ARINC Specification 653 Part 4 was prepared to support controllers and relatively simple avionics. Part 4 is true subset of services specified in ARINC 653 Part 1. In the Subset Services, partition scheduling is restricted to only one partition time window within the partition's period. This simplifies the management of process release points. Process management uses a dual-process model with at most two processes within a partition, for example, foreground and background.