1. In emulation of a target system on a host system in which one or more components of the target system are emulated by translation of target system code, a method for invalidating blocks of translated code, the method comprising: translating one or more blocks of target system code with the host system to produce one or more corresponding blocks of translated code; analyzing the one or more blocks of target system code for the presence of one or more native target system instructions indicative of modification of the target system code during execution; and if the one or more native target system instructions are present, marking some or all of the one or more blocks of translated code as potentially invalid; and re-translating one or more blocks of target system code corresponding to one or more blocks of translated code that have been marked as potentially invalid.