An input/output store instruction is handled. A data processing system includes a system nest communicatively coupled to at least one input/output bus by an input/output bus controller. The data processing system further includes at least a data processing unit including a core, system firmware and an asynchronous core-nest interface. The data processing unit is communicatively coupled to the system nest via an aggregation buffer. The system nest is configured to asynchronously load from and/or store data to an external device which is communicatively coupled to the input/output bus. The data processing unit is configured to complete the input/output store instruction before an execution of the input/output store instruction in the system nest is completed.