1) Distributed Operating systems are also referred to as Loosely Coupled systems whereas parallel processin g systems are referred to as tightly coupled systems.
2) A Loosley coupled system is one in which the processors do not share memory and each processor has its own local memory whereas in a tightly coupled system there is a single systemwide primary memory shared by
all the processors.
3) The processors of distributed operating systems can be placed far away from each other to cover a wider geographic area which is not the case with parallel processing systems.
4) The no. of processors that can be usefully deployed is very small in a parallel processing operating system whereas for a ditributed operating system a larger no. of processors can be usefully deployed.......
5)globle clock is used for controlling simd n mimd in parallel..... .in distributed no any global colck present in this synchronization algorithms are used
6)in the distributed operating system there is an unpredictable communication delays between processors whereas the processors in the parallel processing system share over an interconnection network