CODEWITHSUNDEEP
assemblysyntaxgnu-assembler
automaton
automaton

Reputation: 1121

GAS syntax, how to pushl?

GAS is rejecting all my pushl's with

"wrong instruction prefix"

. I can't figure out why...(sample dummy code)

.section .text
        .global _start
_start:
        movl $10, %eax
        pushl %eax
        movl $1, %eax
        int $exit
.data
.equ exit, 0x80

I also couldn't do

int 0x80

directly in the script above.It says

operand size mismatch for `int'

I've tried int 80H, doesn't work too.

What am I missing?

Upvotes: 1

Views: 1365

Answers (2)

Michael
Michael

Reputation: 58517

int 0x80 should be int $0x80 (note the $, since it's an immediate operand)

Regarding pushl, I'm guessing that you're using a version of the GNU assembler that was configured for 64-bit targets. push r32 is non-encodable in 64-bit mode as you can see in the description for push in Intel's Software Developer's Manual. You could use pushq %rax instead.

However, looking at your code it seems like you're writing a 32-bit application, so you should probably tell the assembler to use 32-bit mode instead (pass --32 to as, or -m32 to gcc).

If you're linking with ld you'll also have to specify an emulation mode for ld with -m, e.g. -m elf_i386 (to find out which emulation modes you have available, use ld -V). An alternative would be to link with gcc, e.g. gcc -m32 foo.o.

Upvotes: 3

Andreas Fester
Andreas Fester

Reputation: 36649

It looks like you are on an x86_64 architecture, but trying to assemble x86 specific code. You need to pass the --32 flag to gas:

$ as --32 -ofile.o file.asm

With this command line, I was able to assemble your code - without the flag, I am getting the wrong instruction prefix error.

I also couldn't do

int 0x80

The int instruction requires one immediate operand (the interrupt vector number), and gas requires immediate operands to be prefixed with $ (which you correctly did for your mov instructions):

int $0x80

Upvotes: 2

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