System Unit
The main part, processing unit and devices, of a microcomputer is a system unit. A system unit includes a board called a motherboard that holds a microprocessor chip (or a CPU), memory chips, and expansion slots. Electronic circuitry is printed on the board and it connects between two main parts of a microcomputer, the microprocessor and primary storage and other parts. The system unit is housed within the system cabinet. A system unit includes the following parts:
Motherboard
Microprocessor
Memory Chips
System Clock
Buses
Ports
Expansion Slots and CardsMotherboard

A Motherboard or system board is the main printed, flat circuit board in an electronic device such as microcomputers. The board contains expansion slots (sockets) that accept additional boards (expansion Cards). In a microcomputer, the motherboard contains the microprocessor, the primary storage chips (or main memory cards), the buses, and all the chips used for controlling the peripherals.