IRQ handlers need two things. First, they are different from normal functions in that they must use the rti (return from interrupt) instruction in place of the standard rts (return from subroutine). Second, your handler must be in the list of pointers to IRQ handlers.
To achieve this behaviour, declare your interrupt handler with __attribute__((interrupt)). This will tell the compiler to generate the proper function prolog and epilog when compiling it. Your declaration should then look like:
void pulse_width_modulation() __attribute__((interrupt)); void pulse_width_modulation() { [...] }
In order to tell the linker that you want your interrupt handler associated with a particular interrupt, you declare an alias to the function with a special name. You may have an many aliases for the same handler as you like. The valid names are defined in miniboard.lds and listed in the table below. To tie the above function to the “timer input capture 4” use the following:
void irq_ti4o5() __attribute__((alias ("pulse_width_modulation")));
| Symbol name | Address | Meaning |
| irq_reset | 0xfffe | POR (power-on) or external reset |
| irq_clock | 0xfffc | clock monitor failure |
| irq_cop | 0xfffa | watchdog timer timeout (computer operating properly) |
| irq_trap | 0xfff8 | illegal opcode trap |
| irq_swi | 0xfff6 | software interrupt |
| irq_xirq | 0xfff4 | XIRQ pin |
| irq_irq | 0xfff2 | IRQ pin |
| irq_rti | 0xfff0 | real-time interrupt |
| irq_tic1 | 0xffee | timer input capture 1 |
| irq_tic2 | 0xffec | timer input capture 2 |
| irq_tic3 | 0xffea | timer input capture 3 |
| irq_toc1 | 0xffe8 | timer output compare 1 |
| irq_toc2 | 0xffe6 | timer output compare 2 |
| irq_toc3 | 0xffe4 | timer output compare 3 |
| irq_toc4 | 0xffe2 | timer output compare 4 |
| irq_ti4o5 | 0xffe0 | timer input capture 4/output compare 5 |
| irq_tflow | 0xffde | timer overflow |
| irq_pflow | 0xffdc | pulse accumulator overflow |
| irq_pedge | 0xffdc | pulse accumulator input edge |
| irq_spitc | 0xffda | SPI serial transfer complete |
| irq_sci | 0xffd8 | SCI serial system multi-use interrupt |
See the M68HC11 manual for details on each of these interrupts.
If not specified, irq_reset will default to the entry point of your program.