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.