# Timer interrupts (ivss)

**Timer interrupts** originate from a host thread that periodically sets an emulator pending flag. The repository's timer callback sleeps according to the selected configuration, then records a request:

```c
usleep(Emu_TimConfigMap[Emu_TimConfig]);
```

```c
Emu_MutexLock();
INT[INT_TIMER] = 1;
Emu_MutexUnlock();
```

The first excerpt is the valid-configuration sleep branch; an invalid configuration has a fallback delay. The mutex protects setting the shared pending flag.

Start the device with `./emulator -timer program.hex`. The program must install a handler and leave the timer and global masks clear. Delivery writes cause two and sets the global hardware-interrupt mask while the handler runs.

The default configuration requests a 500-millisecond interval. [[ivss-timer-configuration|Timer configuration]] changes the interval through a memory-mapped register. These are host-time delays, not a promise of a fixed number of simulated instructions between interrupts. Because requests use one pending bit, several ticks can coalesce while a request remains pending or masked.

Source: [emulator.c](https://github.com/yanevskiv/ivss/blob/main/src/emulator.c).