As to noise: if an unused port pin read an arbitrary value WHO CARES, your program should ignore the state of unused pins.

As to the misnomers "input mode" and "output mode":
The key is that the ports are pseudo-bidirectional.
* The '51 pins (in "standard" derivatives) are always "output pins".
* So, there is no such thing as "output mode" or "input mode".
* When they "output a high" they are capable of reading input.

So, while there is no such thing as "output mode" and "input mode" to make a port input capable the port latch must contain a '1' so the port output a 1.

The above may be confusing, but there should be no confusion if you just read about the ports in "the bible" chapter 3


Erik

here are the links to "the bible"
Chapter 1
http://www.semiconductors.philips.com/acrobat/v...ARCH_1.pdf

chapter 2
http://www.semiconductors.philips.com/acrobat/v...UIDE_1.pdf


chapter 3
http://www.semiconductors.philips.com/acrobat/v...WARE_1.pdf