An "old-style" BASIC interpreter, written in standard C, with a focus on being easy to extend and port.

Introduction

This is a BASIC interpreter written from scratch in C. It runs on Windows, Linux, OSX and a number of microcontroller platforms. It aims to fully implement the old-style dialects of BASIC, meaning, line-number based code. It should be quite compatible with early 1980's interpreters, such as Microsoft BASIC (for 8080/8085/Z80, 6502, 6800 etc), EhBASIC, Commodore BASIC, and so on.

Most of the common BASIC commands are supported:

• CLEAR, CLR
• CONT
• DELETE filename
• DIR
• LIST linenr[-linenr]
• LOAD filename
• NEW
• RUN [linenr]
• SAVE filename

Most of the common BASIC statements (instructions) are supported:

• CLS
• COLOR
• DATA
• DEF
• DIM variable "(" expression ")"
• END
• FOR numeric_variable '=' numeric_expression TO numeric_expression [ STEP number ]
• GET variable
• GOTO expression
• GOSUB expression
• IF relation-expression THEN statement [ELSE statement]
• INPUT ["prompt";] variable-list
• LET variable = expression
• LOCATE row,col
• NEXT [variable]
• ON variable { GOTO,GOSUB }
• PRINT expression-list [ ; , ]
• READ variable-list
• REM [text]
• RETURN
• RESTORE
• SLEEP milliseconds
• STOP

These are the functions currently implemented:

• ABS, ATN, COS, EXP, FRE, INT, LOG, POW, RND, SGN, SIN, SQR, TAN
• ASC, CHR$, LEFT$, LEN, MID$, RIGHT$, STR$, VAL

Also, a number of system variables are implemented:

• LINE, ERRLIN, PI
• DATE$, TIME$, VERSION$

Extending the language.

Extending the language is easy - you can register a new BASIC function, as shown here with a sleep function for the XMEGA:

register_function_1(FUNC_KEYWORD, "SLEEP", do_sleep, kind_numeric);
...
static int
do_sleep(basic_var *rv, const basic_var *delay)
{
  delay_ms(delay->value.number);
  
  rv->kind = KIND_NUMERIC;
  rv->value.number = 0;

  return 0;
}

Let's use that new keyword.

C:\> basic
VARCem BASIC Interpreter version 1.1.1
Copyright 2015-2016 Johan Van den Brande
Copyright 2023 Fred N. van Kempen
 _               _
| |__   __ _ ___(_) ___
| '_ \ / _` / __| |/ __|
| |_) | (_| \__ \ | (__
|_.__/ \__,_|___/_|\___|

READY.
load "hello"
READY.
list
10 INPUT "YOUR NAME?"; NAME$
20 CLS
30 FOR I=1 TO LEN(NAME$)
40 PRINT MID$(NAME$,I,1);
50 SLEEP(500)
60 NEXT
70 PRINT
READY.

Port

It should be easy to port the interpreter to other architectures. As an example there is a port to an XMega 128A4U included using the Batsocks breadmate board.

Use

There is a simple REPL for the BASIC interpreter. You can use it in an interactive way, just as you would do on a 80's era computer.

C:\> basic
VARCem BASIC Interpreter version 1.1.1
Copyright 2015-2016 Johan Van den Brande
Copyright 2023 Fred N. van Kempen
 _               _
| |__   __ _ ___(_) ___
| '_ \ / _` / __| |/ __|
| |_) | (_| \__ \ | (__
|_.__/ \__,_|___/_|\___|

READY.
10 PRINT "HELLO"
20 GOTO 10

You can give it a BASIC file on the command line.

$> basic examples/diamond.bas
         *
        ***
       *****
      *******
     *********
    ***********
   *************
  ***************
 *****************
  ***************
   *************
    ***********
     *********
      *******
       *****
        ***
         *

You can also use the shebang operator to make standalone BASIC scripts

#!/usr/bin/env basic

10 RADIUS=10
20 FOR I=1 TO RADIUS-1
30 W=INT(RADIUS*SIN(180/RADIUS*I*3.1415/180))
40 PRINT SPC(RADIUS-W);:FOR J=1 TO 2*W:PRINT "*";:NEXT J:PRINT
50 NEXT I

$> ./examples/circle.bas
       ******
     **********
  ****************
 ******************
********************
 ******************
  ****************
     **********
       ******

It is easy to embed the interpreter into your own application.

    /* Create an instance of the interpreter. */
    basic_init(16*1024, 2*1024, my_error, my_ready);

    basic_eval("10 PRINT \"HELLO\"");
    basic_eval("RUN"); 
  
    basic_destroy();  

where my_error and my_ready are the local implementations of the ERROR and READY handlers.

On OSX/POSIX you can use the 'BASIC_PATH' environment variable to set the folder used for loading and saving BASIC programs. The 'BASIC_PATH' defaults to '.'.

BASIC programs are expected to end with '.bas'. You can use LOAD, SAVE, DELETE and DIR.

Authors and Credits

The original code was written for the AVR X/Mega platform by Johan Van den Brande, who also made that work on Apple's OSX. Support for the Linux platform was added by clarking clarkaaron@hotmail.com in August of 2022, and the project was revived and merged into the VARCem emulator project by Fred van Kempen in 2023.

Copyright

Copyright 2023 Fred N. van Kempen, The VARCem Group LLC Copyright 2015-2016 Johan Van den Brande