Parameter storage management

An apparatus for processing data 2 contains multiple power domains which may be in a non-retaining power state or a retaining power state. If a power domain is in a non-retaining power state in which it is not able to retain a copy of a stored parameter value and it is switched into a retaining power state in which it requires a copy of that parameter value, then it fetches the parameter value from a store within another power domain. One of the power domains contains a master copy of the parameter value to which writes changing in the parameter value are made. At least one of the other power domains fetches a copy of the parameter value if required from a power domain other than the power domain containing the master copy.