTER

CONT Causes procedure execution to continue.

Example: c o n t ENTER

DEG/RAD Selects either degrees or radians as the unit of
measurement for trigonometric functions. DEG
and RAD affect only the current procedure.
Examples: d e 9 ENTER
r a d ENTER

5-2
The Debug Mode / 5

DIR

Displays the name, size, and variable storage requirements of each procedure in the workspace. The current working procedure has an asterisk before its name. Ail packed procedures have a dash before their names. DIR also shows the available memory in the workspace.

If you provide a pathlist, DIR sends its data to the specified file.

Example: d i r ENTER

dir procedures

LET

LIST

PRINT

STATE

Terminates execution of the procedure, closes any open paths, and exits to the System mode.

Example: q ENTER

Assigns a new value to a variable. You must specify variable names that are already initialized by your program. In the Debug mode, you cannot use LET to copy one array to another array as you can in BASIC procedures.

Example: 1 e t a : = 0 ENTER
1 et fruit : = "oranqe5" ENTER

Displays a source listing of the suspended procedure. The display is formatted and includes I-code addresses. An asterisk appears to the left of the last executed statement.

Example: 1 i s t ENTER

Displays the values of variables used in the suspended procedure. You cannot introduce new variable names in the Debug mode, and you cannot display array structures. -

Example: print fruit ENTER

Lists the nesting order of active procedures. STATE displays the highest-level procedure at the bottom of the calling list. The lowest-level procedure is the suspended procedure.

Example: 5 t a t e ENTER

5-3
BASIC09 Reference

STEP Causes execution of the suspended procedure
in specified increments. For example, typing
STEP 5 ENTER executes the equivalent of the
next five statements. If you enter STEP with
out an increment value, the step rate is 1.
Using STEP with the trace function lets you
observe the source lines as they execute.
Because compiled I-code contains actual state
ment memory addresses, the top or bottom
statements of loop structures execute only
once. For example, in FOR/NEXT loops, FOR
executes once, and the statement following
FOR appears to be the top of the loop.

TRON/TROFF Turns on or turns off the trace function. Trace on (TRON) causes the system to reconstruct the compiled code of each statement line into source code. Debug displays the source code before the statement is executed. If the statement causes the evaluation of one or more expressions, Debug displays each result following the statement. The result is preceded by an equal sign.

The trace function is local to the current procedure. If the suspended procedure calls another procedure, Debug suspends the trace function until control returns to the original procedure. However, once you turn on trace for a procedure, it continues in effect until you turn it off. This means that if you turn trace on in a called procedure, and another procedure subsequently calls it, trace continues to display the called procedure's operations.

Example: t r o n ENTER

t r o f f ENTER

When Things Go Wrong

Programming errors show up in two ways. Either your procedure produces incorrect results, or it terminates prematurely.

5-4

The Debug Mode / 5

In the first instance, you can stop your procedure and enter Debug by pressing CTRL ff).

However, sometimes your program executes too quickly to allow you to stop it at the appropriate place. In this case, you can use the Edit mode to insert a PAUSE command where you wish the procedure to stop. PAUSE causes the procedure to halt execution and enter the Debug mode.

Once in Debug, you can use the PRINT command to examine the procedure variables. You can use LET to manipulate the variable values to determine where the error or errors occur. Perhaps you forgot to initialize a variable or forgot to increase a loop counter.

Using the Trace Function

Sometimes, errors are more difficult to discover. If so, the next step is to use the trace function. To do this, type:

t r o n ENTER

Now press ENTER . Each time you press ENTER , Debug executes one line of the procedure. You can see the original source statement, and if an expression is evaluated, Debug prints the result of the expression, preceded by an equal sign.

In this manner, you can step through the entire procedure, or any part of it, examining variable values as you go.

What About Loops?

The STEP command is helpful if you find yourself tracing the operation of a loop. Once you determine that the loop works correctly, you can avoid tedious, step-by-step repetitions by turning trace off and using STEP to quickly run through the loop. Then, turn trace back on and resume single-step debugging. For instance, type:

t r o f f ENTER step 2 0 0 ENTER t r o n ENTER

5-5

BASIC09 Reference

In Multiple Procedures

Although the trace function is local to a procedure, you can pause and turn on the trace function in as many procedures as you wish. Trace continues to operate in each procedure until you turn it off using TROFF.

To cause a procedure to halt execution when it is called by another procedure, use the BREAK command.

5-6