MESSAGE PASSING SYSTEM o two operations ·send ·receive o message size ·fixed 。variable o communication link ·physical o shared memory o hardware bus o network ·logical o direct/indirect o synchronous/asynchronous o automatic/explicit bufferingMESSAGE PASSING SYSTEM  two operations  send  receive  message size  fixed  variable  communication link  physical  shared memory  hardware bus  network  logical  direct/indirect  synchronous/asynchronous  automatic/explicit buffering