Source : Free On-Line Dictionary of Computing

shared memory
     
        1. Memory in a {parallel computer}, usually {RAM}, which can
        be accessed by more than one processor, usually via a shared
        {bus} or network.
     
        It usually takes longer for a processor to access shared
        memory than to access its own private memory because of
        contention for the processor-to-memory connections and because
        of other overheads associated with ensuring synchronised
        access.  Computers using shared memory usually have some kind
        of local {cache} on each processor to reduce the number of
        accesses to shared memory.  This requires a {cache
        consistency} {protocol} to ensure that one processor's cached
        copy of a shared memory location is invalidated when another
        processor writes to that location.
     
        The alternative to shared memory is {message passing} where
        all memory is private to some particular processor and
        processors communicate by sending messages down special
        links.  This is usually slower than shared memory but it
        avoids the problems of contention for memory and can be
        implemented more cheaply.
     
        2. Memory which can be access by more than one process in a
        {multitasking} {operating system} with memory protection.
        Some {Unix} variants, e.g. {SunOS} provide this kind of shared
        memory.
     
        {Unix manual pages}: shmop(2), shmctl(2), shmget(2).
     
        (1994-10-20)