Wishful Coding

Didn't you ever wish your computer understood you?

NXT Forth compiler in Clojure

My first project at Hacker School was to write a compiler for Forth for the LEGO NXT. There are a few real programming languages for the NXT, but before I found Mirah, I never really enjoyed them.

Initially, I wanted to write a Lisp, but the NXT bytecode is so static that even the concept of a cons cell is not viable.

Forth is a very simple stack based language that uses just that, a stack and subroutines(called words). Surpisingly, the only dynamic feature in the NXT are resizable arrays. I think it’s a good fit.

This is the complete stack implementation in NBC.

#define push(stack, val) \
  replace stack.data stack.data stack.offset val \
  add stack.offset stack.offset 1

#define pop(stack, val) \
  sub stack.offset stack.offset 1 \
  index val stack.data stack.offset

My implementation follows the Boostrapping a Forth in 40 lines of Lua code approach of defining a Forth word that evals the host language.

The only difference is that this Forth is compiled, so there is a word for Clojure and one for NBC.

The main Clojure file just defines an atom to store Forth words, and defines the clj word, which evaluates a Clojure expression.

As soon as that is in place, you’re in Forth land.

First thing we do in Forth is some more Clojure defining “:*” to mean “define clojure word”, and then using “:*” to define 3 more words.

  • ”:” start of a Forth word
  • ”;” end of a Forth word
  • “nbc” write a line of assembly

What follows are a few forth words defined in assembly. At this point, the following works

: square
  dup *;

2 square dot

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Pepijn de Vos