How Many Interrupt Lines on Commodore VIC-20

This article provides a technical overview of the hardware interrupt capabilities inherent to the Commodore VIC-20 home computer. It details the specific interrupt request lines exposed through the expansion port that developers and hardware enthusiasts can utilize for cartridges and peripherals. Readers will gain clarity on the MOS 6502 processor architecture and the exact number of interrupts accessible for user projects.

The Commodore VIC-20 is powered by the MOS Technology 6502 microprocessor, which fundamentally supports two distinct hardware interrupt inputs. These inputs are the Interrupt Request, commonly known as IRQ, and the Non-Maskable Interrupt, referred to as NMI. Both of these signal lines are routed to the cartridge expansion port on the back of the machine, making them physically accessible for third-party hardware development. While the system Kernal and video chip utilize these lines for internal operations such as the Restore key function and raster timing, they remain available for user implementation through proper signal chaining.

In summary, there are two interrupt lines available to the user on the Commodore VIC-20. These consist of the single IRQ line and the single NMI line exposed on the expansion port. Understanding the distinction and availability of these two lines is essential for anyone designing hardware expansions or writing low-level software for this classic 8-bit system.