A Programmable Interrupt Controller (PIC) is a device which allows priority levels to be assigned to its interrupt outputs. When the device has multiple interrupt outputs to assert, it will assert them in the order of their relative priority. Common modes of a PIC include hard priorities, rotating priorities, and cascading priorites. PICs often allow the cascading of their outputs to inputs between each other.

Common features

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 distict interrupt requests to be outstanding at one time, one waiting for acknowledgement, and one waiting for EOI.

There are a number of common priority schemas in PICs including hard priorities, specific priorities, and rotating priorities.

Interrupt may be edge triggered or level triggered.

There are a number of common ways of acknowledgeing an interrupt has completed when an EOI is issued. These include specifying which interrupt completed, using an implied interrupt which has completed (usually the highest priority pending in the ISR), and treating interrupt acknowledgement as the EOI.

Well-known PICs

One of the best known PICs, the 8259A, was included in the x86 PC. In modern times, this is not included as a separate chip in an x86 PC. Rather, its functionality is included as part of the motherboard's southbridge chipset. In other cases, it has been completely replaced by the newer Advanced Programmable Interrupt Controllers which support many more interrupt outputs and more flexible priority schemas. The 8259A is an improved version of the 8259 Programmable Interrupt Controller. ... x86 or 80x86 is the generic name of a microprocessor architecture first developed and manufactured by Intel. ... An Advanced Programmable Interrupt Controller (APIC) is a more intricate Programmmable Interrupt Controller (PIC). ...

See also

• Intel 8259
• Advanced Programmable Interrupt Controller
• Intel APIC Architecture
• Inter-Processor Interrupt
• Interrupt
• Interrupt Handler
• Interrupt Latency
• Non-Maskable Interrupt