Abstract: A Verification environment, comprising a testbench and a test harness, which is used to automatically verify the operation of a processor device as described by a hardware description language (HDL) against the desired operation as specified by the instruction set architecture (ISA). Also described is a method of generating test instructions for use in such a system, in which the verification environment selects an instruction from the processor specification in accordance with one or more first constraints, then configures and encodes this instruction in accordance with one or more second constraints.