Bootstrapping a Forth Interpreter from Handwritten 1KB Binary

This project is to bootstrap a language processor from a hand-written ELF binary. This is a hobby project and is not intended for any practical use, but constructing a language processor completely from scratch is a lot of fun. Please try running, reading and playing with the code!

Bootstrapping proceeds as follows:

1. Implement a simple Forth interpreter (really) by handwriting ELF. 2. Grow it into a relatively expressive language through self-extension.

Since writing a program in machine language by hand is a tough work, the first interpreter is designed to be very simple. Every built-in word is single-letter and the interpreter just repeats that reads a character, looks it up from the dictionary and executes it. Also there is no error checking.

This is the actual code for the first interpreter, which is a 136-byte implementation of the interpreter followed by a built-in dictionary of 888 bytes.

https://github.com/nineties/planckforth/blob/main/planck.xxd

The first interpreter and language is so esoteric that, for example, the Hello World looks like this.

    $ ./planck
    kHtketkltkltkotk tkWtkotkrtkltkdtk!tk:k0-tQ

bootstrap.fs self-extends this. After running bootstrap.fs, the Hello World program can be written like this

    $ ./planck < bootstrap.fs
    ." Hello World!" cr

At the beginning of bootstrap.fs, you can only use one-letter words, so the code is difficult to read as shown below.

    $ head bootstrap.fs
    h@l@h@!h@C+h!k1k0-h@$k:k0-h@k1k0-+$h@C+h!ih@!h@C+h!kefh@!h@C+h!l!
    h@l@h@!h@C+h!k1k0-h@$k h@k1k0-+$h@C+h!ih@!h@C+h!kefh@!h@C+h!l!
    
    h@l@ h@!h@C+h! k1k0-h@$ k\h@k1k0-+$ h@C+h!
        i       h@!h@C+h!
        kkf     h@!h@C+h!
        kLf     h@!h@C+h!
        k:k0-   h@!h@C+h!
        k=f     h@!h@C+h!
        kJf     h@!h@C+h!

However, at the end, you can write a normal Forth program as follows. In this way, the interpreter and the language are extended during the execution of the single script.

    $ tail bootstrap.fs
            next-arg dup argv @ !
            included
        else
            ." Welcome to PlanckForth " version type
            ."  [" runtime type ." ]" cr
            copyright
            ." Type 'bye' to exit." cr
            s" /dev/tty" included
        then
    ; execute

Various functionalities like compile mode, immediate mode, control-flow structures, literals, variables, constants, file I/O, heap memory, etc. will be available after running bootstrap.fs.

Also, the language and bootstrap.fs are designed to be as environment-independent as possible, so you can implement the runtime in other languages as well. As an example, I added an implementation of the runtime in C and Python 3.

There is no space for detailed explanation here. So please read the following source code and comments.

https://github.com/nineties/planckforth/blob/main/bootstrap....

Also, there is a commentary article in Japanese here. I hope you can read it with google translate. (I will write it in English if I have time. someday.)

https://qiita.com/9_ties/items/349b2ed65b7cd8a7d580

Feedbacks and pull requests such as runtime implementations in other languages are very welcome.

Thank you for reading.