How can a register detect that the value it containes is either data, or the address of data?

As far as I know, when a data is too long, the address of a data is written to a register. When it is not long for a register, the real value of the data can be written to a register. How can a register detect that the value in the register is whether a the value of a data o开发者_Go百科r the address of a data? If it could not detect, it would process with the wrong value.

I have found a related question ( How is data, address and Instruction differentiated in Processor/Register/memory?). However the answers are not too satisfied. Are there more satisfied answers? (Probably, it will be too complicated )

Thanks.

This is what I want to learn. http://cs.oberlin.edu/~jdonalds/317/IntelEncoding.htm