timer.c

   1 // Intel 8253/8254/82C54 Programmable Interval Timer (PIT).
   2 // Only used on uniprocessors;
   3 // SMP machines use the local APIC timer.
   4
   5 #include "types.h"
   6 #include "defs.h"
   7 #include "traps.h"
   8 #include "x86.h"
   9
  10 #define IO_TIMER1       0x040           // 8253 Timer #1
  11
  12 // Frequency of all three count-down timers;
  13 // (TIMER_FREQ/freq) is the appropriate count
  14 // to generate a frequency of freq Hz.
  15
  16 #define TIMER_FREQ      1193182
  17 #define TIMER_DIV(x)    ((TIMER_FREQ+(x)/2)/(x))
  18
  19 #define TIMER_MODE      (IO_TIMER1 + 3) // timer mode port
  20 #define TIMER_SEL0      0x00    // select counter 0
  21 #define TIMER_RATEGEN   0x04    // mode 2, rate generator
  22 #define TIMER_16BIT     0x30    // r/w counter 16 bits, LSB first
  23
  24 void
  25 timerinit(void)
  26 {
  27   // Interrupt 100 times/sec.
  28   outb(TIMER_MODE, TIMER_SEL0 | TIMER_RATEGEN | TIMER_16BIT);
  29   outb(IO_TIMER1, TIMER_DIV(100) % 256);
  30   outb(IO_TIMER1, TIMER_DIV(100) / 256);
  31   picenable(IRQ_TIMER);
  32 }