case "UXTH":
			if a.Type == obj.TYPE_MEM {
				return errors.New("invalid shift for the register offset addressing mode")
			}
			a.Reg = arm64.REG_UXTH + Rnum
		case "UXTW":
			// effective address of memory is a base register value and an offset register value.
			if a.Type == obj.TYPE_MEM {
				a.Index = arm64.REG_UXTW + Rnum
			} else {
				a.Reg = arm64.REG_UXTW + Rnum
			}
		case "UXTX":