It does not treat it as an illegal instruction. It keeps running their code just fine. This is how Imps fight. The most basic imp is defined as mov 0 1 It

It does not treat it as an illegal instruction. It keeps running their code just fine. This is how Imps fight. The most basic imp is defined as mov 0 1 It copies itself to one instruction ahead of itself. On the off chance it hits the opponents pointer, the opponent becomes an imp too and they both just circle around the core forever. See elwin's comment below on vampires (for now, hopefully above soon).