注意：重定位时要保证编译后的二进制文件中，操作 flash 和初始化 SDRAM 的程序必须在前4K中。否则会出错。

一、判断启动flash：

int isBootFromNorFlash(void) {

    volatile unsigned int *p = (volatile unsigned int *)0;

    unsigned int val = *p;    // 读出0地址存放的值

    *p = 0x66;

    if (*p == 0x66) { // 写成功, 对应nand启动

        *p = val; // 回写

        return 0;

    }

    else {

        return 1;

    }

}

二、重定位：

void copy2sdram(void) {

    extern int __code_start, __bss_start;    // 从链接脚本中获得起始结束地址

    volatile unsigned int *dest = (volatile unsigned int *)&__code_start;

    volatile unsigned int *end = (volatile unsigned int *)&__bss_start;

    volatile unsigned int *src = (volatile unsigned int *)0;

    int len = ((int)&__bss_start) - ((int)&__code_start);

    if (isBootFromNorFlash()) {

        while (dest < end) {

            *dest++ = *src++;

        }

    }

    else {

        nand_init();

        nand_read(src, dest, len);

    }

}

三、清除bss段：

void clean_bss(void) {

    extern int _end, __bss_start;

    volatile unsigned int *start = (volatile unsigned int *)&__bss_start;

    volatile unsigned int *end = (volatile unsigned int *)&_end;

    while (start <= end) {

    *start++ = 0;

    }

}