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Decision times of infinite computations

The decision time of an infinite time algorithm is the supremum of its halting times over all real inputs. The decision time of a set of reals is the least decision time of an algorithm that decides the set; semidecision times of semidecidable sets are defined similary. It is not hard to see that $ω_1$ is the maximal decision time of sets of reals. Our main results determine the supremum of countable decision times as $σ$ and that of countable semidecision times as $τ$, where $σ$ and $τ$ denote the suprema of $Σ_1$- and $Σ_2$-definable ordinals, respectively, over $L_{ω_1}$. We further compute analogous suprema for singletons.