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.
*/
#include <stdio.h>
#include <stdlib.h>
#include "system.h"
#include "sys/alt_sys_init.h"
#include "sys/alt_irq.h"
#include "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 function 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 function "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_description/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 function (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() function, 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. * *
******************************************************************************/ | 
