I tried to  use crash without entering  the system.map or the vmlinux  since the live system use the same kernel but I got this output:

root@o:/home/amer# crash

crash 4.1.0
Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009  Red Hat, Inc.
Copyright (C) 2004, 2005, 2006  IBM Corporation
Copyright (C) 1999-2006  Hewlett-Packard Co
Copyright (C) 2005, 2006  Fujitsu Limited
Copyright (C) 2006, 2007  VA Linux Systems Japan K.K.
Copyright (C) 2005  NEC Corporation
Copyright (C) 1999, 2002, 2007  Silicon Graphics, Inc.
Copyright (C) 1999, 2000, 2001, 2002  Mission Critical Linux, Inc.
This program is free software, covered by the GNU General Public License,
and you are welcome to change it and/or distribute copies of it under
certain conditions.  Enter "help copying" to see the conditions.
This program has absolutely no warranty.  Enter "help warranty" for details.
 
crash: cannot find booted kernel -- please enter namelist argument


Usage:
  crash [-h [opt]][-v][-s][-i file][-d num] [-S] [mapfile] [namelist] [dumpfile]



when I enter the system.map and the vmlinux , it works perfectly and it reads from the /dev/crash. but if I specify the /dev/crash in the argument like this: 

root@o:/home/amer/Desktop# crash  /boot/System.map-2.6.32-25-generic  vmlinux  /dev/crash (or dd /dev/crash > image.dd)

I got this output:
crash: /dev/crash: not a supported file format

I don't know If I'm missing something ,but the link below  shows that dd /dev/crash > image.dd can  work in crash

http://gleeda.blogspot.com/2009/08/devcrash-driver.html

Thanks for help Dave, and looking forward to your feedback,

Amer