The following is a 12-character text kernel for the Atari 2600 that, although flickery, is usable.
I am uncertain about the identity of the original author of this kernel, but it is utilized in the following projects:
- https://github.com/tdididit/a26-flashcart
- https://github.com/robinhedwards/UnoCart-2600
Please find a screenshot of the kernel here: 