全文地址：http://blog.csdn.net/henhen2002/archive/2010/03/09/5360162.aspx
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/http://www.cnblogs.com/http://www.cnblogs.com/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/http://www.cnblogs.com/http://www.cnblogs.com/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。
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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"), *
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* The above copyright notice and this permission notice shall be included in *
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* 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 *
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* *
* 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. * *
******************************************************************************/