1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
|
---
title: "Calling assembly functions from C"
url: calling-assembly-functions-from-c.html
date: 2024-06-17T16:12:13+02:00
type: note
draft: false
tags: [c]
---
This is using the portable GNU assembler and TinyCC compiler but GCC or Clang
can be used as well.
First lets define a simple function in assembly.
```asm
# sum.s
.intel_syntax noprefix
.global sum
.text
sum:
add rdi, rsi
mov rax, rdi
ret
```
Lets compile this with GNU assembler `as sum.s -o sum.o`.
Now we need a C program that calls this function.
```c
// main.c
#include <stdio.h>
// We need to define the signature of the function.
int sum(int a, int b);
int main() {
for(int i=0; i<10; ++i) {
printf("SUM of 5+%d is %d\n", i, sum(5, i));
}
return 0;
}
```
Now lets compile and link into final program with `tcc main.c sum.o -o main`.
Use this table to see what each register is being used for.
| Register | Conventional use | Low 32-bits | Low 16-bits | Low 8-bits |
|----------|---------------------------------|-------------|-------------|------------|
| %rax | Return value, caller-saved | %eax | %ax | %al |
| %rdi | 1st argument, caller-saved | %edi | %di | %dil |
| %rsi | 2nd argument, caller-saved | %esi | %si | %sil |
| %rdx | 3rd argument, caller-saved | %edx | %dx | %dl |
| %rcx | 4th argument, caller-saved | %ecx | %cx | %cl |
| %r8 | 5th argument, caller-saved | %r8d | %r8w | %r8b |
| %r9 | 6th argument, caller-saved | %r9d | %r9w | %r9b |
| %r10 | Scratch/temporary, caller-saved | %r10d | %r10w | %r10b |
| %r11 | Scratch/temporary, caller-saved | %r11d | %r11w | %r11b |
| %rsp | Stack pointer, callee-saved | %esp | %sp | %spl |
| %rbx | Local variable, callee-saved | %ebx | %bx | %bl |
| %rbp | Local variable, callee-saved | %ebp | %bp | %bpl |
| %r12 | Local variable, callee-saved | %r12d | %r12w | %r12b |
| %r13 | Local variable, callee-saved | %r13d | %r13w | %r13b |
| %r14 | Local variable, callee-saved | %r14d | %r14w | %r14b |
| %r15 | Local variable, callee-saved | %r15d | %r15w | %r15b |
| %rip | Instruction pointer | | | |
| %rflags | Status/condition code bits | | | |
You can read more about [x86-64 Machine-Level Programming](/assets/notes/asm64-handout.pdf).
|
