86 CMP指令差异 [英] x86 CMP Instruction Difference

问题描述

问

什么是以下两个x86指令之间的（非平凡）的区别？

What is the (non-trivial) difference between the following two x86 instructions?

39 /r    CMP r/m32,r32   Compare r32 with r/m32
3B /r    CMP r32,r/m32   Compare r/m32 with r32

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背景

我要建一个Java汇编器，这将是我的编译器的中间语言被用来生产Windows的32可执行文件。

I'm building a Java assembler, which will be used by my compiler's intermediate language to produce Windows-32 executables.

目前我有以下code：

Currently I have following code:

final ModelBase mb = new ModelBase(); // create new memory model
mb.addCode(new Compare(Register.ECX, Register.EAX)); // add code
mb.addCode(new Compare(Register.EAX, Register.ECX)); // add code

final FileOutputStream fos = new FileOutputStream(new File("test.exe"));
mb.writeToFile(fos);
fos.close();

要输出一个有效的可执行文件，其中包含在文本部分有两个CMP指令。输出到text.exe会做什么有趣的可执行文件，但是这不是重点。类比较围绕 CMP 指令的包装。

To output a valid executable file, which contains two CMP instruction in a TEXT-section. The executable outputted to "text.exe" will do nothing interesting, but that's not the point. The class Compare is a wrapper around the CMP instruction.

以上code生产（使用OllyDbg的检查）：

The above code produces (inspecting with OllyDbg):

Address   Hex dump                 Command
0040101F  |.  3BC8                 CMP ECX,EAX
00401021  |.  3BC1                 CMP EAX,ECX

所不同的是微妙：如果我用 39 字节运算code：

Address   Hex dump                 Command
0040101F  |.  39C1                 CMP ECX,EAX
00401021  |.  39C8                 CMP EAX,ECX

这让我疑惑其同义，为什么这甚至存在。

Which makes me wonder about their synonymity and why this even exists.

推荐答案

这不要紧，如果你比较两个寄存器使用哪种运code。唯一不同的是比较具有存储器的操作数的寄存器时，如运code用于将被减去从中

It doesn't matter which opcode you use if you compare two registers. The only difference is when comparing a register with a memory operand, as the opcode used determines which will be subtracted from which.

至于为什么此存在：x86指令格式使用MODR / M字节表示是存储器地址或寄存器。每条指令只能有一个MODR / M值，这意味着它只能访问一个内存地址（不包括像MOVSB​​特殊说明）。因此，这意味着不能是一般 CMP R / M32，R / M32 指令，我们需要两个不同的运codeS： CMP R / M32，R32 和 CMP R32，R / M32 。作为一个副作用，比较两个寄存器时会带来一些冗余。

As for why this exists: The x86 instruction format uses the ModR/M byte to denote either a memory address or a register. Each instruction can only have one ModR/M value, which means it can only access one memory address (not including special instructions like MOVSB). So this means that there can't be a general cmp r/m32, r/m32 instruction, and we need two different opcodes: cmp r/m32, r32 and cmp r32, r/m32. As a side effect, this creates some redundancy when comparing two registers.

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