Assembly in Linux

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.