定时器中断问题

#include

#include
***it wela=P2^6;
***it dula=P2^7;
#define uint unsigned int
#define uchar       unsigned char
uchar code table[]=
      {0x3f,0x06,0x5b,0x4f,0x66,
      0x6d,0x7d,0x07,       0x7f,0x6f};
uchar code wed[]=
      {0xfe,0xfd,0xfb,0xf7,
      0xef,0xdf,0xbf,0x7f,
      };
uint tt,b;
void delay (uint c);
void main()
{
            TMOD=0x01;
            TH0=(65536-50000)/256;
            TL0=(65536-50000)%256;
            EA=1;//¿ª×ÜÖÐ¶Ï
            ET0=1;//¿ªÆô¶¨Ê±Æ÷0ÖÐ¶Ï
            TR0=1;//Æô¶¯¶¨Ê±Æ÷0ÖÐ¶Ï
         wela=1;
         P1=0x55;
         wela=0;
            dula=1;
            PO=0x3f;
            dula=0;
               if(tt==20);
               { tt=o;
               b++;
               if(b==9)
               b=0;
            dula=1;
            P0=table[b];
               dula=0;
               }



}

void  exter0() interrupt 1
{ TH0=(65536-50000)/256;
      TL0=(65536-50000)%256;
      tt++;
}
为什么位选可以按照要求2,4,6,8亮，位选不会按照0到9之间变化，如果用for语句则又可以实现定时器延迟的效果，灯也会亮？