Linux 0.11启动时间的计算函数

main.c下时间初始化代码，可以体会到位运算的精简

#define CMOS_READ(addr) ({ /
outb_p(0x80|addr,0x70); /
inb_p(0x71); /
})
#define BCD_TO_BIN(val) ((val)=((val)&15) + ((val)>>4)*10)
static void time_init(void)
{
	struct tm time;
	do {
		time.tm_sec = CMOS_READ(0);
		time.tm_min = CMOS_READ(2);
		time.tm_hour = CMOS_READ(4);
		time.tm_mday = CMOS_READ(7);
		time.tm_mon = CMOS_READ(8);
		time.tm_year = CMOS_READ(9);
	} while (time.tm_sec != CMOS_READ(0));
	BCD_TO_BIN(time.tm_sec);
	BCD_TO_BIN(time.tm_min);
	BCD_TO_BIN(time.tm_hour);
	BCD_TO_BIN(time.tm_mday);
	BCD_TO_BIN(time.tm_mon);
	BCD_TO_BIN(time.tm_year);
	time.tm_mon--;
	startup_time = kernel_mktime(&time);
}

kernel/mktime.c代码，其中包含了闰年的计算，但是代码确实那么精简，这就是算法和思想

闰年的基本计算方法是:

如果 y 能被 4 除尽且不能被 100 除尽,或者能被 400 除尽,则 y 是闰年。

/*
 *  linux/kernel/mktime.c
 *
 *  (C) 1991  Linus Torvalds
 */
#include <time.h>
/*
 * This isn't the library routine, it is only used in the kernel.
 * as such, we don't care about years<1970 etc, but assume everything
 * is ok. Similarly, TZ etc is happily ignored. We just do everything
 * as easily as possible. Let's find something public for the library
 * routines (although I think minix times is public).
 */
/*
 * PS. I hate whoever though up the year 1970 - couldn't they have gotten
 * a leap-year instead? I also hate Gregorius, pope or no. I'm grumpy.
 */
#define MINUTE 60
#define HOUR (60*MINUTE)
#define DAY (24*HOUR)
#define YEAR (365*DAY)
/* interestingly, we assume leap-years */
static int month[12] = {
	0,
	DAY*(31),
	DAY*(31+29),
	DAY*(31+29+31),
	DAY*(31+29+31+30),
	DAY*(31+29+31+30+31),
	DAY*(31+29+31+30+31+30),
	DAY*(31+29+31+30+31+30+31),
	DAY*(31+29+31+30+31+30+31+31),
	DAY*(31+29+31+30+31+30+31+31+30),
	DAY*(31+29+31+30+31+30+31+31+30+31),
	DAY*(31+29+31+30+31+30+31+31+30+31+30)
};
long kernel_mktime(struct tm * tm)
{
	long res;
	int year;
	year = tm->tm_year - 70;
/* magic offsets (y+1) needed to get leapyears right.*/
	res = YEAR*year + DAY*((year+1)/4);
	res += month[tm->tm_mon];
/* and (y+2) here. If it wasn't a leap-year, we have to adjust */
	if (tm->tm_mon>1 && ((year+2)%4))
		res -= DAY;
	res += DAY*(tm->tm_mday-1);
	res += HOUR*tm->tm_hour;
	res += MINUTE*tm->tm_min;
	res += tm->tm_sec;
	return res;
}