The size of the cpu registers don't matter here. What matters is that 'int' is a bigger type than uint8_t and smaller than uint64_t: the resulting unexpected be

The size of the cpu registers don't matter here. What matters is that 'int' is a bigger type than uint8_t and smaller than uint64_t: the resulting unexpected behaviour is mandated by the C standard regardless of the register size of the cpu (for instance both 32-bit i386 and 64-bit x86_64 will behave the same, because both typically have 32 bit 'int'). More generally, reasoning about this kind of thing by thinking about the CPU leads you down the wrong path -- modern C as implemented by compilers today is an implementation of a standardized language, and they behave as that standard says, which is not always as you might expect from the hardware behaviour. (Simple example: left shifting of negative numbers does a simple straightforward thing on practically all hardware, but the C spec says it's UB and if you write code that assumes the h/w behaviour you'll get bitten.)