It is defined the component which is control the system. It consists of router, bridge, switch, hub, repeater. All the components are acted at the same time.
A system unit is the main component of a computer that houses the motherboard, processor, memory, and other essential hardware components. It does not include peripherals such as the monitor, keyboard, or mouse.
There is a wide difference between the internal system unit and the external system unit. The internal system unit is usually mounted on the motherboard inside the computer while the external system unit is independent of the computer like an external drive.
The basic component inside the system unit is the Central Processing Unit.
A system unit, also known as a S.U., is the main unit of a personal computer, typically consisting of a metal or plastic enclosure containing the motherboard, power supply, cooling fans, internal disk drives, and the memory modules and expansion cards that are plugged into the motherboard.
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It sounds like you are asking two different questions. See the links below for "What is the definition of system unit?" and "What is the definition of a motherboard?".
The ALU of the CPU
In the SI measurement system, a meter is a "base unit". However, the definition relies on the prior definition of the second.
The ALU of the CPU
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The external part is the part of system unit which can be seen, include the system case and the power button. And the internal part refers to the things inside the system unit example the motherboard, the expansion slot and disk driver