Understanding Assembly Code Post: 302931995

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Top Forums Programming Understanding Assembly Code Post 302931995 by Azrael on Sunday 18th of January 2015 01:04:54 AM

01-18-2015

Registered User

Understanding Assembly Code

As the title suggests, I want to better understand the following assembly code:

Code:

section .text
    global main          ; must be declared for linker (gcc)
main:                        ; tell linker entry point
    mov    edx, len     ; message length
    mov    ecx, msg    ; message to write
    mov    ebx, 1         ; file descriptor (stdout)
    mov    eax, 4         ; syscall for write (sys_write)
    int      0x80           ; call kernel

    mov    edx, 9        ; message length
    mov    ecx, s2       ; message to write
    mov    ebx, 1        ; file descriptor (stdout)
    mov    eax, 4        ; syscall number for write (sys_write)
    int      0x80          ; call kernel
    mov    eax, 1        ; system call (sys_exit)
    int      0x80          ; call kernel

section .data
msg db 'Displaying 9 stars', 0xa ; a message
len equ $ - msg            ; length of message
s2 times 9 db '*'

As you can see I already have descriptions in the comments from the tutorial I found here. Here are some of the things I don't understand:

1. - What is s2? Is this just a variable or a register I know nothing about?

2. When '1' is moved into ebx, is this a parameter to the sys_exit later called in eax? I found a listing of Linux syscalls here and it does seem sys_exit does take one parameter in ebx that's an integer. If this is the case why not exit cleanly with zero?

I just want to make sure I understand everything correctly in this. Thanks in advance!

Azrael

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