1. A processor that cyclically executes a plurality of tasks in accordance with an execution time period allotted to each of the tasks, the processor comprising: a storage unit operable to store pieces of execution control information respectively corresponding to the tasks; a plurality of register groups; an execution unit operable to sequentially use one of the register groups to execute the tasks based on the execution control information held in the register group being used; a selection unit operable to select a task to be executed next among the tasks; a restoration unit operable to, while one of the tasks is being executed for an execution time period allotted thereto, restore a piece of the execution control information corresponding to the selected task into one of the register groups other than the register group being used; a detection unit operable to detect a suspension cause for the one of the tasks being executed; and a control unit operable to, (i) if the suspension cause is detected before the restoration starts, have the execution unit suspend the execution and execute one of the tasks based on a piece of the execution control information held in one of the register groups other than the register group having been used for the execution, and (ii) if the register group holding the piece of the execution control information corresponding to the selected task is the register group the restoration into which is to be started, have the restoration unit cancel the restoration.