Task handling apparatus for a computer system

Task handling apparatus in a computer system is structured to be common to system control tasks, user tasks and I/O tasks. Although the task handling apparatus contains a task priority structure, all tasks are handled in the same manner, and there are no fixed interrupt levels for I/O tasks. There are N levels of priority, and N is variable. Each task is a server for a functional request. Task dispatching elements (TDE's) are enqueued in priority sequence on a task dispatching queue (TDQ). A task dispatcher functions to dispatch the highest priority TDE on the TDQ, if any, and to perform task switching. Intertask communication is accomplished by send message, send count, receive message and receive count mechanisms, and is coupled with task synchronization. Task synchronization is achieved by dequeueing and enqueueing TDE's on the TDQ. An active task becomes inactive dispatchable when a higher priority TDE is enqueued on the TDQ by send message or send count mechanisms. The active task becomes inactive waiting when a receive message or receive count is not satisfied. An inactive dispatchable task becomes the active task when it becomes the highest priority enqueued TDE on the TDQ by the receive message or receive count mechanisms. An inactive waiting task becomes either the active task or an inactive dispatchable task after being enqueued on the TDQ by the send message or send count mechanisms, depending upon whether it is the highest or other than the highest priority TDE on the TDQ.