PaloAlto BASIC in Ruby less then in 500 lines

PaloAlto BASIC (also known as Tiny BASIC) interpreter less than in 500 lines writen in Ruby. The original program takes only 3KB of ROM and runs on Intel 8080, Motorola 6800 and MOS Technology 6502 processors.

This BASIC understands a few statements and all variables there are 16 or 32-bit integers.

Take a look on Wikipedia page.

Just take a look on some code examples

Print statement: PRINT "HELLO WORLD", PRINT A or PRINT "X: ", X, ", Y: ", Y, ", Z: ", Z

Input staement: INPUT A, or you can use list of variables: INPUT A, B, C

Variable defenition/assigment: LET A = 10, LET B = (2 + 6) * 4 or LET C = (A * B) + (B / A) - 1

Goto: 10 GOTO 10 is endless cycle. GOTO A * 10 + 230 is also possible.

If: IF A = B THEN PRINT "EQUALITY", IF A > B * 2 THEN LET C = C + (B - A) or IF A <> B THEN PRINT "UNEQUALITY"

GoSub: 10 IF A = 1 THEN GOSUB 50 ... 50 LET X = X * X ... 60 RETURN

Important Note: To run the program just type in RUN and hit <Enter>!

EBNF Grammer

The grammer is also pretty simple and can be described in EBNF just in a very few lines:

CR stands for Carret Return (\r\n or \n, depends on OS)

empty stands for nothing.

space stands for any white space ASCII symbol.

(...)* means that ... may be repeated zero or more times.

digit ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

lowercase ::= a | b | c ... x | y | z

uppercase ::= A | B | C ... X | Y | Z

letter ::= lowercase | uppercase

number ::= digit (digit)*

string ::= " (letter | digit | space)* "

var ::= uppercase

var-list ::= var (, var)*

factor ::= var | number | ( expression )

term ::= factor ((* | /) factor)*

expression ::= (+ | - | empty) term ((+ | -) term)*

expression-list ::= (expression | string) (, expression | string)*

ralational-operator ::= (< (> | = | empty)) | (> (< | = | empty)) | =

statement ::= (PRINT expression-list) | (INPUT var-list) | (LET var = expression) | (GOTO expression) | (GOSUB expression) | (RETURN) | (IF expression relational-operator expression THEN statement) | (END)

line ::= ((number statement) | (statement)) CR

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To run the programm just download paloaltobasic.rb and run it using ruby paloaltobasic.rb. Program works stable on Ruby version 2.6.

Attention! Program works unstable under GIT Bash. Try using some other terminal (ex. Windows CMD)

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