M0 macro assembly, it is implemented here: http://git.savannah.nongnu.org/cgit/stage0.git/tree/stage1/M... Which is Built in Hex2 which is implemented here: http://git.savannah.nongnu.org/cgit/stage0.git/tree/stage1/s... Which is built in Hex1 which is implemented here: http://git.savannah.nongnu.org/cgit/stage0.git/tree/stage1/s... Which is self-hosting and implemented here: http://git.savannah.nongnu.org/cgit/stage0.git/tree/stage1/s...

And was written using the bare metal text editor found here: http://git.savannah.nongnu.org/cgit/stage0.git/tree/stage1/S...