ForthScript

My own spinoff Forth-esque dialect heavily based upon Chuck Moore's work. Written entirely in JavaScript for the purposes of embedded browser applications. Goal of this work is making the ideas in forth as readily accessible as possible to the intrigued user.

In this implimentation the language consists of only 31 or so "words", also known as "primitives". They are based upon, but not strictly following, the F18 instruction set by GreenArray. Everything else is built upon this foundation.

They are as follows:

Opcode	Notes
:	define, terminated by ;
->	jump-ahead to >! or exit name
<-	jump-back to !< or loop name
name	call to name
next	loop to for if R != 0 (decrement R)
if	jump to then if T=0
-if	jump to then if T≥0
@b	fetch addr via B
@+	fetch addr via A (autoincrement A)
@	fetch addr via A
!b	store to addr via B
!+	store to addr via A (autoincrement A)
!	store to addr via A
+*	multiply step
2*	left shift
2/	right shift
inv	invert all bits
+	add
and	bitwise and
xor	bitwise exclusive or
drop	pop the data stack
dup	push T to the data stack
over	push S to the data stack
r>	pop R push T
>r	pop T push R
p	fetch from register P
a	fetch from register A
p!	store into register P
a!	store into register A
b!	store into register B
nop	do nothing

Additional words

Opcode	Notes
(	comment, terminated by )
.	print T as decimal
.e	print T as ascii char
include	load .fs files
abort	terminate process with error message
bye	exit jforth

Architecture

The model of this virtual machine is as follows:

• Return stack consists of 8 registers addressed circularly (no fixed top or bottom).
• R extends this stack to 9 entries but only 8 are circular.
• P serves as a “program counter”, holding the address of the next word in the instruction stream. It is incremented after each word is fetched.
• A is a general purpose read/write address or data register.
• B is a write only address register.
• T is the top word of a 10-element data stack.
• S is the second word of a 10-element data stack.
• Data Stack is 8 circularly addressed registers constituting the third through 10th elements of a 10-element stack.

Installation

Download the project then run:

node fs.js

If you want to turn it into an executable. Make sure the following shebang line is included at top of the jforth.js file.

#!/usr/bin/env node

Change the permission of the file.

chmod +x script.js

Move it into your preferred $PATH.

sudo mv jforth.js /usr/local/bin/jforth

Now you should be able to run ForthScript from anywhere.

jforth

Example Program

( Fibonacci Generator )

: print ( n -- n )
  10 .e dup .
;

: swap ( n1 n2 -- n2 n1 )
  over >r nip r>
;

: rot ( n1 n2 n3 -- n2 n3 n1 )
  >r swap r> swap
;

: 2dup ( n1 n2 -- n1 n2 n1 n2 )
  over over
;

: 2drop ( n1 n2 -- )
  drop drop
;

: fib ( u -- )
  >r 0 1 r>
  for print 2dup + rot drop next
  2drop
;