os_cfg.h

#include "reg51.h"  
#define TIME_PER_SEC 200    //定义任务时钟频率，200Hz  
#define CLOCK 22118400    //定义时钟晶振，单位Hz  
#define MAX_TASK 4      //定义任务数量   
//函数变量声明，在需要用以下函数或变量的文件中包含此头文件即可  
extern void task0(void);  
extern void task1(void);  
extern void task2(void);  
extern void task3(void);  
extern unsigned char task_delay[MAX_TASK];  
extern void run(void (*ptask)());  
extern void os_timer0_init(void);  

os_c.c

#include "os_cfg.h"  
unsigned char task_delay[MAX_TASK];  //定义任务延时量变量  
//定时器0初始化  
void os_timer0_init(void)  
{  
    unsigned char i;  
   for(i=0;i<MAX_TASK;i++) task_delay[i]=0;  
    TMOD = (TMOD & 0XF0) | 0X01;    //定时器 0工作在模式 1，16Bit 定时器模式   
    TH0 = 255-CLOCK/TIME_PER_SEC/12/256;    
//CRY_OSC,TIME_PER_SEC在 os_cfg.h中定义  
    TL0 = 255-CLOCK/TIME_PER_SEC/12%256;    
   TR0 =1;   
 ET0 =1;         //开启定时器和中断  
}  

// 系统 OS定时中断服务  
void os_timer0(void) interrupt 1  
{  
    unsigned char i;  
    TH0 = 255-CLOCK/TIME_PER_SEC/12/256;  
    TL0 = 255-CLOCK/TIME_PER_SEC/12%256;         
    for(i=0;i<MAX_TASK;i++) if(task_delay[i]) task_delay[i]--;   
//每节拍对任务延时变量减1 ，减至 0  后，任务就绪。    
}   

//指向函数的指针函数  
void run(void (*ptask)())  
{   
   (*ptask)();   
} 

main.c

#include "os_cfg.h"   
#define TASK_DELAY0 TIME_PER_SEC/1      //任务执行频度为1Hz  
#define TASK_DELAY1 TIME_PER_SEC/2      //任务执行频度为2Hz  
#define TASK_DELAY2 TIME_PER_SEC/10      //任务执行频度为10Hz  

#define TASK_DELAY3 TIME_PER_SEC/20     //任务执行频度为20Hz  
void (* code task[])() = {task0,task1,task2,task3};  //获得任务PC指针  

sbit LED0 = P1^0;  //演示用 LED接口定义  
sbit LED1 = P1^1;  
sbit LED2 = P1^2;  
sbit LED3 = P1^3;  

/*main主函数*/  
void main(void)  
{  
    unsigned char i;    
    os_timer0_init();   //节拍发生器定时器初始化  
    EA = 1;       //开总中断  

   while(1)  
   {  
     for(i=0;i<MAX_TASK;i++)   
   if (task_delay[i]==0) {run(task[i]); break;}  //就绪任务调度  
   }  //上一行 break有特殊作用，详细解释见后文  
}  

void task0(void)  //任务 0  
{     
    LED0 = !LED0;    
 task_delay[0] = TASK_DELAY0;  
}  

void task1(void)  //任务 1  
{     
    LED1 = !LED1;    
 task_delay[1] = TASK_DELAY1;  
}  

void task2(void)  //任务 2  
{     
    LED2 = !LED2;    
 task_delay[2] = TASK_DELAY2;  
}  

void task3(void)  //任务内分段设计   
{     
      static unsigned char state=0;  //定义静态局部变量  
 switch (state)  

{  
  case 0:  
   LED3 = !LED3;    
   state = 1;  
   task_delay[3] = TASK_DELAY3;  
   break;  

  case 1:  
   LED3 = !LED3;    
   state = 2;  
   task_delay[3] = TASK_DELAY3*2;  
   break;  

  case 2:  
   LED3 = !LED3;    
   state = 0;  
   task_delay[3] = TASK_DELAY3*4;  
   break;  

  default:  
   state = 0;  
   task_delay[3] = TASK_DELAY3;  
   break;  
 }   
}