1: When any REX prefix is used, SPL, BPL, SIL and DIL are used. Otherwise, without any REX prefix AH, CH, DH and BH are used. Legacy Prefixes Each instruction can have up to four prefixes. Sometimes a prefix is required for the instruction while it loses it's original meaning (i.e. a 'mandatory prefix'). The following prefixes can be used, the order does not matter: Prefix group 1 0xF0: LOCK prefi