Original research provided mostly by lifehackerhansol.

• KEY1 gap1 length: 0
• KEY1 gap2 length: 24
• KEY2 encryption: Enabled for data and commands
• Transfer clock rate: 6.7 MHz
• Data block size: 4 bytes (512 bytes for SD commands)

31                 bit                 0
 ???? ???? ???? ???? ???? ???m mmmm msss
                             | |||| ||||
                             | |||| |+++- Initialization status:
                             | |||| |      0 - (in progress)
                             | |||| |      1 - SD/TF card not found
                             | |||| |      2 - SD/TF card read error
                             | |||| |      3 - could not find _DS_MENU.DAT
                             | |||| |      4 - success
                             +-++++-+---- Model:
                                           0x3A - M3DS Simply
                                           0x3E - R4 Revolution for DS

SD read request, but using the “save” FAT entry table.

SD read data, but using the “save” FAT entry table.

• (MSB) sd sc sb sa (LSB) - as follows:

31                 bit                 0
 ssss ssss ssss ssss ssss ssss sss? ???t
 |||| |||| |||| |||| |||| |||| |||     |
 |||| |||| |||| |||| |||| |||| |||     +- Type:
 |||| |||| |||| |||| |||| |||| |||         0 - ROM
 |||| |||| |||| |||| |||| |||| |||         1 - Save
 ++++-++++-++++-++++-++++-++++-+++------- Directory table address
                                           (aligned to 0x20 per FAT spec)

SD read request, but using the “ROM” FAT entry table.

Points to _DS_MENU.DAT on initialization.

SD read data, but using the “ROM” FAT entry table.

• (MSB) sd sc sb sa (LSB) - sector address, in bytes

Returns zero when data is ready to access, non-zero if ongoing.

Returns the 512 bytes read via the “SD read start” command.

• (MSB) sd sc sb sa (LSB) - sector address, in bytes

Followed by 512 bytes of SD card data.

• (MSB) sd sc sb sa (LSB) - sector address, in bytes

Returns zero if done, non-zero if ongoing.

SD write start, but using the “save” FAT entry table.

SD write status, but using the “save” FAT entry table.

Same as SD read data, but applying a XOR-based obfuscation algorithm to the buffer's contents.

This is used by the stage 1 loader to decrypt _DS_MENU.DAT.

The algorithm was initially discovered by Yasu in 2007, and example source code is provided as follows, where key is a 16-bit constant:

uint8_t[512] in;
uint8_t[512] out;
uint16_t key1 = key ^ currentSector;
    
for (int i = 0; i < 512; i++)
{
    // Derive key2 from key1.
    uint8_t key2 = ((key1 >> 7) & 0x80)
        | ((key1 >> 6) & 0x60)
        | ((key1 >> 5) & 0x10)
        | ((key1 >> 4) & 0x0C)
        | (key1 & 0x03);
        
    // Decrypt byte.
    dest[i] = src[i] ^ key2;

    // Derive next key1 from key2.
    uint16_t tmp = ((src[i] << 8) ^ key1);
    uint16_t tmpXor = 0;
    for (int ii = 0; ii < 16; ii++)
        tmpXor ^= (tmp >> ii);

    uint16_t newKey1 = 0;
    newKey1 |= ((tmpXor & 0x80) | (tmp & 0x7C)) << 8;
    newKey1 |= ((tmp ^ (tmpXor >> 14)) << 8) & 0x0300;
    newKey1 |= (((tmp >> 1) ^ tmp) >> 6) & 0xFC;
    newKey1 |= ((tmp ^ (tmpXor >> 1)) >> 8) & 0x03;

    key1 = newKey1;
}