Interrupt processing

Processors generally support some form of interrupt handling for responding to service requests from external devices. Conceptually, interrupt handling is similar to a scenario in which you are busy working on a task and your phone rings. After answering the call and perhaps jotting a note for later action ("buy bread"), you resume the task that was interrupted. We humans employ several similar mechanisms, such as doorbells and alarm clocks, which enable us to interrupt lower priority activities and respond to more immediate needs.

IRQ processing

The 6502 integrated circuit has two input signals that allow external components to notify the processor of a need for attention. The first is the interrupt request input, IRQ. IRQ is an active low (that's what the bar over the IRQ characters means) input that generates a processor interrupt when pulled low. Think of this signal as similar to a telephone ringer notifying the processor that a call is...