A method of operating a data processing system comprises: processing data words and switching between contexts; assigning a context signature Sig to any pair formed of a data word and a context; reading, within a current context, a data record from a memory unit, the data record comprising a payload data word and a protection signature; providing, as a verification signature, the context signature Sig of the payload data word and the current context; checking the verification signature against the protection signature; and generating an error signal if the verification signature differs from the protection signature.