You don't need FM(G)S or FACs or PRIMs to calculate your stalling speed. Total pressure, static pressure, AoA and ADC to make some sense of it all are quite enough and that's what drives low speed cue on Q400. It's interesting to watch it bounce in turbulent approach or climb in turns, just as Alpha max red strip does on A320.
Basically, low speed awareness tapes are just alpha information superimposed on speed tape. If you are trying to fly 110 ton jet with 100t approach speed, you'll need higher AoA to fly it down the glide, that's how ADC knows you are flying closer to stall.