From the FCOM;
S Speed
Minimum speed at which the slats may be retracted at takeoff.
In approach, used as a target speed when the aircraft is in CONF 1.
Represented by "S" on the PFD airspeed scale.
Equal to about 1.23 VS of clean configuration.
The FAC computes :
-The minimum and maximum speeds :
-VSW (stall warning)
-VLS
-VFE and VFE for the next configuration
-VLE
-VMO/MMO
The maneuvering speeds :
-Green Dot Speed
-S speed
-F speed
So, the way I read it, it isn’t the FMGC – it’s the FAC that computes and displays on the speedtape (and the speedtape speeds can (and often do) differ slightly from the FMGC Appr page) and thatGreen Dot, F and S are not protection speeds but target manoeuvring speeds.
Personally I cannot see the point of flying below the calculated manoeuvre speed.