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

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.

System Clock

The clock is a device that generates periodic, accurately spaced signals used for several purposes such as regulation of the operations of a processor or generation of interrupts. The clock circuit uses the fixed vibrations generated from a quartz crystal to deliver a steady stream of pulses to the processor. The system clock controls the speed of all the operations within a computer.
The clock speed is the internal speed of a computer. The clock speed is expressed in megahertzes (MHz). 33 MHz means 33 million cycles per second. A computer processor's speed is faster if it has higher clock speed. For example, a 100-Mhz processor is four times as fast internally as the same processor running at 25MHz.