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Introduction to x86 assembly

exit syscall

This program call the syscall n°1 (value of the register eax) which represents the syscall exit (syscalls list). The register ebx holds the argument of the function exit, here the program will do exit(0xc).

code.asm

section     .text
global      _start
_start:
    mov     ebx, 0xc
    mov     eax, 1
    int     0x80
$ nasm -f elf64 code.asm    # assemble the program
$ ld -s -o code code.o      # link the object file nasm produced into an executable
$ ./code
$ echo $?
12

If we look at the exit code of the program, we can see it's 12 (0xc).

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