今天，我在看AT89S51的datasheet，看了下面的文字，我还不是很清楚，什么时候，P0口需要外加上拉电阻，什么时候又不需要呢，请高手解答，不胜感激呀！！！
Port 0 can also be configured to be the multiplexed low-order address/data bus during accesses
to external program and data memory. In this mode, P0 has internal pull-ups.
Port 0 also receives the code bytes during Flash programming and outputs the code bytes dur-
ing program verification. External pull-ups are required during program verification
