Linux 0.11启动时间的计算函数

ukanxpvlee

　　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;}