Methods and systems for transmission control are provided. In one example, a transmission system operating method includes maintaining a displacement set-point of a variable displacement hydraulic motor in a hydrostatic assembly within a working zone, where at least a portion of a boundary of the working zone is determined based on a kinematic constraint of the hydrostatic assembly. the transmission system includes the hydrostatic assembly that includes the variable displacement hydraulic motor that is hydraulically coupled in parallel with a variable displacement hydraulic pump and a gearbox mechanically coupled to the hydrostatic assembly and including one or more clutches.