A from-scratch BASIC interpreter with a focus on being easy to extend and port.

This is a BASIC interpreter written from scratch in C. For the moment two architectures are supported, POSIX and AVR XMega via the avr-gcc toolchain.

Most of the common BASIC keywords are supported:

• PRINT expression-list [ ; ]
• IF relation-expression THEN statement
• GOTO expression
• INPUT variable-list
• LET variable = expression
• GOSUB expression
• RETURN
• FOR numeric_variable '=' numeric_expression TO numeric_expression [ STEP number ]
• CLEAR, NEW
• LIST
• RUN
• END
• DIM variable "(" expression ")"
• ABS, AND, ATN, COS, EXP, INT, LOG, NOT, OR, RND, SGN, SIN, SQR, TAN
• LEN, CHR$, MID$, LEFT$, RIGHT$, ASC

It should be easy to register a new BASIC function, as shown here with a sleep function for the XMEGA.

register_function_1(basic_function_type_keyword, "SLEEP", do_sleep, kind_numeric); ... int do_sleep(basic_type* delay, basic_type* rv) { delay_ms(delay->value.number); rv->kind = kind_numeric; rv->value.number = 0; return 0; }

Let's use that new keyword.

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

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.

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) 2015-2016 Johan Van den Brande 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.

 basic_init(2048, 512); // memory size, stack size basic_register_io(putchar, getchar); basic_eval("10 PRINT \"HELLO\""); basic_eval("RUN"); basic_destroy(); 

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.

(c) 2015 - 2016 Johan Van den Brande