各位高手，本人是刚接触单片机编程，关于51单片机的中断这块还不是很懂，下面有个小程序，望哪位高手能帮忙分析下，谢谢！
#include<reg51.h>

#define uchar unsigned char
#define uint unsigned int

sbit dula=P2^6;
sbit wela=P2^7;
sbit beep=P2^3;

unsigned int pp;
unsigned char code table[]={0x3f,0x06,0x5b,0x4f,0x66,0x6d,0x7d,
                        0x07,0x7f,0x6f,0x77,0x7c,0x39,0x5e,0x79,0x71};
uchar j,k,l=255;
uchar a1,a0;
uchar shijian,deng=2;
void delay(unsigned int i)
{
  for(j=i;j>0;j--)
    for(k=125;k>0;k--);
}

void display(unsigned char sh_c,unsigned char g_c) //此语句是数码管显示用的，可以不理它
{
   dula=0;
   P0=table[sh_c];
   dula=1;
   dula=0;
   
   wela=0;
   P0=0xfe;
   wela=1;
   wela=0;
   delay(5);
   
   P0=table[g_c];
   dula=1;
   dula=0;
   
   P0=0xfd;
   wela=1;
   wela=0;
   delay(5);
}

void main()
{
    TMOD=0x01;  //定时/计数器工作方式1
    TR0=1;    //TR0置1时，T0开始工作
    TH0=(65536-46080)/256;// 由于晶振为11.0592,故所记次数应为46080，计时器每隔50000微秒发起一次中断。
    TL0=(65536-46080)%256;//46080的来历，为50000*11.0592/12

    ET0=1;     //定时/计数器T0中断允许位
    EA=1;     //CPU开中断（即EA=1） //此处开中断，程序是先执行void time0() interrupt 1   还是执行下面while里面的内容？  
   
    while(1)
    {   
        if(shijian==0)  //此处shijian变量没有赋初值啊？怎么判断等不等于0？
        {
            switch(deng)
            {
                case 0: shijian=50;          //红灯
                        deng=1;
                        break;
                case 1: shijian=15;          //黄灯
                        deng=2;
                        break;
                case 2: shijian=50;          //绿灯
                        deng=3;
                        break;
                case 3: shijian=15;          //黄灯
                        deng=0;
                        break;
            }
        }
        if(pp==20)
        {    pp=0;
            shijian--;
        }
        a0=shijian%10;
        a1=shijian/10;
        display(a1,a0);
        switch(deng)
        {
            case 1: P1=0X00;             //红灯
                    break;
            case 2:             //黄灯
                    if(shijian%2==0)
                    P1=0x00;
                    if(shijian%2==1)
                    P1=0xff;
                    break;
            case 3:         //绿灯
                    if(shijian%2==0)
                    P1=0x55;
                    if(shijian%2==1)
                    P1=0xaa;
                    break;
            case 0:    if(shijian%2==0)      //黄灯
                    P1=0x00;
                    if(shijian%2==1)
                    P1=0xff;
                    break;   
        }
    }
}

void time0() interrupt 1       //定时/计数器工作方式1
{    TH0=(65536-46080)/256;       //初值
    TL0=(65536-46080)%256;
    pp++;
}