Q. What do you mean by Shared Memory?

In shared memory scheme more focus is on controlling parallelism instead of data parallelism. In this model numerous processes run independently on numerous processors but they share a common address space available to all as displayed in Figure.

Figure: Shared memory

The processors communicate with one another by one processor writing data into a location in memory and other processor reading the data. Any change in memory location effected by one processor is visible to all other processors. As shared data is able to be accessed by more than one process at the same time, some control techniques like semaphores/locks must be devised to make sure mutual exclusion. This model is frequently referred to as SMP (Symmetric Multi Processors) named so since a common symmetric operation is used for various processors of the similar type to access the similar shared memory.  Some multi-processor systems employ a shared-memory programming model illustrations comprise: DELL PowerEdge 8450, NEC SX-5, SGI Power Onyx/ Origin 2000; Hewlett-Packard V2600/HyperPlex; SUN HPC 10000 400 MHz.