a data processing system employing registers for holding data to be processed is described . the registers are grouped into sets of registers 2 . a register selecting instruction word controls selection of a particular ones of the registers from within the sets of registers 2 to be available for data processing . the register selecting instruction word includes control fields for each set of registers , the bits of each control field specifying a register swap within the a respective set of register . the new register to be selected is derived from an exclusive or operation performed upon a currently selected register identification word and the control field for that set of registers .