Controlling tasks includes: receiving ordering information that specifies at least a partial ordering among a plurality of tasks; and generating instructions for performing at least some of the tasks based at least in part on the ordering information. Instructions are stored for executing a first subroutine corresponding to a first task, including a first control section that controls execution of at least a second subroutine corresponding to a second task, the first control section including a function configured to change state information associated with the second task, and to determine whether or not to initiate execution of the second subroutine based on the changed state information. Instructions are stored for executing the second subroutine, including a task section for performing the second task and a second control section that controls execution of a third subroutine corresponding to a third task.