When in 64-bit mode, operand size determines the number of valid bits in the destination general-purpose register:
• 64-bit operands generate a 64-bit result in the destination general-purpose register.
• 32-bit operands generate a 32-bit result, zero-extended to a 64-bit result in the destination general-purpose
register.
• 8-bit and 16-bit operands generate an 8-bit or 16-bit result. The upper 56 bits or 48 bits (respectively) of the
destination general-purpose register are not modified by the operation. If the result of an 8-bit or 16-bit
operation is intended for 64-bit address calculation, explicitly sign-extend the register to the full 64-bits.