An operating system for an architectural covering is provided. The operating system allows at least three modes of operation of an architectural covering. A transmission may be included between an input assembly and an output drive member, and the transmission may be selectively engaged to place the operating system into one of the at least three modes of operation. The operating system may include a first drive section including an input, a second drive section including an output, and a control mechanism arranged to selectively lock an element of the first and second drive sections to control movement of the output of the operating system upon actuation of the input. A shift lock is also disclosed herein. In use, the shift lock operates to restrict shifting operation of the operating system from one operating mode to another.