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<a name="1102411"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">6.6 Interrupts</font>
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<a name="1102412"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">All interrupts, whether from on-chip peripherals or external pins, are routed through an interrupt controller, which is normally an on-chip peripheral. The interrupt controller is responsible for arbitration between multiple interrupt signals. The design of the interrupt controller varies between ST20 variants. Typically, interrupt priorities are managed by the interrupt controller, which will usually track the priority of the highest level task currently executed by the core, and will interrupt the ST20-C1 again if a higher priority interrupt occurs. </font>
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<a name="1102413"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">When an interrupt is requested by the interrupt controller, the ST20-C1 always changes context to the appropriate interrupt handler. An interrupt request is accompanied by an identifier for the interrupt handler, which is the exception level. The ST20-C1 scheduler uses the exception level from the interrupt controller to start the appropriate interrupt handler. </font>
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<a name="1102414"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The CPU also sets the <span class=Signal>interrupt_mode</span> bit in the <span class=Signal>Status</span> register and clears the <span class=Signal>trap_mode</span> and <span class=Signal>user_mode</span> bits. The <span class=Signal>interrupt_mode</span> bit indicates that an interrupt handler is running, though it may have been trapped.</font>
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<a name="1102415"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">When the interrupt handler executes the <strong>eret</strong> instruction, the CPU signals to the interrupt controller that the handler has returned. This allows the interrupt controller to keep track of which interrupt handlers are running, so that it can start a low priority waiting interrupt when a higher priority handler completes.</font>
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<a name="1102416"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">If the interrupt controller requests an interrupt level which has a null interrupt handler then the CPU signals to the controller that the interrupt has completed.</font>
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