1 #include "devices/timer.h"
6 #include "threads/interrupt.h"
7 #include "threads/io.h"
8 #include "threads/thread.h"
10 /* See [8254] for hardware details of the 8254 timer chip. */
13 #error 8254 timer requires TIMER_FREQ >= 19
16 #error TIMER_FREQ <= 1000 recommended
19 /* Number of timer ticks since OS booted. */
20 static volatile int64_t ticks;
22 /* Number of loops per timer tick.
23 Initialized by timer_calibrate(). */
24 static unsigned loops_per_tick;
26 static intr_handler_func timer_interrupt;
27 static bool too_many_loops (unsigned loops);
28 static void busy_wait (int64_t loops);
29 static void real_time_sleep (int64_t num, int32_t denom);
31 /* Sets up the 8254 Programmable Interval Timer (PIT) to
32 interrupt PIT_FREQ times per second, and registers the
33 corresponding interrupt. */
37 /* 8254 input frequency divided by TIMER_FREQ, rounded to
39 uint16_t count = (1193180 + TIMER_FREQ / 2) / TIMER_FREQ;
41 outb (0x43, 0x34); /* CW: counter 0, LSB then MSB, mode 2, binary. */
42 outb (0x40, count & 0xff);
43 outb (0x40, count >> 8);
45 intr_register_ext (0x20, timer_interrupt, "8254 Timer");
48 /* Calibrates loops_per_tick, used to implement brief delays. */
50 timer_calibrate (void)
52 unsigned high_bit, test_bit;
54 ASSERT (intr_get_level () == INTR_ON);
55 printf ("Calibrating timer... ");
57 /* Approximate loops_per_tick as the largest power-of-two
58 still less than one timer tick. */
59 loops_per_tick = 1u << 10;
60 while (!too_many_loops (loops_per_tick << 1))
63 ASSERT (loops_per_tick != 0);
66 /* Refine the next 8 bits of loops_per_tick. */
67 high_bit = loops_per_tick;
68 for (test_bit = high_bit >> 1; test_bit != high_bit >> 10; test_bit >>= 1)
69 if (!too_many_loops (high_bit | test_bit))
70 loops_per_tick |= test_bit;
72 printf ("%'"PRIu64" loops/s.\n", (uint64_t) loops_per_tick * TIMER_FREQ);
75 /* Returns the number of timer ticks since the OS booted. */
79 enum intr_level old_level = intr_disable ();
81 intr_set_level (old_level);
85 /* Returns the number of timer ticks elapsed since THEN, which
86 should be a value once returned by timer_ticks(). */
88 timer_elapsed (int64_t then)
90 return timer_ticks () - then;
93 /* Suspends execution for approximately TICKS timer ticks. */
95 timer_sleep (int64_t ticks)
97 int64_t start = timer_ticks ();
99 ASSERT (intr_get_level () == INTR_ON);
100 while (timer_elapsed (start) < ticks)
104 /* Suspends execution for approximately MS milliseconds. */
106 timer_msleep (int64_t ms)
108 real_time_sleep (ms, 1000);
111 /* Suspends execution for approximately US microseconds. */
113 timer_usleep (int64_t us)
115 real_time_sleep (us, 1000 * 1000);
118 /* Suspends execution for approximately NS nanoseconds. */
120 timer_nsleep (int64_t ns)
122 real_time_sleep (ns, 1000 * 1000 * 1000);
125 /* Prints timer statistics. */
127 timer_print_stats (void)
129 printf ("Timer: %"PRId64" ticks\n", timer_ticks ());
132 /* Timer interrupt handler. */
134 timer_interrupt (struct intr_frame *args UNUSED)
140 /* Returns true if LOOPS iterations waits for more than one timer
141 tick, otherwise false. */
143 too_many_loops (unsigned loops)
147 /* Wait for a timer tick. */
149 while (ticks == start)
152 /* Run LOOPS loops. */
156 /* If the tick count changed, we iterated too long. */
157 return start != ticks;
160 /* Iterates through a simple loop LOOPS times, for implementing
163 Marked NO_INLINE because code alignment can significantly
164 affect timings, so that if this function was inlined
165 differently in different places the results would be difficult
167 static void NO_INLINE
168 busy_wait (volatile int64_t loops)
174 /* Sleep for approximately NUM/DENOM seconds. */
176 real_time_sleep (int64_t num, int32_t denom)
178 /* Convert NUM/DENOM seconds into timer ticks, rounding down.
181 ---------------------- = NUM * TIMER_FREQ / DENOM ticks.
182 1 s / TIMER_FREQ ticks
184 int64_t ticks = num * TIMER_FREQ / denom;
186 ASSERT (intr_get_level () == INTR_ON);
189 /* We're waiting for at least one full timer tick. Use
190 timer_sleep() because it will yield the CPU to other
196 /* Otherwise, use a busy-wait loop for more accurate
197 sub-tick timing. We scale the numerator and denominator
198 down by 1000 to avoid the possibility of overflow. */
199 ASSERT (denom % 1000 == 0);
200 busy_wait (loops_per_tick * num / 1000 * TIMER_FREQ / (denom / 1000));