Windows ntdll and ntoskrnl export a multibyte capable version of toupper (not tolower though!) The internal version is kept as it is, because the multibyte version requires unicode tables to be set up and doesn't support IRQL > APC_LEVEL. Why though would toupper (but not tolower) support raw, undecoded multibyte characters, you might ask. Well I don't know, but someone at MS must have decided that this is a good idea, and winetests show this is how it behaves.