The processor can be viewed as integration of two things : a data path and a control unit.
The data path includes the ALU, registers including program counter, memories (memory bus paths), and multiplexers used in processing elements of the processor. All these components of datapath perform various operations depending upon the instruction encountered. The coordination among these components is the task of the control unit.
When we call the ALU as heart of the processor, we can see the control unit as the 'brain' of processor. Control unit performs the major functionality in the processor, and is perhaps the most complex part of any processor. The control unit is also termed as decode unit since it decodes the instruction. The instruction fetched from memory (contained in the Instruction Register), is fed to the control unit and appropriate control signals are generated by it, to execute that instruction. These control signals are fed to various parts of datapath as their control signals. e.g. ALU needs the alu-opcode, multiplexers need select lines etc.
Some components of datapath may feed-back certain status signals to the control unit, using which some conditional instruction might be executed.
Design of the datapath is concerned with the processor organization. A good datapath is the one through which data signal takes minimum end-to-end path for execution, giving highest possible speed.
Also, the control unit which is almost biggest part (in terms of logic) of processor has to be designed carefully to get best performance.
Let's further explore these concepts while analysing an example processor.
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