MPI is the new standard which defines a set of message passing operations for multicomputers and clustered systems. In comparison to other popular message passing systems, MPI provides a richer collection of functions, allowing efficient implementations, portability and excellent support for the development of parallel libraries. In this paper, we describe the implementation and performance of MPI on the Fujitsu AP1000 multicomputer. To produce an efficient implementation, the operating system on the AP1000 had to be modified to better support MPI. These modifications are presented, along with the hardware operations that were utilised. A selective broadcast operation was developed from the modifications which allowed very efficient group-wide broadcast. The performance of the implementation in comparison to native AP1000 calls is presented with benchmarks of the collective routines implemented using the selective broadcast operation. 1 The MPI Standard The message passing paradigm h...