glibc/sysdeps/nios2/start.S

/* Startup code for Nios II
   Copyright (C) 1995-2021 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 Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   In addition to the permissions in the GNU Lesser General Public
   License, the Free Software Foundation gives you unlimited
   permission to link the compiled version of this file with other
   programs, and to distribute those programs without any restriction
   coming from the use of this file. (The GNU Lesser General Public
   License restrictions do apply in other respects; for example, they
   cover modification of the file, and distribution when not linked
   into another program.)

   Note that people who make modified versions of this file are not
   obligated to grant this special exception for their modified
   versions; it is their choice whether to do so. The GNU Lesser
   General Public License gives permission to release a modified
   version without this exception; this exception also makes it
   possible to release a modified version which carries forward this
   exception.

   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
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library.  If not, see
   <https://www.gnu.org/licenses/>.  */

/* This is the canonical entry point, usually the first thing in the text
   segment.

	Note that the code in the .init section has already been run.
	This includes _init and _libc_init

	The stack pointer, sp, will point to the argument count on the stack.
	The initial state of the stack when a userspace process is started is:

	    Purpose			Start Address	Length
	    Unspecified			High Addresses
	    Referenced strings, etc.			Varies
	    Unspecified
	    Null auxilliary vector entry		4bytes
	    Auxilliary vector entries			8bytes each
	    NULL terminator for envp			4bytes
	    Environment pointers	sp+8+4*argc	4bytes each
	    NULL terminator for argv	sp+4+4*argc	4bytes
	    Argument pointers		sp+4		4bytes each
	    Argument count		sp		4bytes
	    Unspecified			Low Addresses

	If the application should register a destructor function with atexit,
	the pointer will be placed in r4. Otherwise r4 will be zero.

	The contents of all other registers are unspecified. User code should
	set fp to zero to mark the end of the frame chain.

	The auxilliary vector is a series of pairs of 32-bit tag and 32-bit
	value, terminated by an AT_NULL tag.
*/

	.text
	.globl _start
	.type _start,%function
_start:
	/* Set up the global pointer.  */
	movhi	gp, %hiadj(_gp)
	addi	gp, gp, %lo(_gp)

	/* Save the stack pointer.  */
	mov	r2, sp

	/* Create room on the stack for the fini, rtld_fini and stack_end args
	   to __libc_start_main. */
	subi	sp, sp, 12

	/* Push stack_end */
	stw	r2, 8(sp)

	/* Push rtld_fini */
	stw	r4, 4(sp)

	/* Set up the GOT pointer.  */
	nextpc	r22
1:	movhi	r2, %hiadj(_gp_got - 1b)
	addi	r2, r2, %lo(_gp_got - 1b)
	add	r22, r22, r2

	/* Used to be fini.  */
	stw	zero, 0(sp)

	/* Used to be init.  */
	mov	r7, zero

	/* r6 == argv */
	addi	r6, sp, 16

	/* r5 == argc */
	ldw	r5, 12(sp)

	/* r4 == main */
	movhi	r4, %call_hiadj(main)
	addi	r4, r4, %call_lo(main)
	add	r4, r4, r22
	ldw	r4, 0(r4)

	/* fp == 0 */
	mov	fp, zero

	/* __libc_start_main (main, argc, argv, init, fini, rtld_fini,
			      stack_end) */

	/* Let the libc call main and exit with its return code.  */
	movhi	r2, %call_hiadj(__libc_start_main)
	addi	r2, r2, %call_lo(__libc_start_main)
	add	r2, r2, r22
	ldw	r2, 0(r2)
	callr	r2

	/* should never get here....*/
	movhi	r2, %call_hiadj(abort)
	addi	r2, r2, %call_lo(abort)
	add	r2, r2, r22
	ldw	r2, 0(r2)
	callr	r2

/* 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