The next instruction to be fetched is located in main memory. But in case of virtual memory system, it may be either in main memory or secondary memory (disk). In most cases, the next instruction to be fetched immediately follow the current instruction. In those cases, there is no explicit reference to the next instruction.When an explicit reference is needed, then the main memory or virtual memory address must be given.
Source and result operands can be in one of the three areas: