It is a Forth (why Forth?) operating system and a collection of tools and documentation with a single purpose: preserve the ability to program microcontrollers through civilizational collapse. Found via Michael Schierl's port https://github.com/schierlm/collapseos-uxn

Main page is http://cowlark.com/cowgol/
It used to support the Apollo Guidance Computer used in the Apollo spacecraft, but I had to remove the code generator while rewriting the compiler and I haven’t reworked the AGC backend.
http://cowlark.com/2019-07-20-cowgol-agc/index.html
The author also ported the Fuzix operating system to the Raspberry π pico.
http://cowlark.com/2021-02-16-fuzix-pi-pico/index.html

His tool https://github.com/davidgiven/narcissus to remap keys might be interesting too.
So is his Forth system http://cowlark.com/fforth/

FlashForth is a Forth stamp system implemented on the Microchip 8-bit PIC18F and 16-bit PIC24, 30, 33 and the Atmel Atmega microcontroller families.

FlashForth also works on the popular Arduino UNO and MEGA boards.

FF (FlashForth) allows you to write and debug complex real-time applications. The complete system including the compiler is executing on the microcontroller.

A Forth interpreter, compiler, assembler, multitasker and user definable interrupts are provided.

A computer with a terminal emulator is used for communicating with FF via a serial or USB link.

Asynchronous, multi (40!) core Forth-bases processor with fast ADC and DAC interfaces, might be interesting for software radio applications.

Just a backup of the very nice Richard W.M. Jones Forth interpreter. On Eric Raymond's blog http://esr.ibiblio.org/?p=6839#more-6839 somebody recommended reading jonesforth.{S,f}