Introduction to Parallel Processing -part 2

A shared memory model is one in which processors communicate by reading and
writing locations in a shared memory that is equally accessible by all processors.
Each processor may have registers, buffers, caches, and local memory banks as
additional memory resources. A number of basic issues in the design of shared
memory systems have to be taken into consideration. These include access control,
synchronization, protection, and security. Access control determines which process
accesses are possible to which resources. Access control models make the required
check for every access request issued by the processors to the shared memory,
against the contents of the access control table. The latter contains flags that
determine the legality of each access attempt. If there are access attempts to
resources, then until the desired access is completed, all disallowed access attempts
and illegal processes are blocked. Requests from sharing processes may change the
contents of the access control table during execution. The flags of the access control
with the synchronization rules determine the system’s functionality. Synchronization
constraints limit the time of accesses from sharing processes to shared
resources. Appropriate synchronization ensures that the information flows properly
and ensures system functionality. Protection is a system feature that prevents processes
from making arbitrary access to resources belonging to other processes. Sharing
and protection are incompatible; sharing allows access, whereas protection
restricts it.