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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LEARN ABOUT OSX

qc2movie

qc2movie(1)						    BSD General Commands Manual 					       qc2movie(1)

NAME

     qc2movie -- Quartz Composer export tool

SYNOPSIS

     qc2movie inCompositionPath|repositoryCompositionIdentifier outMoviePath width height duration [--parameterKey1 value1 ...]

DESCRIPTION

     qc2movie is a tool that exports a Quartz Composer composition into a QuickTime movie by adding a track containing this composition. It is the
     command line equivalent of exporting a composition for the Quartz Composer application to a QuickTime movie.

OPTIONS

     inCompositionPath|repositoryCompositionIdentifier
			   specifies which composition you want to export. Ar inCompositionPath is the path to the composition. Ar reposito-
			   ryCompositionIdentifier is the identifier of a composition which lies in the Quartz Composer Repository.

     outMoviePath	   specifies movie file path to which the composition will be exported to

     width		   defines the width of the export movie

     height		   defines the height of the export movie

     duration		   defines the length of the export movie

     [--parameterKey1 value1 ...]
			   passes composition specific parameters through the command line tool to the composition

EXAMPLES

     -To create a 10 second QuickTime movie containing a Quartz Composer track of the Particle System composition:
	      qc2movie "/Developer/Examples/Quartz Composer Compositions/Particle Systems/Particle System.qtz" ~/example.mov 640 480 10

     -To merge two video tracks (30s each) together using an image transition protocol abiding composition (Swing for example) in the Quartz
	      Composer Repository (using its identifier: /swing):
	      qc2movie /swing ~/merged-example.mov 640 480 60 --inputMoviePath1 ~/firstVideo.mov --inputMoviePath2 ~/secondVideo.mov --duration 2

     -To apply an image filter protocol abiding composition to the video track of a movie:
	      qc2movie /colorpencil ~/effect-example.mov 640 480 30 --input ~/Video.mov

SEE ALSO

     pcastaction(1)

Mac OS								    July 9 2007 							    Mac OS