section .data
hello: db 'Hello world!',10 ; 'Hello world!' plus a linefeed character
helloLen: equ $-hello ; Length of the 'Hello world!' string
section .text
global _start
_start:
mov ecx,5 ; display the string 5 times
_loop:
mov eax,4 ; The system call for write (sys_write)
mov ebx,1 ; File descriptor 1 - standard output
push ecx ; save ecx as it is gonna be used as param to sys_write
mov ecx,hello ; Put the offset of hello in ecx
mov edx,helloLen ; helloLen is a constant, so we don't need to say
; mov edx,[helloLen] to get it's actual value
int 80h ; Call the kernel
pop ecx ; restore ecx (counter)
loop _loop
mov eax,1 ; The system call for exit (sys_exit)
mov ebx,0 ; Exit with return code of 0 (no error)
int 80h
Steps:
- Save the file as syscall.asm
- Execute: nasm -f elf syscall.asm
- Execute: ld -s -o syscall syscall.o
- run it as: ./syscall
- To check the object file, we can use objdump, elfdump, or readelf. For example:
$ readelf -a ./syscall.o
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: REL (Relocatable file)
Machine: Intel 80386
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 64 (bytes into file)
Flags: 0x0
Size of this header: 52 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 40 (bytes)
Number of section headers: 7
Section header string table index: 3
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .data PROGBITS 00000000 000160 00000d 00 WA 0 0 4
[ 2] .text PROGBITS 00000000 000170 00002b 00 AX 0 0 16
[ 3] .shstrtab STRTAB 00000000 0001a0 000031 00 0 0 1
[ 4] .symtab SYMTAB 00000000 0001e0 000080 10 5 7 4
[ 5] .strtab STRTAB 00000000 000260 000029 00 0 0 1
[ 6] .rel.text REL 00000000 000290 000008 08 4 2 4
$ readelf -a ./syscall.o
ELF Header:s:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32unknown)
Data: 2's complement, little endianpecific)
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: REL (Relocatable file)
Machine: Intel 80386
Version: 0x1 0x290 contains 1 entries:
Entry point address: 0x0Value Sym. Name
Start of program headers: 0 (bytes into file)
Start of section headers: 64 (bytes into file)
Flags: 0x0e.
Size of this header: 52 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0Vis Ndx Name
Size of section headers: 40 (bytes)UND
Number of section headers: 7DEFAULT ABS syscall.asm
Section header string table index: 3DEFAULT 1
3: 00000000 0 SECTION LOCAL DEFAULT 2
Section Headers: 0 NOTYPE LOCAL DEFAULT 1 hello
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .data PROGBITS 00000000 000160 00000d 00 WA 0 0 4
[ 2] .text PROGBITS 00000000 000170 00002b 00 AX 0 0 16
[ 3] .shstrtab STRTAB 00000000 0001a0 000031 00 0 0 1
[ 4] .symtab SYMTAB 00000000 0001e0 000080 10 5 7 4
[ 5] .strtab STRTAB 00000000 000260 000029 00 0 0 1
[ 6] .rel.text REL 00000000 000290 000008 08 4 2 4
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
There are no section groups in this file.
There are no program headers in this file.
Relocation section '.rel.text' at offset 0x290 contains 1 entries:
Offset Info Type Sym.Value Sym. Name
00000011 00000201 R_386_32 00000000 .data
There are no unwind sections in this file.
Symbol table '.symtab' contains 8 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 00000000 0 FILE LOCAL DEFAULT ABS syscall.asm
2: 00000000 0 SECTION LOCAL DEFAULT 1
3: 00000000 0 SECTION LOCAL DEFAULT 2
4: 00000000 0 NOTYPE LOCAL DEFAULT 1 hello
5: 0000000d 0 NOTYPE LOCAL DEFAULT ABS helloLen
6: 00000005 0 NOTYPE LOCAL DEFAULT 2 _loop
7: 00000000 0 NOTYPE GLOBAL DEFAULT 2 _start
No version information found in this file.
