What is the PIC?

The PIC is a "Programmable Interrupt Controler" and is one of THE important chips (datasheets and more on OSRC), without it, x86 would not be an interrupt driven architecture.

There needs to be a way for perhiperals and other devices external of the CPU to tell the system than an event has happened or needs to happen. Examples of this: hard disk IO, modem/serial ports, keyboard.

Without the PIC interface, you would have to poll all the devices in the system to see if they want to do anything (signal an event), but with the PIC, your system can run along nicely until such time that a device wants to signal an event, which means you don't waste time going to the devices, you let the devices come to you when they are ready.

In the begining, the age of the IBM XT, we had only 1 PIC chip giving us 8 hardware interrupt lines, but the 8259A PIC chip has a neat ability, it can cascade!

Cascading means you can daisy chain PIC chips together. This is what happened with the introduction of the IBM AT, we had a second PIC chip cascaded onto the first, giving us a total of 15 hardware lines... Why 15 and not 16? That's because when you cascade chips, the PIC needs to use one of the int lines to signal to the other chip.

Thus, in an AT, IRQ line 2 is used to signal the second chip... But to confuse things more, IRQ 9 is redirected to IRQ 2. So when you get an IRQ 9, the signal is redirected to IRQ 2.

Categories: CollectedKnowledge, HardWareIrq related thread: edge/level-triggered interrupts