Debugging Basic with the Python Debugger

Debugging Basic with the Python Debugger

You can debug a Basic program using python's built-in debugger pdb. Start the pcbasic interpreter in the python debugger:

python3 -m pdb /path/to/pcbasic your-program.bas --interface=none -d

Alternatively you may want to use the native interface to not interfere with the debugger. Or you may want to specify the --input option to make pcbasic read from a file instead of the console.

Of course you want to be familiar with the python debugger which has excellent documentation

Stopping at a Basic line number

In pcbasic in the file interpreter.py around line 105 there is a line reading:

if self.tron:

This line checks if the Basic interpreter built-in trace facility is active and at this point the line number of the basic program is known. So we can set a breakpoint in the python debugger there:

break /usr/lib/python3/dist-packages/pcbasic/basic/interpreter.py:105

As said before, your version of pcbasic may need a different line number if the statment above has changed position.

Now you can single-step through your Basic program with the pdb command continue abbreviated c. You can display the current Basic line number using:

p struct.unpack_from('<H', token, 2)[0]

and we can print the line number every time the breakpoint is hit:

commands 1
p struct.unpack_from('<H', token, 2)[0]
end

The commands 1 instructs the debugger to associate the following commands (until a line with only end) with the breakpoint numbered 1.

To run the basic program until a specific line number is reached, we can put a condition on the breakpoint:

condition 1 struct.unpack_from('<H', token, 2)[0] == 42

When you enter c after this command, the program will be run until it has reached line number 42 and stop before executing that line.

To make the breakpoint unconditional again (so we can single-step through our program) enter:

condition 1

Inspecting variables

You can display a scalar variable using:

p self._scalars.get (b'U2!').to_value()

Note that variables are stored with their sigil, see the the documentation on sigils for details. The example above is the single precision floating point variable U2!.

This extends to arrays:

p self._memory.arrays.get(b'J2!', [1,1]).to_value()

will list the item at position 1,1 in the two-dimensional array J2!. You can display a complete array using:

p self._memory.arrays.to_list (b'J2!')

At the time of this writing the to_list method has a bug that does not display the last item in every dimension, there is a bug report you may want to check.

You can check the dimension of an array with:

p self._memory.arrays.dimensions (b'J2!')

Or display a sorted list of all defined scalar variables:

p sorted (self._scalars._vars.keys())

These are the basics for getting started, of course you may use other features of the debugger and/or the pcbasic interpreter for debugging your programs.