To unlock this register, one must write 0xA55A to it first.

15       bit       0
 ---- ---- ---- ----
 ???? ???? ???? ?mm?
                 |||
                 ||+- ?
                 ||
                 ++-- Mode:
                      0 - ?
                      1 - CompactFlash
                      2 - ?
                      3 - ?

For CompactFlash card access, the SuperCard CF mimics the GBA Movie Player interface.

SD card 4-bit data bus writes.

15       bit       0
 ---- ---- ---- ----
 oooo .... .... ....
 ||||
 ++++---------------- Written value.

SD card 4-bit data bus reads.

TODO: Most likely operates as a 16-bit shift register. This means that, to read sixteen bits in order, the address must be accessed four times.

15       bit       0
 ---- ---- ---- ----
 oooo .... .... ....
 ||||
 ++++---------------- Written value.

SD card 1-bit command bus.

15       bit       0
 ---- ---- ---- ----
 .... ..?? o... ...i
        || |       |
        || |       +- Input (from card)
        || |
        || +--------- Output (to card)
        ||
        ++----------- Related to card insertion?