Beginner's All-purpose Symbolic Instruction Code

BASIC is one of the oldest high-level computer languages. It was designed to be easy to learn and use, and (in this author's opinion) is the best language for an aspiring programmer to start learning how to tell the computer to do things.

See the accompanying PDF document for information about using the BASIC interpreter.

Building

The BASIC interpreter should compile on any BSD or Linux system with a GNU C compiler, GNU Bison, and Flex (including the libfl library, typically provided by “libfl-devel”). You may also need some version of Make.

To build the interpreter, run:

 make basic

If all goes well, you can copy or link the basic executable to your ~/bin/ directory or the system /usr/local/bin/ directory.

Running BASIC

When you run basic, the interpreter will wait for BASIC commands. You can enter statements to be run immediately or enter a program by entering statements with line numbers.

Several example BASIC programs are included with the basic source code in the examples directory. To load these programs, use the LOAD command within the BASIC interpreter; for example:

 LOAD "examples/Blackjack.BASIC"

When the program has been loaded, type “RUN” to run it.

To exit out of the BASIC interpreter, enter the command “BYE”.

Debugging

There are sure to be bugs in this interpreter, but there may be bugs in your BASIC code as well. This implementation of BASIC adds a TRACE command which tells BASIC to show what it's doing with various levels of detail. The syntax is:

TRACE {LINES|STATEMENTS|EXPRESSIONS|GRAMMAR|PARSER} {ON|OFF}

where the first argument is the type of detail to trace and the second turns tracing for that aspect on or off. The first three details that can be traced help you debug your BASIC program:

• LINES: Lists each line of the program before executing its statements.
• STATEMENTS: Lists each statement before executing it. This may be more useful than LINES if your program contains multiple statements on most lines; otherwise LINES is better at orienting your place in the program.
• EXPRESSIONS: Displays the result of evaluating expressions.

The next two details that can be traced are for debugging the BASIC interpreter itself:

• GRAMMAR: Shows how Bison interprets the BASIC statements it is given and builds token lists from it. Using this requires knowledge of the generated token values (which are enumerated in basic.tab.h) and the grammar (declared in basic.y).
• PARSER: Shows the result of Flex parsing input to BASIC into tokens, variable identifiers, and constants. This level of detail should rarely be needed.