[int_19h]

You can implement byte addressing on any architecture, of course. That's what I meant by "the C memory model doesn't have to reflect the underlying hardware architecture". But as you point out yourself, this requires pointers which are basically not raw hardware addresses, and which are more expensive to work with, because they require the compiler to do the same kind of stuff it has to do for bit fields. So the natural implementation - with no unexpected perf gotchas - tends towards pointers as raw hardware addresses, and thus char as the smallest unit those can address.