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If you have learnt high level language(such as C++, Java), you may think that the executions of instruction is simple. This is not the turth.

Actually, a single statement in high-level language can be hurdreds of operations to be done by the CPU.

Take "a = b + c;", such a simple addition, as an example:
The CPU need to:

• Load the content of memory cell associate with B to the Accumulator
• Add the content of memory cell associate with C to the Accumulator
• Store the content of accumulator to memory cell associate with A

You may once again think that the 3 operations above is simple.
Actually, to load the content of B to Accumulator, the CPU already needs to:

• Transfer the content of PC to MAR
• Decode the address held by MAR
• Transfer the instruction to MDR
• Instruction are carried to IR by MDR
• Increment of PC
• Decode the Instruction in IR by Instruction Decoder
• Operand part of Instruction is transfered to MAR
• Decode the Address held by MAR
• Transfer the content to MDR
• MDR carries the content to Accumulator

You can now see how complicate it is.