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assemblyx86disassemblyopcodeobjdump

Why does objdump show a null opcode for this assembly language instruction?

I have the following assembly code:

global _start
section .text
_start:

add byte [eax], al

After compiling and linking, I try to see the opcode:

$ objdump -d eax.o

eax.o:     file format elf32-i386


Disassembly of section .text:

00000000 <_start>:
        ...
$

Why do I get a null opcode?

Solution

  • This instruction:

    add byte [eax], al

    Is encoded as the two byte ADD instruction 0x00 0x00:

    Opcode    Instruction     Op/En   64-bit Mode Compat/Leg Mode Description
00 /r     ADD r/m8, r8    MR      Valid       Valid           Add r8 to r/m8.

    OBJDUMP by default will print blocks of repeated zeros as .... To change this behavior try using the -z option as described in the manual:

    -z --disassemble-zeroes

    Normally the disassembly output will skip blocks of zeroes. This option directs the disassembler to disassemble those blocks, just like any other data.

    Your command could look like this:

    objdump -z -d eax.o

    The output should look something like:

    Disassembly of section .text:

00000000 <_start>:
   0:   00 00                   add    %al,(%eax)