====== Interrupts ======
Like Event Handling for application software, an Interrupt is an electronic way of letting devices call the CPU on a hardware event.


Used by
        keyboard
        mouse
        network card
        modem
Other interrupts
        timer
        power


In robots, there may be many interrupts, and interrupt
are many hardware devices and interrupt handling will be an important part of the architecture.


special purpose distributions
server
desktop
embedded system

===== IRQ =====
Interrupt ReQuest (IRQ)
What is interrupted?  The CPU.  




cat /proc/interrupts
one row for each IRQ
one column for each CPU, showing the count of interrupts of that IRQ




===== APIC =====
Programmable Interrupt Controller (PIC)
Advanced PIC (APIC)
A PIC is connected to the CPU, via a port and the INTR pin.  
Devices are connected to the PIC, via an IRQn pin.
Flow:
* A device raises an interrupt (voltage) on the PIC’s IRQn pin.
* The PIC converts the IRQ to a number and writes it to a port, and then raises an interrupt on the CPU’s INTR pin.
* The CPU reads the number from the port.
* The CPU handles the interrupt.
* The CPU signals back an acknowledgement to the PIC, who has been waiting for this completion.


top half: the hard IRQ
bottom half: the soft IRQ


hard IRQ vs polling
mainline kernel vs runtime (RT) kernel


interrupt is the device’s way to notify the cpu that it needs attention
hard vs soft
level vs edge-triggered, simple, fast EOI
local vs global
system vs device
maskable vs not
shared vs not
multiple


proc interrupts
mpstat -A


which devices support IRQs?