Search code examples
assemblyx86nasmgnu-assembleropcode

same x86 assembly instruction differs between NASM and GAS

the assembly code is 

mov eax, 0x3a14a5
jmp eax

GAS produces an opcode of 

0xB8, 0xA5, 0x14, 0x3A, 0x00 
0xFF, 0xE0

while NASM produces and opcode of 

0x66, 0xB8, 0xA5, 0x14, 0x3A, 0x00 
0x66, 0xFF, 0xE0

So you see NASM preappends a 0x66 before the code. Within my program (which I won't go into details about), the GAS opcode works correctly, and the NASM code causes a crash indicating that these two opcodes are not equal. Why does NASM preappend the 0x66 and how can I get rid of it?

update: The bits 32 directive worked. Thanks for the quick reply, links, and explanations!

Solution

  • Put a bits 32 directive at the beginning of your assembly file, and NASM should generate the same machine code as GAS for this instruction.

    More info in the NASM manual:

    In BITS 32 mode ... 32-bit instructions require no prefixes, whereas instructions using 16-bit data need an 0x66 and those working on 16-bit addresses need an 0x67.