The FA, N2000, N2020, and FG) work in program mode with AI
(and AIS, AF, E, P, etc.) lenses - AIS lenses are not required
for program mode to work on these Nikon bodies. Shutter priority
mode also works with the FA with AI lenses. The AIS lens diaphram
mechanism does simplify the process of getting correct exposures
in P and S modes, but at least the FA (maybe the others?) has
provision for reading through the stopped-down diaphram and making
corrections to the shutter speed just before exposure, so that P and
S modes produce correct exposure with AI lenses.
Hope This Helps