Partial register stalls occurred when you assigned something to say `eax`, then read `ax`, or vice versa - if you wrote `ax` then read `eax`. If you wrote to `ax` then read `ax`, there was no stall. This was due to the register renaming implementation.

AFAIK, this was only an issue in some older CPUs and isn't a problem today.

32-bit `int` is still cheaper than 16-bits though, because 16-bit instructions require an operand size override prefix (0x66), or address size override prefix (0x67), or both. Technically, these aren't "16 bit prefixes" - if the machine was running in 16-bit protected mode, then you would need those prefixes to use 32-bit instructions and the non-prefixed ones would be 16-bit and thus cheaper, so `int` would be better as 16-bits in 16-bit protected mode - though this mode is essentially unused today, so for all intents and purposes the prefixes are used to issue 16-bit instructions and 16-bits is more expensive (in code size, i-cache usage, which may impact performance).