#include <REG51.H>

void delay(void) {
    unsigned int i;
    for (i = 0; i < 500; i++);
}

void main(void) {
    P0 = P1 = P2 = P3 = 0x33;
    while (1) {
        P0 = ~P0;
        P1 = ~P1;
        P2 = ~P2;
        P3 = ~P3;
        delay();
    }
}
