关于HOT-51单片机数码管显示问题求助

#include <reg52.h>
#define uchar unsigned char
#define uint unsigned int
uint a,count;
uchar code table[]={
0x3f,0x06,0x5b,0x4f,
0x66,0x6d,0x7d,0x07,
0x7f,0x6f,0x77,0x7c,
0x39,0x5e,0x79,0x71,
0x76,0x79,0x38,0x38,0x3f};
uint i,j;
void delay(uint z)
{
uint x,y;
for(x=z;x>0;x--)
for(y=110;y>0;y--);
}
void main()
{
TMOD=0x01;
TH0=(65536-50000)/256;
TL0=(65536-50000)%256;
EA=1;
ET0=1;
TR0=1;


while(1)
{

if(a>20){

for(i=2,j=16;i<8 && j<21;i++,j++)
{
P2=i;
P0=table[j];

delay(1);
}
}

if(a<20){
for(i=0,j=0;i<8 && j<16;i++,j++)//为什么数码管只依次显示0-7，而不是0-F,我晕死了，好像逻辑上没错啊，有谁能给我一 把手？
{
P2=i;
P0=table[j];
if(i==8)
i=0;
if(j==16)
j=0;
delay(300);


}

}

}
}
void timer0() interrupt 1
{
TH0=(65536-50000)/256;
TL0=(65536-50000)%256;
count++;
if(count==4)
{
count=0;
a++;
}
}