A semiconductor processing apparatus is described. The semiconductor processing apparatus includes a gas supply system. The gas supply system has at least one gas supply unit including a process gas source, a gas distribution manifold having an annular gas distribution conduit provided with an inlet and valved outlets; and a gas supply conduit fluidly connecting the process gas source to the inlet of the gas distribution manifold. The semiconductor processing apparatus also includes a plurality of reactors, each fluidly connected to a respective valved outlet of the gas distribution manifold, such that process gas from the process gas source of the at least one gas supply unit is selectively suppliable to a respective reactor via the gas supply conduit, the gas distribution manifold, and a respective valved outlet. A method of providing a plurality of reactors with process gas is also described.