System for restructuring input/output control system

A system for restructuring an input/output control system having a first (10) and a second (20) input/output controllers each of which respectively connects a first (15) or a second (25) group of peripheral devices in a lower level and a data processing apparatus in an upper level, and controls data transfer operation between each of the corresponding group of peripheral devices and the data processing apparatus, wherein each of the first (15) and second (25) group of peripheral devices is also connected to the input/output controller corresponding to the other group of peripheral devices (25,15) so as to realize a cross call function. Each of the first and second input/output controllers (10,20) respectively holds its own control data, which is used for controlling a data transfer operation between the corresponding peripheral devices (15,25) and the data processing apparatus. When a situation which requires one of the input/output controllers to be in an off-line state arises, that fact is notified to the other input/output controller and the control data held in the one input/output controller is transferred to the other input/output controller. The other input/output controller receives the transferred conrol data and restructures the control data so that the other input/output controller can control the data transfer operation between the data processing apparatus, and all the peripheral devices which were originally under its control and all the devices originally under the control of the one input/output controller wherein the above situation has arisen.