An apparatus for processing data includes signature generation circuitry 30, 32 for generating a signature value indicative of the current state of the apparatus in dependence upon a sequence of immediately preceding return addresses generating during execution of a stream of program instructions to reach that state of the apparatus. Prefetch circuitry 10 performs one or more prefetch operations in dependence upon the signature value that is generated. The signature value may be generated by a hashing operation (such as an XOR) performed upon return addresses stored within a return address stack 28.