实现了对SD卡的SPI方式下读写操作，已经测试了，可以直接用-The realization of the SD card to read and write SPI operation mode has been tested, can be directly used

关于读写SD卡的规范和协议；有助于进一步了解关于SD构造-Norms and agreements on the SD card reader shed further light on the SD structure