Abstract: A parallel multiprocessor data processing system having a plurality of nodes for processing data and a switch connected to each of said nodes for switching messages between the nodes, each node having a node processor for defining messages under program control to be sent to another node. Each of the nodes has an I/O processor for controlling the sending of messages to another node via the switch, and a shared memory which can be accessed by both the node processor and the I/O processor. Instructions for the messages to be sent by the I/O processor are stored in mailboxes in the shared memory by the node processor. A comparing circuit compares addresses on the bus to the contents of a plurality of address registers and sets the corresponding bit in a results register for each match. The adapter processor reads the contents of the results register such that the adapter processor may, with a single bus access, determine all mailboxes that have been accessed by the node processor.