https://github.com/judofyr/rcpu
It's a DCPU assembler (written in a Ruby DSL) and an emulator/debugger (which allows you to step/run through the execution).
Next up I'm going to implement library support and write some I/O functions. Much of the code is already in place though (e.g. https://github.com/judofyr/rcpu/blob/master/examples/screen.... which maps 0x8000-0x8400 to the terminal).