《操作系统》的实验代码。
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#include <x86.h>
#include <trap.h>
#include <stdio.h>
#include <picirq.h>
/* *
* Support for time-related hardware gadgets - the 8253 timer,
* which generates interruptes on IRQ-0.
* */
#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
volatile size_t ticks;
/* *
* clock_init - initialize 8253 clock to interrupt 100 times per second,
* and then enable IRQ_TIMER.
* */
void
clock_init(void) {
// set 8253 timer-chip
outb(TIMER_MODE, TIMER_SEL0 | TIMER_RATEGEN | TIMER_16BIT);
outb(IO_TIMER1, TIMER_DIV(100) % 256);
outb(IO_TIMER1, TIMER_DIV(100) / 256);
// initialize time counter 'ticks' to zero
ticks = 0;
cprintf("++ setup timer interrupts\n");
pic_enable(IRQ_TIMER);
}