Lua instruction is a 32 bits double word, and there are:

a fixed 6 bits op-code.
8 bits A operation
9 bits B operation
9 bits C operation

26 bits Ax operation
18 bits Bx operation
18 bits sBx operation

There are the following compositions:

C B A OP (9 9 8 6)
Ax     OP (26 6)
Bx  A Op (18 8 6)
sBX A Op (18 8 6)

For example, a = b + 1, instruction is 0x0040400D

(cast(OpCode, ((0x0040400D)>>POS_OP) & MASK1(SIZE_OP,0)))

-> (0x0040400D >> 0) & ((~((~(Instruction)0)<<(6)))<<(0))
-> 0x0040400D & 0x3F
-> 0x0D

So, GET_OPCODE(instruction) is 0x0D = OP_ADD

The SET_OPCODE is REVERSE work macro. If you can't understand that macro, please reference your C language textbooks.

貌似还是不是很懂。。请指教``