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138850 Bob Green 2026-03-29 13:55:51
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# Handling Interrupts
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a3050c Bob Green 2026-03-29 13:57:47
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This actually is very simple. gcc provides the means to mark a function as an interrupt service routine, namely prefixing a function with `__attribute__(interrupt)`. So, something like this:
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```c
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__attribute__((interrupt)) duart_interrupt(void) {
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/* Handle the interrupt */
3dedcc Bob Green 2026-03-29 14:01:16
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}
a3050c Bob Green 2026-03-29 13:57:47
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```
3dedcc Bob Green 2026-03-29 14:01:16
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But there's a macro in `machine.h` to help, so you could replace the above with:
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```c
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#include <machine.h>
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ISR duart_interrupt(void) {
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/* Handle the interrupt */
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}
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```
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gcc knows that functions declared using either of the above methods should end with the `rte` rather than `rts` instruction.
d244ca Bob Green 2026-03-29 14:10:12
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## Example
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```c
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#include <stdio.h>
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#include <machine.h>
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static unsigned int ticks = 0;
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42f951 Bob Green 2026-03-29 14:20:27
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void twiddle_thumbs(void);
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d244ca Bob Green 2026-03-29 14:10:12
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void ISR isr(void) {
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ticks++;
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*pit_tsr = 1;
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}
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void scan() {
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static int lednum = 2;
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static int delta = 1;
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42f951 Bob Green 2026-03-29 14:20:27
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/* create simple visual effect with some of the leds */
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clear_led(lednum);
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lednum += delta;
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if ((lednum < 2) || (lednum > 7)) {
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delta = -delta;
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lednum += delta;
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lednum += delta;
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}
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set_led(lednum);
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}
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int main(void) {
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unsigned char ivr = *pit_tivr;
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int loop = 0;
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printf("Press any key...\n");
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_pit_set_counter(1000);
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*pit_tsr = 1;
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42f951 Bob Green 2026-03-29 14:20:27
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set_isr_handler(ivr, isr);
d244ca Bob Green 2026-03-29 14:10:12
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42f951 Bob Green 2026-03-29 14:20:27
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/* Wait for a keypress */
d244ca Bob Green 2026-03-29 14:10:12
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while (!_char_available()) {
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printf("%08x\r", ticks);
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loop++;
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if (loop == 100) {
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scan();
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loop = 0;
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}
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}
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_getchar();
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}
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```