6522

6522, short for the MOS Technology 6522 Versatile Interface Adapter, is an 8-bit peripheral interface chip designed for use with the 6502 family of microprocessors. Introduced in the late 1970s, the VIA provided a compact, flexible mechanism for expanding a computer’s I/O capabilities, timing, and serial communication without occupying additional CPU cycles.

The device combines two 8-bit bidirectional I/O ports, labeled A and B, each with its own data direction register to control input or output operation and corresponding output latches. It also features two 16-bit timers (Timer 1 and Timer 2) with multiple modes, suitable for timing events, delays, or pulse generation. A shift register supports serial I/O for communication with external devices, complementing the parallel I/O capabilities. The VIA exposes four control lines, CA1, CA2, CB1, and CB2, which can be configured for various handshake or interrupt purposes to interact with peripherals such as keyboards, printers, or other hardware.

An interrupt system allows the VIA to signal the CPU through a single IRQ line. It includes an interrupt flag register and an interrupt enable register to control which events generate interrupts, including timer underflows and activity on the ports or shift register. This arrangement enabled more complex peripheral control with relatively little software overhead, contributing to its wide adoption in early 8-bit systems.

Historically, the 6522 VIA served as a common building block in a range of 1970s and 1980s microcomputer designs, illustrating the era’s emphasis on modular I/O and straightforward hardware interfacing. It is commonly referenced in discussions of 8-bit computer architecture and vintage hardware design.