1. Register file circuitry comprising: a plurality of physical registers; a plurality of tag storing portions, corresponding respectively to the physical registers, each for storing a tag representing a logical register ID allocated to the corresponding physical register; a register selection unit which receives a logical register ID and selects that one of the physical registers whose said tag matches the received logical register ID; and a tag changing unit which changes the stored tags by transferring the tag stored in at least one tag storing portion into another one of said tag storing portions, thereby to change a mapping between at least one logical register ID and one of said physical registers.