This type represents an array of time steps of simulation data. In any simulation, for a given IOArray, only one of the three 'Step' arrays will be used. For (very strong) efficiency reasons this cannot be implemented as a disjoint union: the code that reads and writes IOArray array elements will access the appropriate array. Truthtable simulations use FDataStep everywhere. Normal simulations use UInt32step or BigIntStep according to the size of the relevant bus.