After starting off with a 'toy' CPU like this one at FOSDEM, progress to the next level by using an emulator for a 'real' 8-bit CPU like (say) the 8080 or the Z

After starting off with a 'toy' CPU like this one at FOSDEM, progress to the next level by using an emulator for a 'real' 8-bit CPU like (say) the 8080 or the Z80, and write real code and use real assemblers for real 8-bit programs running on real 8-bit Operating Systems like CP/M. It really is simple enough, and lots of fun.