I've talked today with a fellow student about some system internals and we weren't sure what actually happens. So I needed the assembly code of some example programs.
General Information
It is important to know that I will use AT&T syntax in this article! This is AT&T Syntax:
movl %esp, %ebp
And this is Intel Syntax:
MOVL EBP, ESP
Pointers
- %esp: Stack pointer for top address of the stack.
- %ebp: Stack base pointer for holding the address of the current stack frame.
- %eax: Accumulator
The size of the eax register will always be 32 bit, regardless of the system's register size.[6]
$ Dollar and % Percentage signs
\(i, with $i \in mathbb{N}\), is a constant and percentages mean registers.[10][11]
Instruction
pushl
Suffixes
Many instructions have suffixes. This is what they mean[6]:
- b: byte (8 bit)
- s: short (16 bit integer) or single (32-bit floating point)
- w: word (16 bit)
- l: long (32 bit integer or 64-bit floating point)
- q: quad (64 bit)
- t: ten bytes (80-bit floating point)
Simple example
C-Code
This program simply outputs
#include <stdio.h>
int main(void)
{
printf("%i", 1337*42);
return 0;
}
Assembly
Now I compile it and I save the assembly code:
gcc -S test.c; gcc test.c -o test
This gives me test.s (the assembly code) and an executable called "test".
.file "test.c"
.section .rodata
.LC0:
.string "%i"
.text
.globl main
.type main, @function
main:
pushl %ebp
movl %esp, %ebp
andl $-16, %esp
subl $16, %esp
movl $.LC0, %eax
movl $56154, 4(%esp)
movl %eax, (%esp)
call printf
movl $0, %eax
leave
ret
.size main, .-main
.ident "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
.section .note.GNU-stack,"",@progbits
This is code of the GNU Assembler. I guess other assemblers might produce other code. Could anybody please give me an example of other assemblers?
The first and most important thing you might notice is that neither "1337" nor "42" appear in the assembly code, but 56154 which is 1337*42. I didn't use any optimization options! You might also notice that constants begin with a dollar sign and registers (esp, ebp) begin with a percent sign.
The following ones are called assembly directives. They tell the assembler what to do next. .file, .section, .size and .ident are such directives. .data might be the most well-known one and tells the assembler to store something in the data segment of the program. .LC0 is a label for the immediately following string. .globl indicates that the following label (in this case "main") is a global symbol.
Line 14: I'm not quite sure why you need the 4. I thought the integer size could be the reason (see variable sizes in C), but as I used a string it still worked. As I used a character, it disappeared.
Further information
objdump gives even more information!
Archive header information: objdump -a test
test: file format elf32-i386
test
File header information: objdump -f test
test: file format elf32-i386
architecture: i386, flags 0x00000112:
EXEC_P, HAS_SYMS, D_PAGED
start address 0x08048330
Object specific file header contents: objdump -p test
test: file format elf32-i386
Program Header:
PHDR off 0x00000034 vaddr 0x08048034 paddr 0x08048034 align 2**2
filesz 0x00000100 memsz 0x00000100 flags r-x
INTERP off 0x00000134 vaddr 0x08048134 paddr 0x08048134 align 2**0
filesz 0x00000013 memsz 0x00000013 flags r--
LOAD off 0x00000000 vaddr 0x08048000 paddr 0x08048000 align 2**12
filesz 0x000004d8 memsz 0x000004d8 flags r-x
LOAD off 0x00000f0c vaddr 0x08049f0c paddr 0x08049f0c align 2**12
filesz 0x00000108 memsz 0x00000110 flags rw-
DYNAMIC off 0x00000f20 vaddr 0x08049f20 paddr 0x08049f20 align 2**2
filesz 0x000000d0 memsz 0x000000d0 flags rw-
NOTE off 0x00000148 vaddr 0x08048148 paddr 0x08048148 align 2**2
filesz 0x00000044 memsz 0x00000044 flags r--
STACK off 0x00000000 vaddr 0x00000000 paddr 0x00000000 align 2**2
filesz 0x00000000 memsz 0x00000000 flags rw-
RELRO off 0x00000f0c vaddr 0x08049f0c paddr 0x08049f0c align 2**0
filesz 0x000000f4 memsz 0x000000f4 flags r--
Dynamic Section:
NEEDED libc.so.6
INIT 0x080482bc
FINI 0x080484ac
HASH 0x0804818c
GNU_HASH 0x080481b4
STRTAB 0x08048224
SYMTAB 0x080481d4
STRSZ 0x0000004c
SYMENT 0x00000010
DEBUG 0x00000000
PLTGOT 0x08049ff4
PLTRELSZ 0x00000018
PLTREL 0x00000011
JMPREL 0x080482a4
REL 0x0804829c
RELSZ 0x00000008
RELENT 0x00000008
VERNEED 0x0804827c
VERNEEDNUM 0x00000001
VERSYM 0x08048270
Version References:
required from libc.so.6:
0x0d696910 0x00 02 GLIBC_2.0
Display the contents of the section headers: objdump -h test
test: file format elf32-i386
Sections:
Idx Name Size VMA LMA File off Algn
