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The externals

Values of VHDL signals that are shared in two or more partitions are communicated by means of messages. Several types of messages exist, depending on the simulation algorithm. Normal messages communicate the occurrence of an event on a signal; conservative simulation uses null-messages to avoid or resolve deadlocks [6][1]; optimistic simulation uses anti-messages to discard incorrect messages [2]. All distributed algorithms need extra messages to update the global simulation time, needed for time-ordered I/O and other tasks that are specific to different simulation algorithms.

Special tables have to be generated by the partitioner for each partition. One table containing all incoming signals used in the partition but driven on other ones, and another table containing all outgoing signals driven in the partition and used somewhere else.