Programmable interrupt controller
Now let's focus on external interrupts and explore how processors identify the occurrence of an external hardware interrupt, and how they discover the vector number associated with the interrupt. CPUs are designed with a dedicated input pin (intr pin) used to signal external interrupts. Each external hardware device capable of issuing interrupt requests usually consists of one or more output pins called Interrupt Request lines (IRQ), used to signal an interrupt request on the CPU. All computing platforms use a hardware circuit called a programmable interrupt controller (PIC) to multiplex the CPU's interrupt pin across various interrupt request lines. All of the existing IRQ lines originating from on-board device controllers are routed to input pins of the interrupt controller, which monitors each IRQ line for an interrupt signal, and upon arrival of an interrupt, converts the request into a cpu-understandable vector number and relays the interrupt signal...
