These registers are used to access the system NAND and the inserted SD card. Both devices use the same interface.

Registers

Old3DS	Name	Address	Width
Yes	EMMC_CMD	0x10006000	2
Yes	?	0x10006002	2
Yes	EMMC_CMDARG0	0x10006004	2
Yes	EMMC_CMDARG1	0x10006006	2
Yes	EMMC_STOP	0x10006008	2
Yes	EMMC_BLKCOUNT	0x1000600a	2
Yes	EMMC_RESP0-7	0x1000600c	2*8
Yes	EMMC_STATUS0	0x1000601c	2
Yes	EMMC_STATUS1	0x1000601e	2
Yes	?	0x10006020	2
Yes	?	0x10006022	2
Yes	EMMC_CLKCTL	0x10006024	2
Yes	EMMC_BLKLEN	0x10006026	2
Yes	EMMC_OPT	0x10006028	2
Yes	EMMC_FIFO	0x10006030	2
Yes	EMMC_RESET	0x100060E0	2

The IO interface for SDMC/NAND seems to be very similar to the DSi; see libnds for sample code and documentation.

3DS SDMC/NAND IO registers are located at 0x10006000 and apparently mirrored at 0x10007000. It seems the 0x10007000 mirror is never used on retail units.

EMMC Registers

Registers