Mostly. One difference is that over/underflow is signaled in `CF` (carry bit), and that's used to extend arithmetic: to add a pair of 128-bit numbers you `ADD` the lower 64 bits, advance to the higher 64 bits, and `ADC` those (the "C" means it uses the carry bit). And `CF` might again be set afterward indicating that it overflowed 128 bits. (This is also why Intel architecture orders bytes/words the way it does.) That it otherwise behaves as modular is in support of this, not in support of a mathematical law, and the behavior might differ on other architectures (e.g. PPC, or if someone somewhere still has a processor using 1s- complement). Which is the most important point: the Report specifies `Integral` types other than `Integer` to behave like the machine type, and you're basically assuming x86-64 (_probably_ also AArch64 but i haven't studied that one, just assuming it's 2s-complement) if you assume either modular arithmetic or any other behavior. The only promise you get is "matches the underlying hardware".