RTEMS Ada User's Guide
A tightly-coupled system is a multiprocessor configuration in which the processors communicate solely via shared global memory. The MPCI can simply place the RTEMS packets in the shared memory space. The two primary considerations when designing an MPCI for a tightly-coupled system are data consistency and informing another node of a packet.
The data consistency problem may be solved using atomic "test and set" operations to provide a "lock" in the shared memory. It is important to minimize the length of time any particular processor locks a shared data structure.
The problem of informing another node of a packet can be addressed using one of two techniques. The first technique is to use an interprocessor interrupt capability to cause an interrupt on the receiving node. This technique requires that special support hardware be provided by either the processor itself or the target platform. The second technique is to have a node poll for arrival of packets. The drawback to this technique is the overhead associated with polling.
RTEMS Ada User's Guide
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