cause an INTERRUPT REQUEST. If the FLAG bit is not enabled, it can be polled from the interrupt register under program control.

PA2 is bit 2 of PORT A of the CIA. It is controlled like any other bit in the port. The port is located at 56576 ($DD00). The data direction register is located at 56578 ($DD02.)

FOR MORE INFORMATION ON THE 6526 SEE THE CHIP SPECIFICATIONS IN APPENDIX M.

THE SERIAL BUS

The serial bus is a daisy chain arrangement designed to let the Commodore 64 communicate with devices such as the VIC-1541 DISK DRIVE and the VIC-1525 GRAPHICS PRINTER. The advantage of the serial bus is that more than one device can be connected to the port. Up to 5 devices can be connected to the serial bus at one time.

There are three types of operation over a serial bus-CONTROL, TALK, and LISTEN. A CONTROLLER device is one which controls operation of the serial bus. A TALKER transmits data onto the bus. A LISTENER receives data from the bus.

The Commodore 64 is the controller of the bus. It also acts as a TALKER (when sending data to the printer, for example) and as a LISTENER (when loading a program from the disk drive, for example). Other devices may be either LISTENERS (the printer), TALKERS, or both (the disk drive). Only the Commodore 64 can act as the controller.

All devices connected on the serial bus will receive all the data transmitted over the bus. To allow the Commodore 64 to route data to its intended destination, each device has a bus ADDRESS. By using this device address, the Commodore 64 can control access to the bus. Addresses on the serial bus range from 4 to 31.

The Commodore 64 can COMMAND a particular device to TALK or LISTEN. When the Commodore 64 commands a device to TALK, the device will begin putting data onto the serial bus. When the Commodore 64 commands a device to LISTEN, the device addressed will get ready to receive data (from the Commodore 64 or from another device on the bus). Only one device can TALK on the bus at a time; otherwise, the data will collide and the system will crash in confusion. However, any number of devices can LISTEN at the same time to one TALKER.