I was tutoring a grade 4 student about CPU and I got confused as there are also the other units of a cpu like arithmetic and logic unit and memory unit. So, is the above statement actually true even if ALU is the one which does the arithmetic and logical operations?
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4$\begingroup$ This is going to come down to what you mean by "control". I'm not sure it's really answerable in its current form. $\endgroup$– David RicherbyCommented Jan 8, 2019 at 18:23
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$\begingroup$ By definition, a control unit controls. Right ? en.wikipedia.org/wiki/Control_unit $\endgroup$– user16034Commented Jul 23, 2022 at 15:10
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3 Answers
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Well true, sort of.
In simple terms: The control unit, regardless of how it is implemented, directs the activities of all of the other processor units.
However: It does so by decoding the instructions fetched from memory, the program.
So ultimately the program controls what gets done.
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Oh well. To be honest, I haven't heard any part of a computer being called a general "control unit" for a while. There are parts like a "power control unit" which controls power, your car may have an "engine control unit", etc., and there are wikipedia articles about a "control unit", but you won't have a control unit in a modern computer.
Your computer has a CPU (Central Processing Unit) which will execute instructions. The CPU will be divided into several units: A unit controlling memory, a unit controlling cache memory, a unit reading and decoding instructions and distributing the instructions to usually several execution units, and so on. Certain units will be connected to certain other ones, instructing one unit what to do and taking instructions from some other unit, but there is no unit having overall control.
So right now, with a current non-trivial CPU, I'd say there is no control unit controlling overall operations of a computer.
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A short answer would be – yes, you can say that the control unit is responsible for the overall operation of the CPU.
More specifically, it controls the retrieval of instruction from the primary memory as well as the sequence of their execution. It is responsible for providing the ALU with the data that needs to be processed as well as the instructions on how the data should be processed. So in the machine instruction cycle (fetch-execute cycle), the CU is responsible for loading new commands and instructions from the primary memory (RAM), decoding these commands, and then later storing them back in the primary memory (through buses). So even though the ALU actually executes the instructions while carrying out the arithmetic and logical calculations, I guess it's safe to say that the overall control of the CPU is done by the control unit.
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1$\begingroup$ Well, if I look at the micro architecture of any newish CPU, I find it very, very difficult to find anything I would call a control unit. Look at a new Intel or ARM chip. Can you find a control unit? $\endgroup$ Commented Jun 29, 2021 at 19:17