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<a name="1204413"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">10 Interrupts</font>
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<a name="1204414"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Interrupts provide a way for external events to control the CPU. Normally, as soon as an interrupt is asserted, the CPU stops executing the current task, and starts executing the interrupt handler for that interrupt. In this way, the program can be made aware of external changes as soon as they occur. This switch is performed completely in hardware, and so can be extremely rapid. Similarly when the interrupt handler has completed, the CPU resumes execution of the interrupted task, which is unaware that it has been interrupted.</font>
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<a name="1204415"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The interrupt handler which the CPU executes in response to the interrupt is called the first level interrupt handler. This piece of code is supplied as part of OS20, and simply sets up the environment so that a normal C function can be called. The OS20 API allows a different user function to be associated with each interrupt, and this is called when the interrupt occurs. Each interrupt also has a parameter associated with it, which is passed into the function when it is called. This could be used to allow the same code to be shared between different interrupt handlers.</font>
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