hello dharmik….
you know basic about port 0 so as we know that port 0 has multiplex address and data bus so
if we not connect external pullup than it will sink current from both one from inside supply as well as outer vcc supply and it would be a large current flow through internal transistor so it will damage port so thats why…..we use external pull up resistor is used in port 0 rathar than other port because other port not have any multiplexing add or data bus.
