新手 求大神帮忙解释一下代码

按键控制图形输出
#include <reg52.h>
#include <intrins.h>
#define uchar unsigned char
#define uint unsigned int
uchar code M[3][8]=
{
        {0x00,0x7e,0x7e,0x7e,0x7e,0x7e,0x7e,0x00},
        {0x00,0x38,0x44,0x54,0x44,0x38,0x00,0x00},
        {0x00,0x20,0x30,0x38,0x3c,0x3e,0x00,0x00}
};
uchar i,j;
void main()
{
        P0 = 0xff;
        P1 = 0xff;
        TMOD = 0x01;
        TH0  = (65535-2000)/256;
        TL0  = (65535-2000)%256;
        IT0 = 1;
        IE  = 0x83;
        i   = 0xff;
        while(1);
}
void Key_Down() interrupt 0
{
        P0 = 0xff;
        P1 = 0x80;
        j = 0;
        i = (i+1)%3;
        TR0 = 1;
}
void Show_Dot_Matrix() interrupt 1
{
        TH0  = (65535-2000)/256;
        TL0  = (65535-2000)%256;
        P1 = _crol_(P1,1);
        P0 = ~M[i][j];
        j=(j+1)%8;       
}

硬件图