无法将.data中的变量移动到Mac x86 Assembly中进行注册 [英] Unable to move variables in .data to registers with Mac x86 Assembly

问题描述

我已经用AT& T语法编写了一小段程序集，并且当前在.data部分中声明了三个变量.但是，当我尝试将这些变量中的任何一个移至诸如%eax的寄存器时，会出现来自gcc的错误.代码和错误消息如下:

I have written a small piece of assembly with AT&T syntax and have currently declared three variables in the .data section. However, when I attempt to move any of those variables to a register, such as %eax, an error from gcc is raised. The code and error message is below:

.data
  x:.int 14
  y:.int 4
  str: .string "some string\n"

.globl _main

_main:
  pushq %rbp
  movq %rsp, %rbp
  subq $16, %rsp
  movl x, %eax; #attempting to move the value of x to %eax;
  leave
  ret

引发的错误是:

call_function.s:14:3:错误:在64位模式下不支持32位绝对寻址

call_function.s:14:3: error: 32-bit absolute addressing is not supported in 64-bit mode

movl x，％eax;

movl x, %eax;

^

我还尝试过先在x前面添加$字符来移动值，但是会出现clang错误:

I have also tried moving the value by first adding the $ character in front of x, however, a clang error is raised:

clang:错误:链接器命令失败，退出代码为1(使用-v查看调用)

clang: error: linker command failed with exit code 1 (use -v to see invocation)

有人知道如何将存储在x中的值成功地移到%eax吗?我在Mac OSX上使用x86程序集，并使用gcc进行编译.

Does anyone know how the value stored in x can be successfully moved to %eax? I am using x86 assembly on Mac OSX and compiling with gcc.

推荐答案

相对于RIP的寻址模式是在MacOS上寻址静态数据的唯一好选择；图像基址大于2 ^ 32，因此即使在与位置相关的代码中也不可用32位绝对地址(与x86-64 Linux不同). RIP相对于静态数据的寻址与位置无关，因此即使在与位置无关的可执行文件(ASLR)和库中也可以使用.

A RIP-relative addressing mode is the only good option for addressing static data on MacOS; the image base address is above 2^32 so 32-bit absolute addresses aren't usable even in position-dependent code (unlike x86-64 Linux). RIP-relative addressing of static data is position-independent, so it works even in position-independent executables (ASLR) and libraries.

movl x(%rip), %eax 是相对于RIP的AT& T语法.

movl x(%rip), %eax is the AT&T syntax for RIP-relative.

mov eax, dword ptr [rip+x] .

mov eax, dword ptr [rip+x] in GAS .intel_syntax noprefix.

或者，要将符号的地址保存到寄存器中，请 lea x(%rip), %rdi

Or, to get the address of a symbol into a register, lea x(%rip), %rdi

NASM语法:mov eax, [rel x]，或使用default rel，因此[x]是RIP相对的.

NASM syntax: mov eax, [rel x], or use default rel so [x] is RIP-relative.

请参见 Mach-O 64位格式不支持32位绝对地址. NASM Accessing Array (NASM访问数组)，了解有关您在OS X上可以执行的操作的更多背景信息，例如movabs x, %eax是可能的，因为目标寄存器是AL/AX/EAX/RAX. (64位绝对地址，但不要这样做，因为它比RIP相对负载大且不快.)

See Mach-O 64-bit format does not support 32-bit absolute addresses. NASM Accessing Array for more background on what you can do on OS X, e.g. movabs x, %eax would be possible because the destination register is AL/AX/EAX/RAX. (64-bit absolute address, but don't do that because it's larger and not faster than a RIP-relative load.)

另请参见 http://felixcloutier.com/x86/MOV.html .

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