System and method for communicating between a plurality of processors

A communication protocol for direct communi- cations between operating system kernels of a plurality of processors in a clustered-type, shared virtual memory, multi-processor data processing system is disclosed. Each processor unit (10) includes a novel intermachine communications software system ("IMCS") which may be used by a trusted kernel of the operating system of the processor, such as the page fault handler, to communicate with a service in another processor, either directly or through an intermediate processor. Communications are initiated when a user prepares a message in accordance with pre-programmed instructions from the IMCS in a standard format which contains both a data field and a header containing specific control information. Once the message is prepared, the user calls IMCS which selects a route for the transmission of the message over a serial communications link (12) to the receiving processor using a "request to send immediate" serial link level protocol. The message is initially directed to a common buffer pool previously assigned to IMCS by the various services in the receiving processor. IMCS calls special software to read the header of the message to determine the identity of the destined server (42) and to determine if the buffer in the buffer pool assigned to the called service has available buffer space to accept the message. If not, IMCS directs the sending of a negative acknowledgment to the sending processor. If sufficient buffer space is available, IMCS directs the sending of an acknowledgement to the sending processor. Upon receipt of the acknowledgement by the sending processor, IMCS calls a notification routine to inform the user of the status of the transmitted message.