If you have two questions, it's best to make two posts.


The value increments Modulo-256

This is clearly stated in the description of the INC instruction in the 80C51 family programmer’s guide:

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

Together with the following 2 documents, this forms what is often known as "the bible" for the 8051:

http://www.semiconductors.philips.com/acrobat/v...ARCH_1.pdf
http://www.semiconductors.philips.com/acrobat/v...WARE_1.pdf

You need to download & study those three documents: they are fundamental to any understanding of the 8051 and its derivatives.

You can't build a good house without solid foundations!

See also: http://www.8052.com/forum/read.phtml?id=66882