An operating system (100) for an architectural covering (102) is provided. The operating system (100) allows at least three modes of operation in order to extend, open or retract/close an architectural covering. A transmission (112) located between an input assembly (106) and an output drive member (13) can be selectively engaged to place the operating system (100) into one of the at least three modes of operation. The operating system includes a first drive section (164) with an input, a second drive section (165) with an output, and a control mechanism (110) arranged to selectively lock an element of the first and second drive sections to control movement of the output of the operating system (100) upon actuation of the input. A shift lock (500) is is able to restrict shifting operation of the operating system (100) from one operating mode to another.