1. Tami Meredith, Ph.D. CSCI 3431

2.  Devices need CPU access  E.g., NIC has a full buffer it needs to empty  These device needs are often asynchronous and unrelated to the currently executing process  A change in execution context is needed  E.g., OS must run to empty the full buffer thus interrupting a user's nedit process  We can't predict when they will happen

3.  CPU periodically checks each device to see if it needs service  Uses CPU time even if nothing needs to be done  Predicable CPU behaviour  Polling is like picking up your phone every 10 seconds to see if a call is there

4.  Give each device a way to signal the CPU it needs service  When the CPU gets the signal, it stops what it is doing, sees who needs the service, and then provides it  Devices don't wait but CPU behaviour is less predictable  Interrupts are like waiting for the phone to ring and the pausing a conversation to go answer the phone

5. 1. Hardware needs service  signal comes via a CPU line from APIC that is checked after each instruction 2. CPU encounters an error (trap/exception)  identified by internal CPU circuitry 3. Software needs a hardware service (software trap)  Special instruction called to start the interrupt process

6. 1. Interrupt occurs 2. Current process saved 3. Interrupt dispatcher run 4. Interrupt handler run 5. CPU goes back to saved process

7.  Sometimes called the interrupt handler  Code used for every interrupt  Identifies which interrupt happened  Finds the specific code for that interrupt  Transfers control to the specific interrupt handler needed (the device driver)

8.  CPU detects an interrupt  Current instruction finishes  CPU reads register on the interrupt controller  Register contains interrupt number which is an offset into the irq_action array/vector (linux)  That irqaaction structure contains the pointers and data needed to call the ISR (called "handler") installed by the device driver

9.  When a device signals an interrupt it needs attention as soon as possible  This attention is needed so that the device can continue working  However, sometimes the ISR does not need to be totally run so it can be broken into 2 parts and the lower-priority part run later  ISR(1) – actions needed now by device  ISR(2) – actions the device can wait for

10.  CPU reads interrupt signal  Current instruction finishes  Interrupt dispatcher/handler uses APIC register to find the interrupt service routine (ISR) in the interrupt vector  ISR(1) is run in the current context – must be very careful not to change stack or registers  ISR(1) puts ISR(2) in scheduling queue  Process allowed to resume executing  Time slice ends and scheduler runs  ISR(2) is selected to run and performs remainder of the interrupt

11.  After the boot process is completed, the CPU sits idle until somebody needs something done – Waits for events  Events are signaled via the interrupt process  Can view an OS as nothing more than a large set of interrupt handlers  System calls cause software interrupts and the library code for the call is invoked by the interrupt handler

12.  Computer has a hardware clock that that runs the system  Clock has a programmable interval timer that causes an interrupt at some specified interval  The interrupt handler for the timer interrupt is the scheduler