在启动过程中，确保At91bootstrap成功地加载了应用程序代码。可以通过一些调试工具来检查问题所在。代码示例：

1. 在启动过程中，设置调试标志：

#define CONFIG_DEBUG_AT91_MCI

2. 在At91bootstrap启动中添加debug打印信息来输出更多的信息：

#define CONFIG_BOARD_LATE_INIT
int board_late_init(void)
{
    char ver[VERSION_SIZE];
    memset(ver, 0, VERSION_SIZE);
    strcpy(ver, "U-BOOT ");
    strcat(ver, PLAIN_VERSION);
    strcat(ver, " (" U_BOOT_DATE " - " U_BOOT_TIME ")");
    debug("U-Boot %s\n", ver);
    return 0;
}

3. 通过AT91 MCI控制器读取SD卡，并确保文件系统成功加载。

#include 

#define CONFIG_GENERIC_ATMEL_MCI
#define CONFIG_SYS_MMC_MAX_DEVICE 1
#define CONFIG_SYS_MMC_MAX_BLK_COUNT 64

int mmc_initialize(bd_t *bis)
{
    int ret;
    struct mmc *mmc;

    ret = atmel_mci_init();
    if (ret != 0) {
        panic("Failed to init MCI controller\n");
        return -1;
    }

    mmc = mmc_create(0, NULL);
    if (mmc == NULL) {
        panic("Failed to create MMC device\n");
        return -1;
    }

    ret = mmc_init(mmc);
    if (ret != 0) {
        panic("Failed to init MMC\n");
        return -1;
    }

    ret = mmc_detect_card(mmc);
    if (ret != 0) {
        panic("Failed to detect MMC card\n");
        return -1;
    }

    ret = blk_register_device(bis, "mmc", 0, CONFIG_SYS_MMC_MAX_BLK_COUNT,
                              mmc_block_dev_desc, 0);
    if (ret < 0) {
        panic("Failed to register MMC block device\n");
        return -1;
    }

    return 0;
}

int board_late_init(void)
{
    struct mmc *mmc;
    int ret;

    ret = mmc_initialize(&bd);
    if (ret != 0)
        return -1;

    mmc = find_mmc_device(0);
    if (mmc == NULL)
        return -1;

    ret = fat_register_device(mmc, 1, CONFIG_SYS_MMC_MAX_BLK_COUNT,
                              "mmc", "/");
    if (ret != 0)
        return -1;

    return 0;
}