Gnu 汇编程序给出了意外的内存操作数

问题描述

GNU 汇编器在汇编 Intel 语法代码时给出了意外的内存操作数.

The GNU assembler gives an unexpected memory operand when assembling Intel syntax code.

我已将我的错误减少到一行单独的代码，并且在过去的三天里，我尝试了任何方法来理解为什么 GNU 汇编器会产生一些我无法理解的东西.我知道这必须(或应该)是微不足道的，但我不知所措.

I have reduced my bug to one single lonely line of code, and for the last three days I've tried anything to understand why the GNU assembler yields something that I cannot understand. I know this must (or should) be trivial, but I'm at a loss.

以下文本驻留在文件 code.asm 中:

The following text resided in the file code.asm:

.intel_syntax noprefix
.global somecode
somecode:
    int 3
    mov        rax,qword [rcx]
    ret
.att_syntax

汇编和反汇编code.asm:

Assembling and disassembling code.asm with:

as code.asm -o code1.obj -64
objdump -Mintel -d code1.obj > code1.asm

code1.asm(带反汇编代码)的内容为:

The content of code1.asm (with the disassembled code) is:

code1.obj:     file format pe-x86-64
Disassembly of section .text:
0000000000000000 <somecode>:
   0:   cc                      int3
   1:   48 8b 41 08             mov    rax,QWORD PTR [rcx+0x8]
   5:   c3                      ret

我使用的是 GNU 汇编器 (GNU Binutils) 2.25 (`x86_64-pc-cygwin').

I'm using GNU assembler (GNU Binutils) 2.25 (`x86_64-pc-cygwin').

问题:为什么在内存操作数 QWORD PTR [rcx+0x8] 中有一个额外的 qword 偏移量(8 字节)?我期待 mov rax,QWORD PTR [rcx].

Question:Why is there an extra one qword offset (8bytes) in the memory operand QWORD PTR [rcx+0x8]? I expect mov rax,QWORD PTR [rcx].

我一定是做错了什么.所以我与另一位受人尊敬的汇编程序 Yasm 进行了交叉检查并运行:

I must be doing something wrong. So I cross-checked with another respected assembler Yasm and ran:

yasm -f x64 -o code2.obj --parser=gas code.asm
objdump -Mintel -d code2.obj > code2.asm

code2.asm 的内容为:

The content of code2.asm is:

code2.obj:     file format pe-x86-64
Disassembly of section .text:
0000000000000000 <somecode>:
   0:   cd 03                   int    0x3
   2:   48 8b 01                mov    rax,QWORD PTR [rcx]
   5:   c3                      ret

关于内存操作数，这正是我所期望的.我如何指示 GNU 做同样的事情?

With regard to the memory operand, this is what I expected. How can I instruct GNU to do the same?

推荐答案

你需要写mov rax, qword ptr [rcx].显然 qword 本身解析为大小，即.8，所以你的代码汇编为 mov rax, 8[rcx].确实，mov rax, qword 也汇编为 mov rax, 8.

You need to write mov rax, qword ptr [rcx]. Apparently qword by itself resolves to the size, ie. 8, so your code assembled as mov rax, 8[rcx]. Indeed, mov rax, qword also assembles as mov rax, 8.

有趣的是你的交叉检查"如何使用正确的语法:)

It's funny how your "cross check" used the proper syntax :)

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