Opcode 最多3字節，最少1字節，最重要的組成部分
1.如歌區分指令定長還是變長？
1.最關鍵的表格 ---------Intel白皮書（“英特爾® 64 和 IA-32 架構開發人員手冊”） A.2.4.1
2.Zz表示法
其中 Eb ,lb或者Eb,Gb 相搭的則為變長指令，反之則為定長指令！
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