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 at least N+1 identifier selection bits, wherein a position of a first marker bit in the at least N+1 identifier selection bits determines M, and an identifier selection unit configured to determine the 2M selected identifiers. The 2M selected identifiers fall within a range defined by a base identifier and a ceiling identifier. N-M bits of the N+1 identifier selection bits form N-M bits of the base identifier, and M zeroes form a further M bits of the base identifier. The ceiling identifier corresponds to the base identifier, except that the M zeroes of the base identifier are replaced by M ones.