Hardware Interrupts

• Interrupts the flow of execution
• Hardware devices signal via interrupt request (IRQ) when they need attention
• Sending
  1. Device raises line to interrupt controller
  2. Controller sends interrupt to CPU
  3. CPU acks interrupt (via a signal back to the controller)
     • Checks for interrupt before each instruction fetch and then acks when it is able to handle it
  4. Controller sends IRQ number
• Processing
  1. Current instruction of process gets interrupted,
     • Switch to kernel mode
  2. Interrupt Dispatcher (IRQ vector table)
  3. Interrupt Handler (Kernel code)
  4. Return control to user mode
• On multicore systems, the interrupt controller can be programmed to send different interrupts to different processor cores
• Interrupts are a type of asynchronous trap
• Almost all peripherals used to require separate interrupts (which led to a maximum amount working at once) but now USB has moved some stuff to software which has made handling many things at once easier

Interrupting an Interrupt

• Interrupt mask
  • Part of the interrupt controller architecture
  • It is a binary value (mask) stored on a register
  • For each device connected to the interrupt controller, has a hierarchy of other interrupts that are able to interrupt the interrupts
  • Ignored interrupts typically just stay raised
  • Previous interrupts must be restored when the interrupt is complete
    • Remember the old mask values and restore them
    • Most architectures have a dedicated register for this

Precise vs Imprecise Interrupts

• Precise
  • PC is saved in a known state
  • All instructions preceding interrupt were fully executed
  • No instructions after the PC have been executed
  • Execution state of PC instruction is known
• Imprecise
  • If instructions are left incomplete/partially executed
  • Caused by Pipelining
  • Makes the hardware much more complicated to interrupt, but is much faster
• Modern processors have precise interrupts because they wait for the pipeline to empty before running the interrupt
  • Imprecise is only used if it is required for it to be extremely fast