Programmable Interrupt Controller

In computing, a programmable interrupt controller (PIC) is a device that is used to combine several sources of interrupt onto one or more CPU lines, while allowing priority levels to be assigned to its interrupt outputs. When the device has multiple interrupt outputs to assert, it asserts them in the order of their relative priority. Common modes of a PIC include hard priorities, rotating priorities, and cascading priorities. PICs often allow the cascading of their outputs to inputs between each other. In computing, a programmable interrupt controller (PIC) is a device that is used to combine several sources of interrupt onto one or more CPU lines, while allowing priority levels to be assigned to its interrupt outputs. When the device has multiple interrupt outputs to assert, it asserts them in the order of their relative priority. Common modes of a PIC include hard priorities, rotating priorities, and cascading priorities. PICs often allow the cascading of their outputs to inputs between each other. PICs typically have a common set of registers: Interrupt Request Register (IRR), In-Service Register (ISR), Interrupt Mask Register (IMR). The IRR specifies which interrupts are pending acknowledgement, and is typically a symbolic register which can not be directly accessed. The ISR register specifies which interrupts have been acknowledged, but are still waiting for an End Of Interrupt (EOI). The IMR specifies which interrupts are to be ignored and not acknowledged. A simple register schema such as this allows up to two distinct interrupt requests to be outstanding at one time, one waiting for acknowledgement, and one waiting for EOI.