Operating system (1) for rotatably controlling retraction and deployment of an architectural covering. The operating system includes a drive pulley wheel (3) having a circumferential rim (5), a length of looped drive cord (9) operatively engaged with the pulley wheel (3) about its circumferential rim (5) and having depending portions on opposite sides of the pulley wheel (3), a cover (7) co-extensive with a portion of the circumference of the pulley wheel (3) and at least one stopper (19) attached to a portion of the length of the drive cord (9) and adapted to engage the cover (7) to stop rotation upon a predetermined amount of rotation of the pulley wheel (3). The pulley wheel is provided with a cavity (17) on its toothed circumference for receiving the at least one stopper (19) to prevent it from engaging the cover (7).