The Art of Assembly Language, 2nd Edition

8.2 Operating on Different-Size Operands

Occasionally you may need to do some computation on a pair of operands that are not the same size. For example, you may need to add a word and a double word together or subtract a byte value from a word value. The solution is simple: just extend the smaller operand to the size of the larger operand and then do the operation on two similarly sized operands. For signed operands, you would sign extend the smaller operand to the same size as the larger operand; for unsigned values, you zero extend the smaller operand. This works for any operation, although the following examples demonstrate this for the addition operation.

To extend the smaller operand to the size of the larger operand, use a sign extension ...