简易计数器LED显示C源程序！请高手注释一下！万分感谢！

#include<reg51.h>
#include<intrins.h>
#include<math.h>

sbit a=P0^0;
sbit b=P0^1;
sbit c=P0^2;
sbit d=P0^3;
sbit e=P0^4;
sbit f=P0^5;
sbit g=P0^6;
sbit h=P0^7;

sbit w1=P1^0;
sbit w2=P1^1;
sbit w3=P1^2;
sbit w4=P1^3;

sbit d1=P2^0;
sbit d2=P2^1;
sbit d3=P2^2;
sbit d4=P2^3;
sbit d5=P2^4;
sbit d6=P2^5;
sbit d7=P2^6;
sbit d8=P2^7;

sbit s1=P3^2;
sbit s2=P3^3;
sbit s3=P3^4;
sbit s4=P3^5;

sbit beee=P3^7;



unsigned int num=0;

unsigned char disp0=0,disp1=1,disp2=2,disp3=3;

unsigned char code ledd[11]={0xc0,0xf9,0xa4,0xb0,0x99,0x92,0x82,0xf8,0x80,0x90,0xff};//0~9,?

void delay(unsigned long i)
{
for(;i!=0;i--)
{
_nop_();
}
}


void timer0() interrupt 1

{
static unsigned char disp_num=0;
TL0=00;
TH0=0xf8;//2ms@12MHz
if(disp_num==0)
{
w1=0;
w2=1;
w3=1;
w4=1;
P0=ledd[disp0];
}
if(disp_num==1)
{
w1=1;
w2=0;
w3=1;
w4=1;
P0=ledd[disp1];
}
if(disp_num==2)
{
w1=1;
w2=1;
w3=0;
w4=1;
P0=ledd[disp2];
}
if(disp_num==3)
{
w1=1;
w2=1;
w3=1;
w4=0;
P0=ledd[disp3];
}
if(disp_num>=3)disp_num=0;
else disp_num++;



}


void main ()
{
TMOD=0x21;
PCON=0x80;

IE=0x82;

TH1=0xfd;//e8;
TL1=0xfd;//;
TH0=0xfc;//2ms@12MHz
TL0=00;
SCON=0x40;
TR0=1;
TR1=1;
while(1)
{
disp3=num%10;
disp2=(num/10)%10;
disp1=(num/100)%10;
disp0=num/1000;
if(num<9999)num=num+2;
else num=0;
SBUF=disp3;
P2=0x55;
delay(4000);
P2=0xaa;
delay(4000);
}
}