I assume you’re referring to the 64KiB allocation granularity? That’s actually not at all arbitrary: https://devblogs.microsoft.com/oldnewthing/20031008-00/?p=42...
> So if allocation granularity were finer than 64KB, a DLL that got relocated in memory would require two fixups per relocatable address: one to the upper 16 bits and one to the lower 16 bits.
…
> Forcing memory allocations at 64KB granularity solves all these problems.
It does, at the cost of silly limitations. A much nicer solution IMO would have been a way to ask for a 64kB aligned allocation for DLLs.
With 64 bits, my real objection is that it’s unnecessarily complicated. There’s some data structure that maps addresses to whatever is mapped at those addresses, and on Windows it also needs to track reserved memory, and it needs to do so at a different granularity. (For huge pages, additional granularities are needed.)