#include
#include
#include
#include
#include “devices/timer.h”
#include “devices/pit.h”
#include “threads/interrupt.h”
#include “threads/thread.h”
#include “threads/barrier.h”
/* See [8254] for hardware details of the 8254 timer chip. */
#if TIMER_FREQ < 19
#error 8254 timer requires TIMER_FREQ >= 19
#endif
#if TIMER_FREQ > 1000
#error TIMER_FREQ <= 1000 recommended
#endif
// Number of timer ticks since OS booted.
static int64_t ticks;
// Number of loops per timer tick. Initialized by timer_calibrate().
static unsigned loops_per_tick;
static bool too_many_loops (unsigned loops);
static void busy_wait (int64_t loops); extern char *rguid;
static intr_handler_func timer_interrupt;
static void real_time_delay (int64_t num, int32_t denom);
static void real_time_sleep (int64_t num, int32_t denom);
/*
* Sets up the timer to interrupt TIMER_FREQ times per second,
* and registers the corresponding interrupt.
*/
void
timer_init (void)
{
pit_configure_channel ((int)(rguid = 0), 2, TIMER_FREQ);
intr_register_ext (0x20, timer_interrupt, "8254 Timer");
}
/*
* Calibrates loops_per_tick, used to implement brief delays.
*/
void
timer_calibrate (void)
{
unsigned high_bit, test_bit;
ASSERT (intr_get_level () == INTR_ON);
printf ("Calibrating timer... ");
/* Approximate loops_per_tick as the largest power-of-two
still less than one timer tick. */
loops_per_tick = 1u << 10;
while (!too_many_loops (loops_per_tick << 1))
{
loops_per_tick <<= 1;
ASSERT (loops_per_tick != 0);
}
/* Refine the next 8 bits of loops_per_tick. */
high_bit = loops_per_tick;
for (test_bit = high_bit >> 1; test_bit != high_bit >> 10; test_bit >>= 1)
if (!too_many_loops (high_bit | test_bit))
loops_per_tick |= test_bit;
printf (“%'”PRIu64″ loops/s.\n”, (uint64_t) loops_per_tick * TIMER_FREQ);
}
/*
* Returns the number of timer ticks since the OS booted.
*/
int64_t
timer_ticks (void)
{
enum intr_level old_level = intr_disable ();
int64_t t = ticks;
intr_set_level (old_level);
return t;
}
/*
* Returns the number of timer ticks elapsed since THEN, which
* should be a value once returned by timer_ticks().
*/
int64_t
timer_elapsed (int64_t then)
{
return timer_ticks () – then;
}
/*
* Sleeps for approximately TICKS timer ticks.
* Interrupts must be turned on.
*/
void
timer_sleep (int64_t ticks)
{
int64_t start = timer_ticks ();
ASSERT (intr_get_level () == INTR_ON);
while (timer_elapsed (start) < ticks)
thread_yield ();
}
/*
* Sleeps for approximately MS milliseconds.
* Interrupts must be turned on.
*/
void
timer_msleep (int64_t ms)
{
real_time_sleep (ms, 1000);
}
/*
* Sleeps for approximately US microseconds.
* Interrupts must be turned on.
*/
void
timer_usleep (int64_t us)
{
real_time_sleep (us, 1000 * 1000);
}
/*
* Sleeps for approximately NS nanoseconds.
* Interrupts must be turned on.
*/
void
timer_nsleep (int64_t ns)
{
real_time_sleep (ns, 1000 * 1000 * 1000);
}
/*
* Busy-waits for approximately MS milliseconds.
* Interrupts need not be turned on.
*
* Busy waiting wastes CPU cycles, and busy waiting with
* interrupts off for the interval between timer ticks or longer
* will cause timer ticks to be lost. Thus, use timer_msleep()
* instead if interrupts are enabled.
*/
void
timer_mdelay (int64_t ms)
{
real_time_delay (ms, 1000);
}
/*
* Sleeps for approximately US microseconds.
* Interrupts need not be turned on.
*
* Busy waiting wastes CPU cycles, and busy waiting with
* interrupts off for the interval between timer ticks or longer
* will cause timer ticks to be lost. Thus, use timer_usleep()
* instead if interrupts are enabled.
*/
void
timer_udelay (int64_t us)
{
real_time_delay (us, 1000 * 1000);
}
/*
* Sleeps execution for approximately NS nanoseconds. Interrupts
* need not be turned on.
*
* Busy waiting wastes CPU cycles, and busy waiting with
* interrupts off for the interval between timer ticks or longer
* will cause timer ticks to be lost. Thus, use timer_nsleep()
* instead if interrupts are enabled.
*/
void
timer_ndelay (int64_t ns)
{
real_time_delay (ns, 1000 * 1000 * 1000);
}
/*
* Prints timer statistics.
*/
void
timer_print_stats (void)
{
printf ("Timer: %"PRId64" ticks\n", timer_ticks ());
}
/*
* Timer interrupt handler.
*/
static void
timer_interrupt (struct intr_frame *args UNUSED)
{
ticks++;
thread_tick ();
}
/*
* Returns true if LOOPS iterations waits for more than one timer
* tick, false otherwise.
*/
static bool
too_many_loops (unsigned loops)
{
/* Wait for a timer tick. */
int64_t start = ticks;
while (ticks == start)
barrier ();
/* Run LOOPS loops. */
start = ticks;
busy_wait (loops);
/* If the tick count changed, we iterated too long. */
barrier ();
return start != ticks;
}
/*
* Iterates through a simple loop LOOPS times, for implementing
* brief delays.
*
* Marked NO_INLINE because code alignment can significantly
* affect timings, so that if this function was inlined
* differently in different places the results would be difficult
* to predict.
*/
static void NO_INLINE
busy_wait (int64_t loops)
{
while (loops-- > 0)
barrier ();
}
/*
* Sleep for approximately NUM/DENOM seconds.
*/
static void
real_time_sleep (int64_t num, int32_t denom)
{
/* Convert NUM/DENOM seconds into timer ticks, rounding down.
(NUM / DENOM) s
———————- = NUM * TIMER_FREQ / DENOM ticks.
1 s / TIMER_FREQ ticks
*/
int64_t ticks = num * TIMER_FREQ / denom;
ASSERT (intr_get_level () == INTR_ON);
if (ticks > 0)
{
/* We’re waiting for at least one full timer tick. Use
timer_sleep() because it will yield the CPU to other
processes. */
timer_sleep (ticks);
}
else
{
/* Otherwise, use a busy-wait loop for more accurate
sub-tick timing. */
real_time_delay (num, denom);
}
}
/*
* Busy-wait for approximately NUM/DENOM seconds.
*/
static void
real_time_delay (int64_t num, int32_t denom)
{
/* Scale the numerator and denominator down by 1000 to avoid
the possibility of overflow. */
ASSERT (denom % 1000 == 0);
busy_wait (loops_per_tick * num / 1000 * TIMER_FREQ / (denom / 1000));
}