// select row
digitalWrite(a, (row & 0x01));
digitalWrite(b, (row & 0x02));
digitalWrite(c, (row & 0x04));
digitalWrite(d, (row & 0x08));
// select row
digitalWrite(a, (row & 0x01));
digitalWrite(b, (row & 0x02));
digitalWrite(c, (row & 0x04));
digitalWrite(d, (row & 0x08));