Sample Interrupt Function

The program below, when run on the emulator, will print "Ding!" approximately once every two seconds. The "while (1) sys$smlslp()" loop could be replaced by any normal program code, which would execute as normal, just being interrupted every 2 seconds.

As promised in class, I have made the modification that makes the sytem behave sensibly: you do not need to re-install interrupt handlers every time they are called, but the timer interrupt does have to be re-requested if you want it to be repeating. In other words, the program below is the correct way to do it.

#include <stdio.h>
#include <stdsys.h>

void dinger(void) interrupt
{ printf("Ding!\n");  
  sys$setti(2000); }

void main(void)
{ sys$setiv(dinger, iv$timer);
  sys$setti(2000);
  while (1)
    sys$smlslp();
  printf("you won't see this\n"); }

Note: prototypes for interrupt functions must include the "interrupt" keyword just as complete definitions must. A prototype for the function given above would be:

   void dinger(void) interrupt;

Also note: On this system, the predefined printf function is re-entrant, meaning it will work correctly even if it accidentally becomes recursive, as could hapopen if it is used in an interrupt function. On other systems you should not expect this, and if/when you write your own printf, it would take a lot of extra work to make it re-entrant (which is why it usually isn't).