There is by now so many '51 derivatives that for the manufacturers to include every bit of information in every datasheet has been deemed impractical. I am not trying to start a discussion on that subject, we can do nothing about it, flaming the manufacturers will not help. The point is that with the datasheets as they are, basic '51 information must be acquired from elsewhere. For a thorough discussion of the basic function of the '51 including ALE and the rest of the signals read the 3 80C51_FAM.. documents. They are at http://www.semiconductors.philips.com/products/...other.html.

If everything else fails, read the manual

Erik