关于alt_main和main

NiosII处理器的启动可采用两种方式： 自动初始化和用户自定义初始化，nios中alt_main 和main的区别也在这里。

如果NiosII处理器自动初始化，ANSI C标准定义应用程序可以通过调用main()来开始执行。在调用main()之前，应用程序假定运行环境和所有的服务系统都被初始化并准备运行。初始化可以被硬件抽象层（HAL）系统库自动执行。程序员不需要考虑系统的输出设备以及如何初始化每一个外设，HAL会自动初始化整个系统。

 如果用户要避免自动初始化。ANSI C标准将提供了一个可变的入口点程序，定义程序员能手动初始化任何所用的硬件。alt_main()函数提供了一个独立式的编程环境，能够完全控制系统的初始化。例如用alt_irq_init (ALT_IRQ_BASE)函数初始化中断控制器。具体方法在例程hello_alt_main中能找到。

另外，能使用main函数作为开头开头时cpu可能需要更大的ram（ on chip memory ）空间。

最简单的例子（ram都设为4k的条件下）：
程序1：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "system.h"
#include "sys/alt_sys_init.h"
#include "sys/alt_irq.h"
#include "priv/alt_file.h"
int main (void) __attribute__ ((weak, alias ("alt_main")));
int alt_main(void)
{
  alt_irq_init (ALT_IRQ_BASE);
  alt_sys_init();
  alt_io_redirect (ALT_STDOUT, ALT_STDIN, ALT_STDERR);
  int i;
  i=1;
  return 0;
}
－－－－－－－－这样就没问题，编译能通过！！！！！
如果改成这样：
程序2
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "system.h"
#include "sys/alt_sys_init.h"
#include "sys/alt_irq.h"
#include "priv/alt_file.h"
int main(void)
{
  int i;
  i=1;
return 0;
}
－－－－－－－－－－编译就不能通过了，提示如下  
Console中的提示——————————————————
**** 00000047] overlaps section .exceptions [00000020 -> 00000ab7] overlaps section .exceptions [00000020 -> 00000ccb] overlaps section .text [000001c8 -> 00000ccb] overlaps section .text [000001c8 -> 0000261b]
/cygdrive/d/MyProgram/altera/kits/nios2_60/bin/nios2-gnutools/H-i686-pc-cygwin/bin/../lib/gcc/nios2-elf/3.4.1/../../../../nios2-elf/bin/ld section .rodata [00000020 -> 00000047] overlaps section .exceptions [00000020 -> 000001c7]
/cygdrive/d/MyProgram/altera/kits/nios2_60/bin/nios2-gnutools/H-i686-pc-cygwin/bin/../lib/gcc/nios2-elf/3.4.1/../../../../nios2-elf/bin/ld section .rwdata [00000048 -> 00000ab7] overlaps section .exceptions [00000020 -> 000001c7]

region onchip_memory_0 is full 这就是提示ram不够，根据程序适当改大一点后就能编译了。

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一个问题整了几天，原来源于main()和alt_main()

