that when you write 0xFF to a port, the port is configured to be an input port.
bad language, should read
that when you write 0xFF to a port, the port can be used as an input port.
Go to links, load the Intel "bible". Read about ports in the hardware description. In the printed version the ports are described IN DETAIL on pages 3-6 through 3-8, if the download version differs search fo PORT STRUCTURES AND OPERATION

Also, I've read, the ports are LATCHED and sometimes the value you read from the latch may not be equal to the value at the pin
see above reference for Read-Modify-Write Feature.

Have fun,

Erik