A data processing apparatus is provided, comprising plural processing units configured to execute plural processes, a storage unit configured to store data required for the plural processes; and a protection unit configured to control access by the plural processes to the storage unit. The protection unit is configured to define an allocated access region of the storage unit for each process of the plural processes, wherein the protection unit is configured to deny access for each the process outside the allocated access region and wherein allocated access regions are defined to be non-overlapping. The protection unit is configured to define each allocated access region as a contiguous portion of the storage unit between a lower region limit and an upper region limit, and the protection unit is configured such that when the lower region limit is modified the lower region limit cannot be decreased and such that when the upper region limit is modified the upper region limit cannot be decreased.