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/* Startup code compliant to the ELF i386 ABI.
Copyright (C) 1995, 1996, 1997 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Library General Public License as
published by the Free Software Foundation; either version 2 of the
License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Library General Public License for more details.
You should have received a copy of the GNU Library General Public
License along with the GNU C Library; see the file COPYING.LIB. If not,
write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA. */
/* This is the canonical entry point, usually the first thing in the text
segment. The SVR4/i386 ABI (pages 3-31, 3-32) says that when the entry
point runs, most registers' values are unspecified, except for:
%edx Contains a function pointer to be registered with `atexit'.
This is how the dynamic linker arranges to have DT_FINI
functions called for shared libraries that have been loaded
before this code runs.
%esp The stack contains the arguments and environment:
0(%esp) argc
4(%esp) argv[0]
...
(4*argc)(%esp) NULL
(4*(argc+1))(%esp) envp[0]
...
NULL
*/
.text
.globl _start
_start:
/* Clear the frame pointer. The ABI suggests this be done, to mark
the outermost frame obviously. */
xorl %ebp, %ebp
/* %edx contains the address of the shared library termination
function, which we will register with `atexit' to be called by
`exit'. I suspect that on some systems, and when statically
linked, this will not be set by anything to any function
pointer; hopefully it will be zero so we don't try to call
random pointers. */
testl %edx,%edx
jz nofini
pushl %edx
call atexit
popl %eax /* Pop value to unused register to remove
argument from stack. */
nofini:
/* Do essential libc initialization. In statically linked
programs under the GNU Hurd, this is what sets up the
arguments on the stack for the code below. */
call __libc_init_first
/* Extract the arguments and environment as encoded on the stack
and set up the arguments for `main': argc, argv, envp. */
popl %esi /* Pop the argument count. */
leal 4(%esp,%esi,4), %eax /* envp = &argv[argc + 1] */
movl %eax, _environ /* Store it in the global variable. */
movl %esp, %edx /* argv starts just at the current stack top.*/
/* Before pushing the arguments align the stack to a double word
boundary to avoid penalties from misaligned accesses. Thanks
to Edward Seidl <seidl@janed.com> for pointing this out. */
andl $0xfffffff8, %esp
pushl %eax /* Push third argument: envp. */
pushl %edx /* Push second argument: argv. */
pushl %esi /* Push first argument: argc. */
/* Call `_init', which is the entry point to our own `.init'
section; and register with `atexit' to have `exit' call
`_fini', which is the entry point to our own `.fini' section. */
call _init
pushl $_fini
call atexit
popl %eax
/* Call the user's main function, and exit with its value. */
call main
pushl %eax
call exit
hlt /* Crash if somehow `exit' does return. */
/* Define a symbol for the first piece of initialized data. */
.data
.globl __data_start
__data_start:
.long 0
.weak data_start
data_start = __data_start
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