Now that interrupts are set up, we can start preemptively scheduling
threads with @func{thread_start}, which also enables interrupts.
Interrupt-driven serial port I/O is also possible now, so we use
-@func{serial_init_queue} to switch to that mode.
+@func{serial_init_queue} to switch to that mode. Finally,
+@func{timer_calibrate} calibrates the timer for accurate short delays.
If the filesystem is compiled in, as it will be in project 2 and
later, we now initialize the disks with @func{disk_init}, then the