The coprocessor interface achieves loose synchronization between the two pipelines by exchanging tokens from one pipeline to the other. These tokens pass down queues between the pipelines and can carry additional information. In most cases the primary purpose of the queue is to carry information about the instruction being processed, or to inform one pipeline of events occurring in the other.
Tokens are generated whenever a coprocessor instruction passes out of a pipeline stage associated with a queue into the next stage. These tokens are picked up by the partner stage in the other pipeline, and used to enable the corresponding instruction in that stage to move on. The movement of coprocessor instructions down each pipeline is matched exactly by the movement of tokens along the various queues that connect the pipelines.
If a pipeline stage has no associated queue, the instruction contained within it moves on in the normal way. The coprocessor interface is data-driven rather than control-driven.