Generalized queues and specialized registers associated with the
generalized queues are disclosed for coordinating the passing of
information between two tightly coupled processors. The capacity of the
queues can be adjusted to match the current environment, with no limit on
the size of the entry as agreed upon between the sending and receiving
processors, and with no practical limit on the number of entries or
restrictions on where the entries appear. In addition, the specialized
registers allow for immediate notifications of queue and other
conditions, selectivity in receiving and generating conditions, and the
ability to combine data transfer and particular condition notifications
in the same attention register.