Opcode 最多3字节，最少1字节，最重要的组成部分
1.如歌区分指令定长还是变长？
1.最关键的表格 ---------Intel白皮书（“英特尔® 64 和 IA-32 架构开发人员手册”） A.2.4.1
2.Zz表示法
其中 Eb ,lb或者Eb,Gb 相搭的则为变长指令，反之则为定长指令！
E A ModR/M byte follows the opcode and specifies the operand. The operand is either a general-purpose
register or a memory address. If it is a memory address, the address is computed from a segment register
and any of the following values: a base register, an index register, a scaling factor, a displacement.
G The reg field of the ModR/M byte selects a general register (for example, AX (000)).
E,G后面都跟有ModR/M byte selects