The system unit of a personal computer typically contains all of the following except

A modem for connecting to phone or network lines is usually a separate add-on, not a standard part of the system unit.

The microprocessor sits on the motherboard inside the system unit as its core.

A disk controller managing the drives is housed within the system unit.

A serial interface port is a built in part of the system unit.

The system unit gathers the core processing and control hardware in one case.

Add-on: internal modem cards do exist, but the modem is the item least tied to the basic system unit.