RejZoR
2
I’m glad you are happy with avast!. It’s a very good antivirus indeed. About detection… its basically the same thing. If you open EICAR.COM you will see it contains the same text as the one you manually copied. So the whole thing is to detect specific pattern inside some file. It gets a bit harder for packed ones (compressed) which have to be decompressed first and then scanned.
I’m not sure if avast! disassembles anything,but then this would be a heuristic method if you want to break the file into logical units for inspection. This part can be better explaind by Alwil team since they are experts in this field.