[api]

Some of the first polymorphic viruses had very advanced code translation engines, basically JITs before they were common. I wonder if any JIT concepts were invented there?

One example is the DAME which stands for Dark Avenger Mutation Engine. It could recompile DOS viruses into equivalent but different machine code on the fly, including the mutation engine itself.

[ddingus]

Link? That is super interesting to me.

[WaxProlix]

Google yields this http://virus.wikidot.com/dark-avenger-mutation-engine

But it sounds a bit more prosaic than OP made it out to be (maybe it's incomplete?)

[hacdaddy]

Also check out season 1 of https://malicious.life/. S1E02 specifically covers The Dark Avenger, but the whole of S1 was awesome (and he rest too. But S1 especially).

[api]

Maybe I'm confusing DAME with later mutation engines that had more elaborate ways of transforming the decryptor and payload code. I know there were some pretty sophisticated JIT-type things in the latter part of the DOS virus era.