1. A system for controlling a key storage unit in a data processing apparatus, comprising: a main storage unit having at least one block; a key storage unit for storing a main storage protection key, at least one reference bit, and at least one change bit for each block of said main storage unit; a processing unit, said processing unit producing at least one access request for said main storage unit; a main storage control unit, operatively connected to said main storage unit and said processing unit, for processing the access request for said main storage unit and producing a key storage access request for said key storage unit, said key storage access request having address data and requesting a change in the reference bit and the change bit in said key storage unit, the reference bit and the change bit corresponding to the accessed block of said main storage, said main storage control unit supplying said key storage access request for said key storage unit every time said main storage control unit accesses said main storage unit in response to the access request for said main storage unit supplied from said processing unit; a key storage control unit, operatively connected to said processing unit and said main storage control unit, for controlling access to said key storage unit, comprising: a key queue, operatively connected to said main storage control unit and said key storage unit, for temporarily holding a plurality of said key storage access requests supplied from said main storage control unit, said key storage control unit registering each of said key storage access requests supplied from said main storage control unit into said key queue in order, and said key storage control unit, independent of the above-mentioned registering process, reading and processing each of said key storage access requests registered in said key queue in order; comparing means, operatively connected to said key queue and said main storage control unit, for comparing the content of said key storage access request supplied from said main storage control unit with the content of each key storage access request registered in said key queue; and inhibiting means, operatively connected to said comparing means and said key queue, for inhibiting said key storage access request supplied from said main storage control unit from being registered in said key queue when the content of said key storage access request supplied from said main storage control unit is equal to the content of one of said key storage access requests registered in said key queue.