A data processing apparatus is provided which is configured to select 2M selected identifiers within a possible range of up to 2N identifiers, where M≦N. The data processing apparatus comprises a selection storage unit configured to store N+1 identifier selection bits, wherein a position of a marker bit in the N+1 identifier selection bits determines M, and an identifier selection unit configured to determine the 2M selected identifiers. The 2M selected identifiers are defined by a base identifier and 2M−1 identifiers incrementally following that base identifier. N−M bits of the N+1 identifier selection bits form N−M most significant bits of the base identifier, and M trailing zeroes form the M least significant bits of the base identifier.