为什么从 AT&T 切换到 Intel 语法会使本教程使用 GAS 出现段错误？

Question

I'm working through some of the tutorials on http://www.ibm.com/developerworks/linux/library/l-gas-nasm/index.html to familiarize myself with x86/x64. This tutorial code compiles and runs without a hiccup using the provided code, which uses AT&T syntax:

.global main
.text
main:                               # This is called by C library's startup code
    mov     $message, %rdi          # First integer (or pointer) parameter in %edi
    call    puts                    # puts("Hello, World")
    ret                             # Return to C library code
message:
    .asciz "Hello, World"           # asciz puts a 0x00 byte at the end

However, when I convert this code to Intel syntax, I get a "Segmentation fault" error.

.intel_syntax noprefix
.global main
.text
main:                               # This is called by C library's startup code
    mov     rdi, message            # First integer (or pointer) parameter in %edi
    call    puts                    # puts("Hello, World")
    ret                             # Return to C library code
message:
    .asciz "Hello, World"           # asciz puts a 0x00 byte at the end

I'm not familiar with x86, so perhaps I'm missing something. Any ideas?

Answer 1

In AT&T syntax, mov $message, %rdi , the $ means immediate , meaning the address of message .

In GAS's Intel syntax, mov rdi, message means absolute addressing, meaning the content at message . To get the actual address of message , you need to supply the offset keyword: mov rdi, offset message .

Disassebly of the two binaries shows the difference:

AT&T:

0000000000000000 <main>:
0:   48 c7 c7 00 00 00 00    mov    $0x0,%rdi

Intel:

0000000000000000 <main>:
0:   48 8b 3c 25 00 00 00 00    mov    0x0,%rdi