1. A fix-up code generation subsystem comprising: A. a fix-up code generator module configured to, in response to detection of an exception condition of a predetermined type during processing of at least one instruction in an instruction stream, generate fix-up code for said at least one instruction, the fix up code being such as to avoid the exception condition, and B. a fix-up code substitution module configured to substitute said fix-up code in said instruction stream for said at least one instruction.