Understanding Assembly Code Post: 302931995

Sponsored Content

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

View Public Profile for Azrael

Find all posts by Azrael

10 More Discussions You Might Find Interesting

1. UNIX for Dummies Questions & Answers

Understanding Code in IF LOOP

Hello All, I would like to know and understand the difference between the below 3 IF loops and also if possible what are the different other parameters i could use other than those mentioed in the below lF LOOP conditions, appreciate your help. Thanks, Sam.

2. UNIX Desktop Questions & Answers

Understanding the code

hello all, May i know what is this "DEBUG_ME $DEBUG_CMD main" doing in the below code. I am confused with alias also "alias DEBUG_ME='#'". Thanks for your help. set -x alias DEBUG_ME='#' if ; then . /product/apps/informatica/v7/pc/ExtProc/debug.ksh "$1" fi # Declaring the...

3. Programming

Need assembly code for C program

Dear Buddies, I need assembly code for a compiled c program in unix. Kindly help me.... Thanking you in advance.

4. Programming

Help with assembly code

I want make simple assembly code for some thing like this a^6+6a^2+2a and range of a is between -3 to 3. I tried but it is not working properly. As this is my first assembly program that I am going to try, I want some help with it. I found this example online but i dont want this kind of...

5. Shell Programming and Scripting

Help understanding Perl code.

Well, I found myself trying to fix some Perl code (Ive never done any Perl in my life) and I pinpointed the place where the bug could be. But to be sure I have to know what does a few line of code mean: $files_lim =~ (/^\d*$/) $files_lim =~ (/^\d*h$/)$files_age =~ s/h//The code where this was...

6. Shell Programming and Scripting

Help understanding some Perl code.

7. Programming

Understanding perl code

What is the difference between the two statements below? A: $a->{"$fruit"}->{"$color"}->{size} = $size B: $size = $a->{"$fruit"}->{"$color"}->{size} Please assist. Thanks!

8. Shell Programming and Scripting

auto-generating assembly code by variables found by script

Hi everybody I'm working on a list of registers(flip-flops to be exact), now i need to extract some value from this list and use them as arguments to pass them to some assembly code for example i have: 118 chain79 MASTER (FF-LE) FFFF 1975829 /TCK F FD1TQHVTT1 ...

9. Shell Programming and Scripting

Help with perl code understanding

Hi, I need to understand below perl code, can some one advise me. perl -MDate::Parse -e'BEGIN{$main::now=time;$main::old=(time-60*30)}' -nE'if(/^(\w+\s+\d+\s+\d+:\d+:\d+)/) {$t=str2time $1; $t > $old && $t < $now && print}'

10. Shell Programming and Scripting

[Solved] Help understanding this code!!

Hi guys, I am still learning awk and much apprecated to shed some light on the following: the questions asked is below! { total = i = 0 do { ++i total += $i } while ( total <= 100 ) print i, ":", total } File used: cat test.do 45 25 60 20 10 105 50 40 33 5 9 67 108 3 5 4

LEARN ABOUT OSX

qtmodernizer

qtmodernizer(1) 					    BSD General Commands Manual 					   qtmodernizer(1)

NAME

     qtmodernizer

     -- Tool to convert legacy movies to modern format with minimal changes.

SYNOPSIS

     qtmodernizer [options ...] pathToSource [pathToDestination]

DESCRIPTION

     qtmodernizer is a command line tool to examine a media file which may be in a legacy format or may contain legacy media tracks. In this case
     a legacy format is one which cannot be read directly by CoreMedia or AVFoundation frameworks.

     qtmodernizer examines all the tracks to determine if they are all in a form that can currently be played. If all tracks are playable and the
     format is readable, qtmodernizer does returns and does nothing. If any of the audio or video tracks are encoded with a codec that CoreMedia
     does not support, that track will be converted to a modern format, usually H.264 for video and AAC for audio (see --useprores for other
     choices).

     Command line options of qtmodernize:

     -h | --help

     -p | --useprores
	      If -p or --useprores is specified, qtmodernizer will use the ProRes video codec and LPCM audio codec instead of H.264 and AAC-LC.
	      For video tracks it will produce AppleProRes422. For audio it will produce uncompressed output (lpcm). Tracks outher than audio or
	      video are not re-encoded and will not be affected. Either AppleProRes422 may be chosen or the AppleProRes4444 option may be chosen
	      but not both.

     -p4 | --useprores4444
	      If -p4 or --useprores4444 is specified, qtmodernizer will use the ProRes4444 video codec and LPCM audio codec instead of H.264 and
	      AAC-LC. For video tracks it will produce AppleProRes4444. For audio it will produce uncompressed output (lpcm). Tracks outher than
	      audio or video are not re-encoded and will not be affected. Either AppleProRes4444 may be chosen or the AppleProRes422 option may be
	      chosen but not both.

     -r | --replace
	      Normally qtmodernizer will not replace any existing files. If --replace is specified, a file already existing at the destination
	      path will be replaced by the new output.	qtmodernizer will not replace the file at original source path used in the command.

     -v | --verbose
	      Normally qtmodernizer only displays important error or warning messages. Specifying --verbose causes other information about the
	      modernization to be displayed on the command line. --infoonly implies --verbose.

     -i | --infoonly
	      Causes qtmodernizer to display information about the tracks contained in the source file and what the modernization process would be
	      for each track (if any) but does not actually produce an output file.

EXAMPLES

	  qtmodernize legacy_movie.mov modern_movie.mov

     Each legacy audio and video track would be re-encoded to a modern (H.264 or AAC) track with other tracks passed through. The resulting movie
     is saved as modern_movie.mov

	  qtmodernize -p legacy_movie.mov modern_movie.mov

     Legacy audio and video tracks would be re-encoded like in the above example but the resulting codecs would be ProRes for video and LPCM for
     audio

	  qtmodernize ~/Movies/legacy_movie.mov

     This command will create a modernized movie in the ~/Movies directory named "legacy_movie-modernized.mov"

HISTORY

     qtmodernizer command first appeared in Mac OS X 10.9.

Mac OS X							   June 2, 2019 							  Mac OS X