0 .interp 00000013 08048134 08048134 00000134 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
1 .note.ABI-tag 00000020 08048148 08048148 00000148 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
2 .note.gnu.build-id 00000024 08048168 08048168 00000168 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
3 .hash 00000028 0804818c 0804818c 0000018c 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .gnu.hash 00000020 080481b4 080481b4 000001b4 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
5 .dynsym 00000050 080481d4 080481d4 000001d4 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
6 .dynstr 0000004c 08048224 08048224 00000224 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
7 .gnu.version 0000000a 08048270 08048270 00000270 2**1
CONTENTS, ALLOC, LOAD, READONLY, DATA
8 .gnu.version_r 00000020 0804827c 0804827c 0000027c 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
9 .rel.dyn 00000008 0804829c 0804829c 0000029c 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
10 .rel.plt 00000018 080482a4 080482a4 000002a4 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
11 .init 00000030 080482bc 080482bc 000002bc 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
12 .plt 00000040 080482ec 080482ec 000002ec 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
13 .text 0000017c 08048330 08048330 00000330 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
14 .fini 0000001c 080484ac 080484ac 000004ac 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
15 .rodata 0000000b 080484c8 080484c8 000004c8 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
16 .eh_frame 00000004 080484d4 080484d4 000004d4 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
17 .ctors 00000008 08049f0c 08049f0c 00000f0c 2**2
CONTENTS, ALLOC, LOAD, DATA
18 .dtors 00000008 08049f14 08049f14 00000f14 2**2
CONTENTS, ALLOC, LOAD, DATA
19 .jcr 00000004 08049f1c 08049f1c 00000f1c 2**2
CONTENTS, ALLOC, LOAD, DATA
20 .dynamic 000000d0 08049f20 08049f20 00000f20 2**2
CONTENTS, ALLOC, LOAD, DATA
21 .got 00000004 08049ff0 08049ff0 00000ff0 2**2
CONTENTS, ALLOC, LOAD, DATA
22 .got.plt 00000018 08049ff4 08049ff4 00000ff4 2**2
CONTENTS, ALLOC, LOAD, DATA
23 .data 00000008 0804a00c 0804a00c 0000100c 2**2
CONTENTS, ALLOC, LOAD, DATA
24 .bss 00000008 0804a014 0804a014 00001014 2**2
ALLOC
25 .comment 00000023 00000000 00000000 00001014 2**0
CONTENTS, READONLY
Display DWARF info in the file: objdump --dwarf test
test: file format elf32-i386
Contents of the .eh_frame section:
00000000 ZERO terminator
By the way, ELF is an executable file format and DWARF is a debugging file format. I guess they had to think quite long to find this backronym.
Fibonacci
C-Code
This is the most simple version of Fibonacci I could find:[7]
#include <stdio.h>
unsigned int fib(unsigned int n)
{
return n < 2 ? n : fib(n-1) + fib(n-2);
}
int main(void)
{
printf("%i", fib(13));
return 0;
}
Assembly
.file "test.c"
.text
.globl fib
.type fib, @function
fib:
pushl %ebp
movl %esp, %ebp
pushl %ebx
subl $20, %esp
cmpl $1, 8(%ebp)
jbe .L2
movl 8(%ebp), %eax
subl $1, %eax
movl %eax, (%esp)
call fib
movl %eax, %ebx
movl 8(%ebp), %eax
subl $2, %eax
movl %eax, (%esp)
call fib
leal (%ebx,%eax), %eax
jmp .L3
.L2:
movl 8(%ebp), %eax
.L3:
addl $20, %esp
popl %ebx
popl %ebp
ret
.size fib, .-fib
.section .rodata
.LC0:
.string "%i"
.text
.globl main
.type main, @function
main:
pushl %ebp
movl %esp, %ebp
andl $-16, %esp
subl $16, %esp
movl $13, (%esp)
call fib
movl $.LC0, %edx
movl %eax, 4(%esp)
movl %edx, (%esp)
call printf
movl $0, %eax
leave
ret
.size main, .-main
.ident "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
.section .note.GNU-stack,"",@progbits
References
- ↑: PUSHL Instruction. The University of Auckland, Department of Computer Science.
- ↑: IA-32 Assembly for Compiler Writers. Douglas Thain, Associate Professor, University of Notre Dame, Department of Computer Science and Engineering.
- ↑: From C To Assembly Language . Hiran Ramankutty, Linux Gazett, Issue 94.
- ↑: About leave in x86 assembly. zneak, Stackoverflow.
- ↑: GCC's assembly output of an empty program on x86, win32. nos, Stackoverflow.
- ↑: Why would one use “movl $1, %eax” as opposed to, say, “movb $1, %eax”. Jason, Stackoverflow.
- ↑: Fibonacci numbers (C). Literate Programs.
- ↑: The 68000's Instruction Set, page 27. Literate Programs.
- ↑: LEAL Assembler instruction. Nils Pipenbrinck, Stackoverflow
- ↑: What does this dollar sign mean in __asm?. Zimbabao, Stackoverflow
- ↑: What do the dollar ($) and percentage (%) signs represent in assembly intel x86?. Necrolis, Stackoverflow