Quoting from the datasheet for the P89C66x: Port 0 is an open-drain .... P0 ... also [access] external memory. In this case it uses strong pullups when emitting 1s.

Again, everyting is ambigous if you do not specify what derivative you are talking about. The above statement refer to the P89C66x and many more derivatives, I can not say if it apply to the one you are using.

As said in 1/2 the previous posts: READ THE DATASHEET.

Erik