The CIA (complex interface adapter) chips perform the majority of the 128’s input and output functions. Between them, the CIAs are responsible for handling communications with the keyboard, joysticks, the serial bus (where disk drives and printers are connected), the RS-232 port (where modems are connected), and the user port. In fact-with the exception of video output provided by the Vic and Vdc chips and the audio output provided by the Sid chip-the list of I/O functions performed by devices other than the CIAs is quite short: the Vic and Vdc chips handle light pen input for their respective displays, the Sid chip reads paddle controllers (although a CIA reads paddle buttons and selects which pair of paddles is to be read), the processor’s on-chip I/O port is used to control some aspects of tape data storage and to read the CAPS LOCK key, and an MMU register line is used to read the 40/80 DISPLAY key.
All CIA registers are set to zero when the system RESET line is pulled low, as when the reset button is pushed. Most of the CIA registers are initialized during the IOINIT routine $E109.
This CIA is used to read the keyboard, joysticks, and other devices connected to the control ports, such as the 128 mouse. It also selects which pair of paddles will be read. The timers and FLAG input line are used in reading from and writing to tape. The chip’s serial data communications hardware is used for fast serial bus I/O.
This CIA is used to support the serial bus, and to provide the RS-232 interface. It also provides programmable I/O lines at the user port for custom interfacing projects. Another vital function of this unit is to select which area of memory is used aS the current VIC video bank.