Technique for maintaining data values in a prediction register that are used to predict whether a branch represented by a branch instruction will be taken

Circuitry comprises a prediction register storing a plurality of entries each having respective data values for association with one or more branch instructions; prediction circuitry to detect, using prediction data derived by a mapping function from the stored data values associated with a given branch instruction, whether or not a branch represented by the given branch instruction is predicted to be taken; update circuitry to modify the stored data values associated with the given branch instruction in dependence upon a resolution of whether the branch represented by the given branch instruction is taken or not; and control circuitry configured to selectively alter one or more of the data values other than data values associated with the given branch instruction.