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 | _ ) /_\ / __|_ _/ __|_  )  \| | | /  \
 | _ \/ _ \\__ \| | (__ / / () |_  _| () |
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>

A Modular BASIC Interpreter for Embedded Systems

Read on for an overview or jump right to information on:

• Integration into projects
• BASIC Dialect details

Introduction

Originally forked from the excellent PyBasic, this version has been restructured to make it easy to use as a module in other proejcts, specifically embedded systems running Circuit/Micro python. It still runs in cPython (required Python 3+), but has been optimized for lower memory environments. The 2040 in Basic2040 refers to a popular Circuit Python compatible microcontroller, the Pi2040, which is available as part of several different boards.

The primary differences from PyBasic are:

• Core BASIC engine code cleanly encapsulated in the basic2040 module so it can be used without modification in other projects
• I/O Abstration to allow various screen/keyboard setups
• Easy ability to add new BASIC functions/keywords for enabling hardware specific functions
• Lower memory usage at the expense of code complexity

Example Implementations

An example interpreter can be invoked as follows:

$ python example.py

You can immediately begin entering program lines or commands at the > prompt

> 10 PRINT "HELLO"
> RUN
HELLO

Interpreter Commands

Programs may be listed using the LIST command:

> LIST
10 LET I = 10
20 PRINT I
>

The list command can take arguments to refine the line selection listed

LIST 50 Lists only line 50

LIST 50-100 Lists lines 50 through 100 inclusive

LIST 50 100 Also Lists lines 50 through 100 inclusive, almost any delimiter works here

LIST -100 Lists from the start of the program through line 100 inclusive

LIST 50- Lists from line 50 to the end of the program

A program is executed using the RUN command:

> RUN
10
>

A program may be saved to disk using the SAVE command:

> SAVE myprogram
Program written to file
>

The program may be re-loaded from disk using the LOAD command:

> LOAD myprogram
Program read from file
>

When loading or saving, the .bas extension is assumed if not provided and files are expected in the BAS directory under the current working path. You can load files from any location, with any extension, but using quotes and providing the full path:

> LOAD "/path/to/program/myprogram.foo"

Individual program statements may be deleted by entering their line number only:

> 10 PRINT "Hello"
> 20 PRINT "Goodbye"
> LIST
10 PRINT "Hello"
20 PRINT "Goodbye"
> 10
> LIST
20 PRINT "Goodbye"
>

The program may be erased entirely from memory using the NEW command:

> 10 LET I = 10
> LIST
10 LET I = 10
> NEW
> LIST
>

Finally, it is possible to terminate the interpreter by issuing the EXIT command:

> EXIT
$

On occasion, it might be necessary to force termination of a program and return to the interpreter, for example, because it is caught in an infinite loop. This can be achieved by using Ctrl-C to force the program to stop:

> 10 PRINT "Hello"
> 20 GOTO 10
> RUN
"Hello"
"Hello"
"Hello"
...
...
<Ctrl-C>
Program terminated
> LIST
10 PRINT "Hello"
20 GOTO 10
>

Terminals

One of the key features of Basic2040 is a that all IO (Keyboard/Screen) is abstracted into a terminal class that can be adapted to many specific use-cases. The command above will start the standard Basic2040 interpreter connected to stdio and should be compatible with all systems that can run python.

Also included is an example curses based terminal class. Basic2040 includes some extra screen control features, such as CLEAR to clear the screen, and the CURSOR command allow printing at specific screen locations. To use these features the terminal class used must support them. The example curses terminal does just this.

$ python example_curses.py

The curses based terminal class also has a scrollback and bell functionality. Use the up/down arrows to scroll back previous commands. A left arrow will clear the current line.

For specific hardware situations the Terminal class in term.py can be implemented to handle IO from non-standard keyboards/screens. This is handy for specific hardware projects using CircuitPython comptible microcontrollers. See my PicoBasic repo for examples using various screen/keyboard options for microcontrollers.

Example programs

A number of example BASIC programs have been supplied in the repository, in the examples directory:

• tests.bas - A program to exercise the key programming language constructs in such a way as to allow verification that the interpreter is functioning correctly. This program is designed to be used with the special testing terminal for automated testing.

• factorial.bas - A simple BASIC program to take a number, N, as input from the user and calculate the corresponding factorial N!.

• rock_scissors_paper.bas - A BASIC implementation of the rock-paper-scissors game.

• PyBStartrek.bas - A port of the 1971 Star Trek text based strategy game.

• adventure-fast.bas - A port of a 1979 text based adventure game.

• bagels.bas - A guessing game.

• eliza.bas - A port of the early chatbot, posing as a therapist, originally created by Joseph Weizenbaum in 1964.

Open issues

• It is not possible to renumber a program. This would require considerable extra functionality.
• Negative values are printed with a space (e.g. '- 5') in program listings because of tokenization. This does not affect functionality.
• Decimal values less than one must be expressed with a leading zero (i.e. 0.34 rather than .34)
• User input values cannot be directly assigned to array variables in an INPUT or READ statement
• Strings representing numbers (e.g. "10") can actually be assigned to numeric variables in INPUT and READ statements without an error, Python will silently convert them to integers.

License

Basic2040 is made available under the GNU General Public License, version 3.0 or later (GPL-3.0-or-later).