为STM32移植FATFS，读取SD卡上FAT12/16/32文件系统

DSTATUS disk_initialize (BYTE);
DSTATUS disk_status (BYTE);
DRESULT disk_read (BYTE, BYTE*, DWORD, BYTE);
DRESULT disk_write (BYTE, const BYTE*, DWORD, BYTE); // 如果实现只读的文件系统就不需要了。
DRESULT disk_ioctl (BYTE, BYTE, void*);

unsigned int i;
BYTE buffer[512];    // file copy buffer
FATFS fs;            // Work area (file system object) for logical drive
FIL fsrc;            // file objects
FRESULT res;         // FatFs function common result code
UINT br;             // File R/W count
USART1_Puts("Now, I'll read file 'i2c/uart.lst'.\n");

// Register a work area for logical drive 0
f_mount(0, &fs);

// Open source file
res = f_open(&fsrc, "i2c/uart.lst", FA_OPEN_EXISTING | FA_READ);
if (res)
{
USART1_Puts("Can't open i2c/uart.lst for read. :-(\n");
goto exit;
}

for (;;) {
res = f_read(&fsrc, buffer, sizeof(buffer), &br);
if (res || br == 0) break;   // error or eof
for( i = 0; i < br; ++i )
USART1_Putc(buffer[i]);
}

f_close(&fsrc);
exit:
// Unregister a work area before discard it
f_mount(0, NULL);