  We prove a general upper bound on the tradeoff between time and space that
suffices for the reversible simulation of irreversible computation. Previously,
only simulations using exponential time or quadratic space were known.
  The tradeoff shows for the first time that we can simultaneously achieve
subexponential time and subquadratic space.
  The boundary values are the exponential time with hardly any extra space
required by the Lange-McKenzie-Tapp method and the ($\log 3$)th power time with
square space required by the Bennett method. We also give the first general
lower bound on the extra storage space required by general reversible
simulation. This lower bound is optimal in that it is achieved by some
reversible simulations.
