As far as dissassemblers, this is the one I have used in the past for Intel based products. Mine was in DOS, but the new one has a GUI and a looks to be windows based...
http://www.hex-rays.com/products/ida/index.shtml Did a great job on creating intel binary files to intel assembly language. However you are left with reams of paper with raw assembly code. which you then have to decipher the calls to various output devices. (Keyboard, Serial Port, Video Screen, LPT1, etc...) Lots of time consuming, tedious work.
As far as taking the next step and going from assembly to a higher level language, (C, BASIC, PASCAL, etc..) I've tried a few,but none were very reliable, and left a lot of guess work..
What kind of processor does the Northern use?