32位cpu有实模式、保护模式和虚拟-86模式,64位有实模式、保护模式和长模式.
虚拟-86模式呢?
因为有传统模式（但是只能支持16bit/32bit长的opcode，因此不能在64位环境下执行）：
https://en.wikipedia.org/wiki/X86-64#Operating_modes
Legacy mode:
the mode used by 16-bit ('protected mode' or 'real mode') and 32-bit operating systems. In this mode, the processor acts like a 32-bit x86 processor, and only 16-bit and 32-bit code can be executed. Legacy mode allows for a maximum of 32 bit virtual addressing which limits the virtual address space to 4 GB.1(p24)(p118) 64-bit programs cannot be run from legacy mode.