TForth

I once discovered this abandoned DOS Forth on my own home page, and noticed that it (after a few bug fixes) wasn't all that bad. It's a direct threaded Forth, with its kernel of less than 4kB (or with everything compiled, about 9kB), it's a pretty small and non-bloated system. I got a lot of inspiration from isForth, but TForth is smaller and is NOT compatible with any other Forth (which, by the way, is a proud tradition within the Forth community). Now I have added support for hashed dictionaries (which makes it quite a lot faster, about 30%), and converted to FASM (which is about 3 times faster than NASM). Together these changes reduce the build time from about 45 to 15 seconds on my 386. TForth is now also tested in my 286 laptop, and it runs fine! Unfortunately FASM requires a 386, so it can't be completely built from scratch on a 286.

Features

Small size, between 4kB and 10kB depending on what you compile
Comes with several example programs, such as a Langton's Ant implementation, a prime number calculator (which uses the most interesting way of calculating ln(x) ever implemented in Forth, I think) and a BrainFuck compiler which allows you to inline BrainFuck code in your TForth programs
Has documentation
Features hash tables for dictionaries (this makes it about 30% faster on my 386, and the mere fat that I list this in the feature list ought to tell you something about the Forth community)
Exception handling

To do

Add internal file buffering (could probably improve the compilation time significantly)
Separate code and data (could create a large speedup on modern chips, and also allow slightly larger programs without things getting too inconvenient
Add a better line editor

Getting started

First of all, read the documentation. But if you just want to give it a try, here is how you run the demos (prime.f, ant.f):

Start tf.com (if you don't have this file, run build.bat to build it)
Type: fload demo\ant.f
Type: use ant
Now the words in ant (or prime, if that's what you loaded) are ready for use. Just type the name of the word you want to use and press enter. The ant.f program uses "run" to start the demonstration.

Downloads

TForth (latest version)
TForth (older version)