The Art of Assembly Language, 2nd Edition

3.7 Register Type Coercion

You can also cast a register to a specific type using the type coercion operator. By default, the 8-bit registers are of type byte, the 16-bit registers are of type word, and the 32-bit registers are of type dword. With type coercion, you can cast a register as a different type as long as the size of the new type agrees with the size of the register. This is an important restriction that does not exist when applying type coercion to a memory variable.

Most of the time you do not need to coerce a register to a different type. As byte, word, and dword objects, registers are already compatible with all 1-, 2-, and 4-byte objects. However, there are a few instances where register type coercion is handy, if not downright necessary. ...