Direction/Sign Extend bit in the encoding of an x86 opcode

[Comment made into answer].

You obviously need a boolean formula over the stream of instruction bytes. I wouldn't know how to define that formula easily; the x86 has a really messy instruction set. I'd expect the key trick is to lookup the opcode byte in a table determined by the prefix bytes. If you are writing some kind of disassembler, I'd expect you to have such tables already anyway.