2007-02-02 09:12

一个计时器中断程序始终跑不起来： volatile alt_u8 led; static void handle_Timer_interrupts(void* context, alt_u32 id) {     volatile alt_u8 *led_ptr = (volatile alt_u8 *)context;     IOWR_ALTERA_AVALON_TIMER_STATUS(TIMER_0_BASE, 0);//清TO标志     IOWR_ALTERA_AVALON_PIO_DATA(PIO_0_BASE, ++*led_ptr); //写到LED输出口 } int main (void) __attribute__ ((weak, alias ("alt_main"))); int alt_main (void) {     alt_irq_register( TIMER_0_IRQ,(void *)&led, handle_Timer_interrupts);          IOWR_ALTERA_AVALON_TIMER_CONTROL(TIMER_0_BASE, 7);     while(1){};         return 0; } alt_irq_init (ALT_IRQ_BASE); 原来是中断控制器没初始化，在alt_main开始加入alt_irq_init (ALT_IRQ_BASE);即搞定了。但是当用main代替alt_main时，不需要alt_irq_init 中断也能运行。 后来才查到：     NiosII处理器的启动可采用两种方式： 自动初始化和用户自定义初始化。ANSI C标准定义应用程序可以通过调用main()来开始执行。在调用main()之前，应用程序假定运行环境和所有的服务系统都被初始化并准备运行。初始化可以被硬件抽象层（HAL）系统库自动执行。程序员不需要考虑系统的输出设备以及如何初始化每一个外设，HAL会自动初始化整个系统。    另外，ANSI C标准也提供了一个可变的入口点程序，以避免自动初始化。ANSI C标准还定义程序员能手动初始化任何所用的硬件。alt_main()函数提供了一个独立式的编程环境，能够完全控制系统的初始化。        如果用户不编写alt_main()函数，则系统会到默认先调用alt_main.c（在<nios2安装目录>/components/altera_hal/HAL/src）里面的alt_main()，这个函数进行如下操作    ① 调用ALT_OS_INIT()来执行任何操作系统所特有的初始化。如果HAL是在操作系统里运行的，那么初始化alt_fd_list_lock命令。它可以控制访问HAL文件系统，初始化中断控制器并执行中断。    ② 调用alt_sys_init()函数，以初始化系统里所有的驱动装置和软件组成部分。    ③ 重新设置C标准I/O通道（stdin，stdout，stderr）,以使用合适的器件。    ④ 调用main()。    ⑤ 调用exit()。main()的返回代码作为exit()的输入。 　也就是说在NiosII IDE工程中，只需简单定义alt_main()就可以实现用户的启动顺序，而且能够选择HAL的服务程序。如果应用程序需要一个alt_main()入口点程序，可以复制默认的执行作为开始点，根据要求来定制它。 ＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝   nios中alt_main 和main的区别 [ 2007-3-31 21:36:00 | By: caopengly ] nios中alt_main 和main的区别? caopengly: 一般程序都只有一个入口地址，main，但是为了程序的初始化方便或利用继承性（c++）中，定义了main的别名，这里的程序入口你可以想象成alt_main()是main（）函数的内部隐藏带有驱动的入口。区别看了就知道了。 请看下面的程序 /* Copyright ?2004 Altera Corporation, San Jose, California, USA. All rights  * reserved. All use of this software and documentation is subject to the License  * Agreement located at the end of this file below.  ******************************************************************************  *  DANGER ** WARNING ** Please read before proceeding! ** WARNING ** DANGER  *            ******************************************************************************  *  * "Hello World Free-Standing" (hello_alt_main) example.  *  * This program is an example of a "free-standing" C application since it  * calls "alt_main()" instead of "main()". The example's purpose is to  * illustrate to the advanced embedded developer the low-level system  * initialization steps needed for a "Hello World" type application. By  * calling "alt_main()" instead of "main()", these system initialization steps  * are NOT linked in automatically, and must be provided by the developer,  * which this example illustrates.  *  * Please refer to file readme.txt for notes on this software example.  */   ＃i nclude <stdio.h> ＃i nclude <stdlib.h> ＃i nclude "system.h" ＃i nclude "sys/alt_sys_init.h" ＃i nclude "sys/alt_irq.h" ＃i nclude "priv/alt_file.h" /*  * The following statement defines "main()" so that when the Nios II IDE  * debugger is set to break at "main()", it will break at the appropriate  * place in this program, which does not contain a called "main()".  * Note that the Nios II IDE debugger can also be set to break at "alt_main()",  * in which case the following statement would be unneccessary since  * "alt_main()" is defined in this program.  */ int main (void) __attribute__ ((weak, alias ("alt_main"))); /*  * _do_ctors() is used to call the C++ constructors.  *  * It is declared weak so that we don't have to force  * inclusion of _do_ctors if there are no C++ constructors  * to call.  *  * Commended out here because this example is not a C++ program:  */ //extern void _do_ctors(void) ALT_WEAK; int alt_main(void) {   /*    * Enable Interrutps    *    * Turn on interrupts, and initialize the low-level interrupt handler:    */   alt_irq_init (ALT_IRQ_BASE);       /*    * Device-Driver Initialization    *    * Initialize all the device drivers for every piece of hardware    * in the current system.     *    * Note that the "alt_sys_init()" is defined in the    * AUTOMATICALLY-GENERATED file alt_sys_init.c. This file is    * generated "on the fly" into your System Library project    * at library-build time (as part of the Make-process). You can find the    * file here:    *    <system library name>/Release/system_deion/alt_sys_init.c    *    * Because it is generated on-the-fly, your library's copy of    * alt_sys_init() is customized to initialize *only* the devices    * in your *particuilar* Nios II system.     *    * Being machine-generated, alt_sys_init() initializes every single    * device in your system--even ones which your application may not    * use. Indeed, this "Hello World" program only uses one--the    * STDOUT device. If you wish to save code-space, you may want    * to initialize only the devices you actually need and use.    *    * To do so, you should define your own (e.g. small_sys_init()).    * DO NOT edit the file alt_sys_init.c! If you do, your changes will be    * overwritten the next time you build the library! Please refer to the    * "readme.html" file that accompanies this software example to see how    * such a customized sys_init routine would look for UART initialization ONLY.    */    alt_sys_init();   /*    * I/O Stream Initialization.    *    * Initialize the STDOUT stream, and associate it with the    * System Library's designated STDOUT device.  Note that the symbols    * ALT_STDOUT (etc) are defined in the (generated) file system.h.    *    */   alt_io_redirect (ALT_STDOUT, ALT_STDIN, ALT_STDERR);   /*    * C++ Constructors.    *    * This particular example is not a C++ program.  But, if it were,    * you would need to call all your static constructors as part of    * the initialization process.  To do so, you would un-comment this    * line:    *    */    //_do_ctors();   printf("Hello from Nios II Free-Standing!/n");   /*    * Exit gracefully.    *    * Many embedded programs run as long as the machine is powered-up.    * Those programs don't need to call exit().  But even this humble    * little "Hello World" application (which, indeed, does terminate)    * needs to call exit().  The exit() , amongst many other    * things, flushes the I/O buffers--a singularly-important service    * for this example:    */   exit(0);  // Return code for "success!" if anyone is checking (they aren't). } /****************************************************************************** *                                                                             * * License Agreement                                                           * *                                                                             * * Copyright (c) 2004 Altera Corporation, San Jose, California, USA.           * * All rights reserved.                                                        * *                                                                             * * Permission is hereby granted, free of charge, to any person obtaining a     * * copy of this software and associated documentation files (the "Software"),  * * to deal in the Software without restriction, including without limitation   * * the rights to use, copy, modify, merge, publish, distribute, sublicense,    * * and/or sell copies of the Software, and to permit persons to whom the       * * Software is furnished to do so, subject to the following conditions:        * *                                                                             * * The above copyright notice and this permission notice shall be included in  * * all copies or substantial portions of the Software.                         * *                                                                             * * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR  * * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,    * * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE * * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER      * * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING     * * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER         * * DEALINGS IN THE SOFTWARE.                                                   * *                                                                             * * This agreement shall be governed in all respects by the laws of the State   * * of California and by the laws of the United States of America.              * * Altera does not recommend, suggest or require that this reference design    * * file be used in conjunction or combination with any other product.          *                                                                 * ******************************************************************************/