read 8051 manual, some ports do not need pullups port0 does in order to read u have to out '1's to i/o lines, when u read it will read pins now if port has internal pullups u just have to pull that pin down to logic '0' otherwise it will read disconnected pin (open switch) as high. in case of port 0 u have to use 10k pull ups as its a open drain output.

check this out www.8052.com/users/bitmaster/p0.txt

abhishek