Registers

Old3DS	Name	Address	Width
Yes	SPI_CNT0	0x10142000	2
Yes	SPI_DATA0	0x10142002	1
Yes	SPI_NEW_CNT0	0x10142800	4
Yes	SPI_NEW_???0	0x10142804	4
Yes	SPI_NEW_BLKLEN0	0x10142808	4
Yes	SPI_NEW_FIFO0	0x1014280C	4
Yes	SPI_NEW_STATUS0	0x101428010	4
Yes	SPI_CNT1	0x10143000	2
Yes	SPI_DATA1	0x10143002	1
Yes	SPI_NEW_CNT1	0x10143800	4
Yes	SPI_NEW_???1	0x10143804	4
Yes	SPI_NEW_BLKLEN1	0x10143808	4
Yes	SPI_NEW_FIFO1	0x1014380C	4
Yes	SPI_NEW_STATUS1	0x101438010	4
Yes	SPI_CNT2	0x10160000	2
Yes	SPI_DATA2	0x10160002	1
Yes	SPI_NEW_CNT2	0x10160800	4
Yes	SPI_NEW_???2	0x10160804	4
Yes	SPI_NEW_BLKLEN2	0x10160808	4
Yes	SPI_NEW_FIFO2	0x1016080C	4
Yes	SPI_NEW_STATUS2	0x101608010	4

SPI_CNT

This is the old NDS/DSi SPI interface.

Old3DS	Name
0-1	Baudrate (0=4MHz/Firmware, 1=2MHz/Touchscr, 2=1MHz/Powerman., 3=512KHz)
2-6	Not used (Zero)
7	Busy Flag (0=Ready, 1=Busy) (presumably Read-only)
8-9	Device Select (0-3)
10	Transfer Size (0=8bit/Normal, 1=16bit/Bugged)
11	Chipselect Hold (0=Deselect after transfer, 1=Keep selected)
12-13	Not used (Zero)
14	Interrupt Request (0=Disable, 1=Enable)
15	SPI Bus Enable (0=Disable, 1=Enable)

This is an alternative faster interface introduced with the 3DS.

Old3DS	Name
6-7	Device Select
13	Transfer Direction? (0=Incoming, 1=Outgoing)
15	Busy/enable

The number of bytes to be sent/read is written to this register.

32-bit FIFO for reading/writing the SPI payload.

Bit0: FIFO busy.