This type represents an array of time steps of simulation data. In any simulation, for a given IOArray, only one of the three step stores will be used. For (very strong) efficiency reasons this cannot be implemented as a disjoint union: the code that reads and writes IOArray elements will access the appropriate store. Truthtable simulations use FDataStep everywhere. Normal simulations use the UInt32 or BigInt store according to the size of the relevant bus. The uint32 and bigint stores are REGIONS OF SHARED SLABS, not arrays of their own: step s of this IO lives at `slab[StepBase + s]`, and `StepLength` steps belong to it. FastCreate's arena packs every step region of a build into a few large slabs (under both runtimes), so a design has dozens of step allocations rather than one per output port, and a port's data is named by an integer offset. Always go through the members below - indexing a slab without adding StepBase reads another port's data, which no bounds check will ever catch.