CPU parts, the Control Unit and ALU, what are those 2 parts of the cpu responsible for?
The Central Processing Unit (CPU) has 2 parts, the Control Unit(CU) and the Arithmetic Logic Unit (ALU), what are those 2 parts of the CPU really responsible for? If anyone can give a detailed description and definition as to what they are responsible for in the CPU it would be appreciated thanks.
The arithmetic logic unit (ALU) is a digital circuit that calculates an arithmetic operation (addition, subtraction, etc.) and logic operations (Exclusive Or, AND, etc.) between two numbers. The ALU is a fundamental building block of the central processing unit of a computer.
Many types of electronic circuits need to perform some type of arithmetic operation, so even the circuit inside a digital watch will have a tiny ALU that keeps adding 1 to the current time, and keeps checking if it should beep the timer, etc...
By far, the most complex electronic circuits are those that are built inside the chip of modern microprocessors like the Pentium. Therefore, these processors have inside them a powerful and very complex ALU. In fact, a modern microprocessor (or mainframe) may have multiple cores, each core with multiple execution units, each with multiple ALUs.
Many other circuits may contain ALUs inside: GPUs like the ones in NVidia and ATI graphic cards, FPUs like the old 80387 co-processor, and digital signal processor like the ones found in Sound Blaster sound cards, CD players and High-Definition TVs. All of these have several powerful and complex ALUs inside.
A control unit is the part of a CPU or other device that directs its operation. The outputs of the unit control the activity of the rest of the device. A control unit can be thought of as a finite state machine.
At one time control units for CPUs were ad-hoc logic, and they were difficult to design. Now they are often implemented as a microprogram that is stored in a control store. Words of the microprogram are selected by a microsequencer and the bits from those words directly control the different parts of the device, including the registers, arithmetic and logic units, instruction registers, buses, and off-chip input/output. In modern computers, each of these subsystems may have its own subsidiary controller, with the control unit acting as a supervisor.
The control unit is the circuitry that controls the flow of information through the processor, and coordinates the activities of the other units within it. In a way, it is the "brain within the brain", as it controls what happens inside the processor, which in turn controls the rest of the PC.
The functions performed by the control unit vary greatly by the internal architecture of the CPU, since the control unit really implements this architecture. On a regular processor that executes x86 instructions natively, the control unit performs the tasks of fetching, decoding, managing execution and then storing results. On a processor with a RISC core the control unit has significantly more work to do. It manages the translation of x86 instructions to RISC micro-instructions, manages scheduling the micro-instructions between the various execution units, and juggles the output from these units to make sure they end up where they are supposed to go. On one of these processors the control unit may be broken into other units (such as a scheduling unit to handle scheduling and a retirement unit to deal with results coming from the pipeline) due to the complexity of the job it must perform.
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