I have a question,
is it possible for multiple processor machine to access data from RAM ( single ram system ) ? for eg machine has 2 processors p1, p2 which are executing in parallel , is it possible that they access same ram for read and write ( ofcos write is not on same location )
i understand that in multi core machines it will not be possible since data bus is shared.
As long as the RAM is mapped to all cores or processors (such as in a multi-threaded application) it may be accessed from any core or processor.
There is no difference if you're discussing single processor/single core, single processor/multi-core, multi-processor/(each with a) single-core or multi-processor/multi-core. Since they have no system RAM of their own - the RAM in caches is not system RAM - the only RAM available to all of them is the system RAM.
The only difference between multi-processor/single core (as in older systems) and single processor/multi-core (modern systems) is that the former needs to coordinate RAM accesses with off-chip logic whereas for the latter all coordination is on-chip and sometimes even on-die which of course results in much faster and more electronically efficient RAM accesses.
In the case of AMD's multi-processor/multi-core solutions each processor owns part of the system RAM. The processors themselves are interconnected with high-speed data (HyperTransport) channels to facilitate accesses to RAM not owned by the processor accessing it.
In any case it is up to the programmer to decide how the processors/cores access the RAM. Sure they can read and/or write to the same location if that is what the programmer wants.