  Landauer principle describes the minimum heat produced by an
information-processing device. Recently a new term has been included in the
minimum heat production: it's called conditional entropy and takes into account
the microstates content of a given logic state. Usually this term is assumed
zero in bistable symmetric systems thanks to the strong hypothesis used to
derive Landauer principle. In this paper we show that conditional entropy can
be nonzero even for bistable symmetric systems and that it can be expressed as
the sum of three different terms related to the probabilistic features of the
device. The contribution of the three terms to conditional entropy (and thus to
minimum heat production) is then discussed for the case of bit-reset.
