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There is no need to differentiate because 8 is not representable in a 4-bit word.1000, if 2's complimented, gives back 1000.How can we differentiate between +8 and -8 ?
By definition of the representation. There may be several equivalent starting points, but one is to postulate that the word represents the number .Why is it, that 1000 is always taken as -8 and the greatest positive number that can be stored is 7 and not 8?
It is 0 and is found using the standard procedure.And what about the 2's compliment of zero?
You are probably familiar with the Wkipedia article about two's complement and especially the section about the most negative number. In fact, this article could be given a more rigorous structure from the mathematical viewpoint (but this would probably make it less readable). It would be nice to give a sequence of definitions and theorems that establish connections between equivalent concepts and operations.