Stub (only covers SPI bus for storage).

SPI data port used for communication with the SD card. Write to send and receive a byte; read to get the last received byte.

7  bit  0
---- ----
b... .als
|     |||
|     ||+- Chip select: 1 = high, 0 = low
|     |+-- 1 to use ~400kHz speed (for init), 0 to use full speed
|     +--- ?
+--------- 1 if SPI byte transfer ongoing

Write 0xA5 to unlock registers; write 0x00 to lock registers.