【2021.01.09】定长指令与变长指令

x86指令编码(硬编码)的结构

1. opcode最少1个字节，最多3个字节。
2. opcode是指令中最重要的组成部分。
3. 前缀指令只能影响自己，而opcode、ModR/M、SIB决定了整条指令的长度。
4. 有没有ModR/M是由opcode决定的，有没有SIB是由ModR/M决定的。

定长指令、变长指令

定长指令：它的长度永远没有变化，只要opcode的长度确定了，那么它的长度就确定了。

变长指令：仅仅通过opcode是没有办法确定长度的。

如何区分指令定长还是变长？

opcode后面没有ModR/M，该指令就是定长指令。如果opcode后面紧跟ModR/M，该指令就是变长指令。

Intel白皮书中的表格

Zz表示法(一个大写字母紧随一个小写字母)

Codes for Addressing Method

字母	说明
A	Direct address: the instruction has no ModR/M byte; the address of the operand is encoded in the instruction. No base register, index register, or scaling factor can be applied (for example, far JMP (EA)).
B	The VEX.vvvv field of the VEX prefix selects a general purpose register.
C	The reg field of the ModR/M byte selects a control register (for example, MOV (0F20, 0F22)).
D	The reg field of the ModR/M byte selects a debug register (for example, MOV (0F21,0F23)).
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.
F	EFLAGS/RFLAGS Register.
G	The reg field of the ModR/M byte selects a general register (for example, AX (000)).
H	The VEX.vvvv field of the VEX prefix selects a 128-bit XMM register or a 256-bit YMM register, determined by operand type. For legacy SSE encodings this operand does not exist, changing the instruction to destructive form.
I	Immediate data: the operand value is encoded in subsequent bytes of the instruction.
J	The instruction contains a relative offset to be added to the instruction pointer register (for example, JMP (0E9), LOOP).
L	The upper 4 bits of the 8-bit immediate selects a 128-bit XMM register or a 256-bit YMM register, determined by operand type. (the MSB is ignored in 32-bit mode).
M	The ModR/M byte may refer only to memory (for example, BOUND, LES, LDS, LSS, LFS, LGS, CMPXCHG8B).
N	The R/M field of the ModR/M byte selects a packed-quadword, MMX technology register.
O	The instruction has no ModR/M byte. The offset of the operand is coded as a word or double word (depending on address size attribute) in the instruction. No base register, index register, or scaling factor can be applied (for example, MOV (A0–A3)).
P	The reg field of the ModR/M byte selects a packed quadword MMX technology register.
Q	A ModR/M byte follows the opcode and specifies the operand. The operand is either an MMX technology 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, and a displacement.
R	The R/M field of the ModR/M byte may refer only to a general register (for example, MOV (0F20-0F23)).
S	The reg field of the ModR/M byte selects a segment register (for example, MOV (8C,8E)).
U	The R/M field of the ModR/M byte selects a 128-bit XMM register or a 256-bit YMM register, determined by operand type.
V	The reg field of the ModR/M byte selects a 128-bit XMM register or a 256-bit YMM register, determined by operand type.
W	A ModR/M byte follows the opcode and specifies the operand. The operand is either a 128-bit XMM register, a 256-bit YMM register (determined by operand type), 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, and a displacement.
X	Memory addressed by the DS:rSI register pair (for example, MOVS, CMPS, OUTS, or LODS).
Y	Memory addressed by the ES:rDI register pair (for example, MOVS, CMPS, INS, STOS, or SCAS).

Codes for Operand Type

字母	说明
a	Two one-word operands in memory or two double-word operands in memory, depending on operand-size attribute (used only by the BOUND instruction).
b	Byte, regardless of operand-size attribute.
c	Byte or word, depending on operand-size attribute.
d	Doubleword, regardless of operand-size attribute.
dq	Double-quadword, regardless of operand-size attribute.
p	32-bit, 48-bit, or 80-bit pointer, depending on operand-size attribute.
pd	128-bit or 256-bit packed double-precision floating-point data.
pi	Quadword MMX technology register (for example: mm0).
ps	128-bit or 256-bit packed single-precision floating-point data.
q	Quadword, regardless of operand-size attribute.
qq	Quad-Quadword (256-bits), regardless of operand-size attribute.
s	6-byte or 10-byte pseudo-descriptor.
sd	Scalar element of a 128-bit double-precision floating data.
ss	Scalar element of a 128-bit single-precision floating data.
si	Doubleword integer register (for example: eax).
v	Word, doubleword or quadword (in 64-bit mode), depending on operand-size attribute.
w	Word, regardless of operand-size attribute.
x	dq or qq based on the operand-size attribute.
y	Doubleword or quadword (in 64-bit mode), depending on operand-size attribute.
z	Word for 16-bit operand-size or doubleword for 32 or 64-bit operand-size.