P0 - is used in external rom mode as a multiplex address and data bus, and therefore has no internal pullups. So to use P0 as a port, external pullups should be connected to +Vcc othervise you could have problems reading the port. When writing 1s to the port, P0 pins are in fact floating if there is no pull up connected. When using as a dedicated input connected to a TTL or CMOS compatible driver, no pull-ups are needed. When using a bidirectional transfer, you should connect external pullups (from 1 - 100 k ohms).