The GNU assembler gives an unexpected memory operand when assembling Intel syntax code.
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.
The following text resided in the file code.asm:
.intel_syntax noprefix
.global somecode
somecode:
int 3
mov rax,qword [rcx]
ret
.att_syntax
Assembling and disassembling code.asm with:
as code.asm -o code1.obj -64
objdump -Mintel -d code1.obj > code1.asm
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
I'm using GNU assembler (GNU Binutils) 2.25 (`x86_64-pc-cygwin').
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].
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
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
With regard to the memory operand, this is what I expected. How can I instruct GNU to do the same?
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
.
It's funny how your "cross check" used the proper syntax :)
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