The fancy word for assuming O(1) access and not needing to spill over into longer addressing modes is, I believe, "transdichotomous model" - https://en.wikipedia.org/wiki/Transdichotomous_model

> In computational complexity theory, and more specifically in the analysis of algorithms with integer data, the transdichotomous model is a variation of the random-access machine in which the machine word size is assumed to match the problem size.