Processor Support for Message Passing

• Processors that support message passing are those processors that contain the special instructions needed to support interprocess message communications. A number of features are required:
  1. A port is a communication channel. It is a reference object for tasks and threads. Two main operations can be performed on ports: send and receive.
  2. Messages are used as communication among objects. A message is divided into a header and a body. The size of the body is variable while that of the header is fixed. A message holds information exchanged between processes.
  3. Port sets: A task can hold multiple access rights (send and receive) on ports.
     • Multiple tasks can hold send access to a single port.
     • On the other hand, one task can hold receive access at a given time.
     • In port set, a task can have either all or none of the access rights to a group of ports. Ports must be mutually exclusive in the sense that a port cannot be in two different sets at a given time.
• The Intel iPAX 432 uses message passing communications and supports them directly. It also uses port objects that work as a competitor to the path of the message. The processor contains a message queue. A message communication can be arranged depending on the following:
  • Time of arrival (such as the ``first-in-first-out'', FIFO);
  • Priority;
  • Deadline within priority.
• The IBM AS/400 supports message passing by having an event object type that contains a field supporting the contents of the message. This field is called the event data field. AS/400 processor operations are send and receive.