各位高手，小弟正在向NIOS上移植FATFS文件系统。移植完成后，调用f_open创建并打开一个文件，这个时候遇到“打开失败”。调试了一下，发现check_fs函数中的LD_WORD
(&fs->win[BS_55AA])!=0xAA55判断失败，返回2。请教其中的原因。谢谢！部分代码如下：
static BYTE check_fs (
/* 0:FAT boor sector, 1:Valid boor sector but not FAT, 2:Not a boot sector, 3 isk error */
FATFS* fs,
/* File system object */
DWORD sect
/* Sector# (lba) to check if it is an FAT boot record or not */
) {
WORD temp;
fs->wflag = 0; fs->winsect = 0xFFFFFFFF;
/* Invaidate window */
if (move_window(fs, sect) != FR_OK)
/* Load boot record */
return 3;
if (LD_WORD(&fs->win[BS_55AA]) != 0xAA55)
/* Check boot record signature (always placed at offset 510 even if the sector size is >512) */
return 2;
if ((LD_DWORD(&fs->win[BS_FilSysType]) & 0xFFFFFF) == 0x544146)
/* Check "FAT" string */
return 0;
if ((LD_DWORD(&fs->win[BS_FilSysType32]) & 0xFFFFFF) == 0x544146)
/* Check "FAT" string */
return 0;
return 1;
} | 
