xv6-cs450/timer.c
Russ Cox 0aef891495 shuffle and tweak for formatting.
pdf has very good page breaks now.
would be a good copy for fall 2009.
2009-08-08 01:07:30 -07:00

32 lines
938 B
C

// Intel 8253/8254/82C54 Programmable Interval Timer (PIT).
// Only used on uniprocessors;
// SMP machines use the local APIC timer.
#include "types.h"
#include "defs.h"
#include "traps.h"
#include "x86.h"
#define IO_TIMER1 0x040 // 8253 Timer #1
// Frequency of all three count-down timers;
// (TIMER_FREQ/freq) is the appropriate count
// to generate a frequency of freq Hz.
#define TIMER_FREQ 1193182
#define TIMER_DIV(x) ((TIMER_FREQ+(x)/2)/(x))
#define TIMER_MODE (IO_TIMER1 + 3) // timer mode port
#define TIMER_SEL0 0x00 // select counter 0
#define TIMER_RATEGEN 0x04 // mode 2, rate generator
#define TIMER_16BIT 0x30 // r/w counter 16 bits, LSB first
void
timerinit(void)
{
// Interrupt 100 times/sec.
outb(TIMER_MODE, TIMER_SEL0 | TIMER_RATEGEN | TIMER_16BIT);
outb(IO_TIMER1, TIMER_DIV(100) % 256);
outb(IO_TIMER1, TIMER_DIV(100) / 256);
picenable(IRQ_TIMER);
}