I think a "field programmable" device is distinct from a "mask programmable" device. The latter is potentially much simpler (no RAM, potentially a "sea of muxes

I think a "field programmable" device is distinct from a "mask programmable" device. The latter is potentially much simpler (no RAM, potentially a "sea of muxes" (or nands) while the former needs RAM, shift register logic, additional I/O and so on.