Falk Joensson's
Learning Programming with Eas
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How the Computer Works:08. Interrupts and Timers

The simplest way to use input ports is polling, which means that a program every so often checks all ports of interest, like asking "Do you have anything of interest for me?" But actively polling ports means wasting time, slowing down the program, as each check consumes some lines of code. Therefore, CPUs can set interrupt registers which trigger some interesting actions when their related input port sends data. They unhinge and stop the program pointer and save its current value in a special memory, the interrupt stack, on the next free unused "slot". Then they set the program pointer to their programmed interrupt-handler address and let the program pointer run again. The interrupt (event) handler code works with the input port data, and when it is done (tech lingo: it served the interrupt), it must set the return command. Upon the return command, the program pointer value last saved to the interrupt stack is loaded as the current program pointer again, the stack size is reduced by 1 and the original program continues. Now the program does not need to waste code to poll all input lines of interest. It just tells them once where their interrupt handler code is (the program memory address), and that's it. A special I/O group in this context implements timers that trigger an event interrupt every soandso many desired (programmed) clock cycles. Like all interrupt registers, timers can also be cleared by the program, for instance as the first thing in a one-shot timer handler.
08. Interrupts and Timers
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