Outputting variable values in x86 asm ?

Question

Hello All-
I'm working on a homework assignment in x86 and it isn't working as I expect (surprise surprise!). I'd like to be able to output values of variables in x86 functions to ensure that the values are what I expect them to be. Is there a simple way to do this, or is it very complex? For what it's worth, the x86 functions are being used by a C file and compiled with gcc, so if that makes it simpler that is how I'm going about it.
Thanks for the help.

Answer 1

It appears that your question is along the lines of "How can I print out variable values in x86 assembler". The x86 itself doesn't know how to do that, because it depends entirely on what output device you're using (and the specifics of the OS-provided interface to that output device).

One way of doing it is to use operating system syscalls, as you mentioned yourself in another answer. If you're on x86 Linux, then you can use the sys_write sys call to write a string to standard output, like this (GNU assembler syntax):

STR:
    .string "message from assembler\n"

.globl asmfunc
    .type asmfunc, @function

asmfunc:
    movl $4, %eax   # sys_write
    movl $1, %ebx   # stdout
    leal STR, %ecx  #
    movl $23, %edx  # length
    int $0x80       # syscall

    ret

However, if you want to print numeric values, then the most flexible method will be to use the printf() function from the C standard library (you mention that you're calling your assembler rountines from C, so you are probably linking to the standard library anyway). This is an example:

int_format:
    .string "%d\n"

.globl asmfunc2
    .type asmfunc2, @function

asmfunc2:
    movl $123456, %eax

    # print content of %eax as decimal integer
    pusha           # save all registers
    pushl %eax
    pushl $int_format
    call printf
    add $8, %esp    # remove arguments from stack
    popa            # restore saved registers

    ret

Two things to note:

• You need to save and restore registers, because they get clobbered by the call; and
• When you call a function, the arguments are pushed in right-to-left order.