Whatever you write to P0 is what the 8052 will output on the corresponding lines. If you write 1s, the 8052 will raise the lines. If you write 0s, the 8052 will lower the lines.

However, it is necessary to write 1s to P0 before you use it as an input port. I.e., you must write 1s to all the pins you want to read before you read them. So first you'd write #0FFh to P0, then you'd read the value of P0.

Hope this helps,
Craig Steiner