256 char ASCII is called "8-bit", "high", or "extended" ASCII. So, pure (7-bit) ASCII is the only thing you can hold in a "one-byte UTF-8 array". The 8th bit (or the first, depending on how you see it) is used to mark multibyte characters, i.e. any character that is not ASCII. So you only can represent 128 possible symbols into a UTF-8 character with length of one byte. In particular, UTF-8 maps these to ASCII (or, US-ASCII) characters, and the byte starts with a 0 bit (In other words, you cannot encode high ASCII into one UTF-8 byte.) For ALL other characters (no matter), the first bit in any (multi-)byte is set to 1. That's why it is easy to scan for length of ASCII chars in UTF-8, but not for any others.