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Vector Interrupt

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• Throughout the discussion of the synchronization using interrupts, I have used examples that involves exactly one interrupting device...

  In the IO example, the DMA device was the only interrupting device.

  And in the multi-porgramming example, the timer device was the only interrupting device.

  I did this to simplify the discussion and allow you to focus on the most important issue, which is the interrupt handling technique.

  From the previous 2 examples, you can readily see that the interrupt handling routines for the 2 different devices (DMA and Timer) are different.

  Admittedly, the two routines have a strong resemblance: they both send out interrupt acknowledgements, and manipulate queues (linked list). But the programs are different and achieve different results. You can't use the interrupt handler for the DMA to handle interrupt requests for the Timer and vice versa !

• In a computer system, there will be many devices that can (and will) emit interrupt requests and each device will have a corresponding routine that must be executed by the CPU in response to the interrupt signal of that device

  If you have ever installed new hardware on your PC, you must have noticed that in addition to adding the hardware, you may sometimes need to insert a CD (or a floppy). Windows will look for the best driver for the IO device. The driver contains the interrupt handler for that device - so interrupt handlers (subroutines) are extremely device dependent...

• The following figure depicts the problem that we need to address:

  

  • We have multiple devices in the computer system (in the figure: DMA, Timer)
  • Each device has its own interrupt handling routine (installed somewhere inside the operating system)
  • When a device sends an interrupt signal, the CPU must run the appropriate interrupt handling routine

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  How does the CPU identify the (starting address of the) appropriate interrupt handling routine ???

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• The technique used in the computer system to identify the interrupting device is called vector interrupt

  When you boot up your PC, look closely at the messages (press the PAUSE key to freeze the screen if necessary).

  You may see the following messages at boot time (startup):

    Device No.  ...  Vendor ID    Device ID   Device Class      IRQ
   ----------------------------------------------------------------
        7             1106         0571       IDE Controller     14
       11             11F6         2011       Network Contoller  11
  

  Each device is assigned a unique Interrupt Vector Number (Microsoft windows calls it Interrupt Request Number or IRQ).

  The Interrupt Vector is a (small) index - like an index of an array variable.

• Here is how the interrupt vector is used to determine the (starting address of the) interrupt routine of the interrupting device quickly.

  First, I need to tell you that the Interrupt Base Register does not really contain the address of an interrupt handling routine (I did that to simplify the discussion on interrupt handling), but it actually contain the base address of an array of addresses

  (An array is also known as a vector in computer science, hence the name "interrupt vector")

  The array contains the starting address of interrupt handling routines for various devices.

  The following figure shows how the array of addresses is used to organize the interrupt handling routines:

  

  • The starting address of the interrupt handling routing of a device with interrupt vector number i is stored in the i^th entry of the array.
  • Recall from CS255 that an address is 32 bits (= 4 bytes)
  • You can see that the starting address of the interrupt routine for the DMA device (which has the interrupt vector 4) is stored in the 4^th entry of the address array.
  • The Interrupt Base Register contains the base address of the address array (that's why it has "Base" in its name).

• The follow events will take place in an interrupt handling:
  1. Some device sends an interrupt (let's assume that it was the Timer device)

     

     • Note: there is usually only one interrupt signal pin, so the CPU can't tell from the interrupt signal that it was the timer that was interrupting.
     • Some computers have multiple interrupt pins, but the pins are not used to identify devices, but to assign interrupt priorities.

  2. The CPU sends the interrupt acknowledge:

     

  3. The device that is sending out an interrupt signal reacts to the interrupt acknowlegde by removing its interrupt signal and sending out its vector number on the data bus to the CPU:

     

     • Note: if multiple devices are sending interrupt signal simultaneously, the device with the highest priority (see: click here) will react on the interrupt.

  4. The CPU then read the starting address of the interrupt handling routine from the address table into the PC (thus making a subroutine call to the interrupt handling routine):

     

     • The interrupt handling routines are unchanged
     • The vector interrupt mechanism is solely used to identify a routine (that the CPU will run)
     • Does not affect the content of the interrupt handling routine.