Passing parameters from C to GNU Assembly function in 64bit - c

Question

I have main function in C that runs code in assembly. I just want to make simple sum:
main.c
#include <stdio.h>
extern int addByAssembly(int first_number, int second_number);
int main (int argc, char **argv)
{
int sum=0;
sum = addByAssembly(5,4);
printf ("%d\n",sum);
return 0;
}
addByAssembly.s
.data
SYSREAD = 0
SYSWRITE = 1
SYSEXIT = 60
STDOUT = 1
STDIN = 0
EXIT_SUCCESS = 0
.text
#.global main
#main:
#call write
#movq $SYSEXIT, %rax
#movq $EXIT_SUCCESS, %rdi
#syscall
#********
.globl addByAssembly
addByAssembly:
pushq %rbp
movq %rsp, %rbp
movq 16(%rsp), %rax
addq 24(%rsp), %rax
movq %rbp, %rsp
popq %rbp
But i got mess in my sum. It looks like i badly pass arguments, beause if I do this:
movq $123, %rax
return value is 123. I 've tried many ways, but cannot find how to make this properly to sum.

Answer 1

Thanks 'Jester' for so much effort and time to get me this explained!
To sum up. Passing parameters from C to As ( and as well from As to C) has its own ABI convention.
As you can see there, params are send on order:
1) rdi
2) rsi
3) rdx
... and so on...
In case you have more parameters than in convention, it will be pushed to stack.
So in my case:
.globl addByAssembly
addByAssembly:
pushq %rbp
movq %rsp, %rbp
--movq 16(%rsp), %rax #this was wrong as my params are
--addq 24(%rsp), %rax # first in %rdi, second in %rsi
++lea (%rdi, %rsi), %rax # in my case this line will do
# %rdi+%rsi -> %rax (learn lea, usefull command)
# REMEMBER return value is always in %rax!
movq %rbp, %rsp
popq %rbp