Hi,
Atalan is high level language supporting rule based translation (compilation). That means, that in theory, you can easily (relatively easily) implement support for new CPUs or improve optimizations or add support for special operations, hardware etc.
The web site has not been updated for a long time, as I am currently rewriting almost all compiler code to improve it and to support some more features.
That's why all commits also go o special branch in the repository, so seemingly nothing has been going on with Atalan for some time.
That is fortunately not true.
Unfortunately at the present moment, I can not give you any other advice than to stay away from Atalan, as there are many errors and I am not able/willing to fix the old version of code. As soon as I feel Atalan is able to do something useful again, I will certainly anounce it on all major 6502 forums