FIRM

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This page describes the file format for the 3DS' Firmware, it contains four 'sections' of ARM code (ARM9 and ARM11). The firmware sections are not encrypted in the FIRM format.

The ARM9 section contains the ARM9 kernel and the ARM9 process(exheader process name is "Process9"). The ARM11 section(s) contains the ARM11 kernel, and the ARM11 process(es). For NATIVE_FIRM/SAFE_MODE_FIRM these ARM11 processes are sm, fs, pm, loader, and pxi. Normally the 4th section is not used. The code loaded from FIRM is constantly running on the system until another FIRM is launched. The ARM11 kernel is hard-coded to always decompress the FIRM ARM11 modules ExeFS .code, the exheader compression bit is not checked.

FIRM Header

OFFSET SIZE DESCRIPTION
0x000 4 Magic 'FIRM'
0x004 4 Reserved1
0x008 4 ARM11 Entrypoint
0x00C 4 ARM9 Entrypoint
0x010 0x030 Reserved2
0x040 0x0C0 (0x030*4) Firmware Section Headers
0x100 0x100 RSA-2048 signature of the FIRM header, using SHA-256. This is only checked when bootrom/Process9 is doing FIRM-launch, not when installing FIRM to the NAND firm0/firm1 partitions.

Firmware Section Headers

OFFSET SIZE DESCRIPTION
0x000 4 Offset
0x004 4 Address
0x008 4 Size
0x00C 4 Firmware Type ('0'=ARM9/'1'=ARM11) Process9 doesn't use this field at all.
0x010 0x020 SHA-256 Hash of Firmware Section

New_3DS FIRM

For New3DS firmwares (NATIVE_FIRM, TWL_FIRM, ..), the ARM9 FIRM binary has an additional layer of crypto. At the end of each ARM9 binary, there's a plaintext loader. The format of the FIRM header is identical to regular 3DS FIRM(the RSA modulo is the same as regular 3DS too).

Before checking 0x10000000 the loader main() does the following:

  • On 9.5.0-X: executes a nop instruction with r0=0 and r1=<address of arm9binhdr+0x50>.
  • Clears bit6 in REG_AESKEYCNT.

If (u8*)0x10000000 bit 1 is clear (which means that this happens only on hard reboots), it does the following things:

  • Clears 0x200-bytes on the stack, then reads NAND sector 0x96(NAND image offset 0x12C00), with size 0x200-bytes into that stack buffer.
  • Checks u8 0x10000000 bit1 again, if it's set then it executes a panic function(set r0-r2=0, execute nop instruction, then execute instruction "bkpt 0x99"). Hashes data from the region 0x10012000-0x10012090 using SHA256 via the SHA hardware.
  • Clears bit6 in REG_AESKEYCNT. Initializes AES keyslot 0x11 keyX, keyY to the lower and higher portion of the above hash, respectively. Due to the above hashed data, the keyX+keyY here are console-unique.
  • Decrypts the first 0x10-byte block in the above read NAND sector with keyslot 0x11 using AES-ECB. 9.6.0-X: Then it decrypts the 0x10-bytes at offset 0x10 in the sector with keyslot 0x11.
  • Then the normalkey, keyX, and keyY, for keyslot 0x11 are cleared to zero. Runs the TWL key-init/etc code which was originally in the ARM9-kernel, then writes 0x2 to REG_SYSPROT9.
  • Then it uses the above decrypted block from sector+0 to set the normalkey for keyslot 0x11. Decrypts arm9_bin_buf+0 using keyslot 0x11 with AES-ECB, and initialises keyX for keyslot 0x15 with it.
  • 9.6.0-X: Then it uses the above decrypted block from sector+0 to set the normalkey for keyslot 0x11. Decrypts a 0x10-byte block from arm9loader .(ro)data using keyslot 0x11 with AES-ECB, and initializes keyX for keyslot 0x18 with it(same block as previous versions).
  • 9.6.0-X: Starting with this version keyslot 0x16 keyX init was moved here, see below for details on this. The code for this is same as 9.5.0-X, except the decrypted normalkey from sector+0x10 is used for keyslot 0x11 instead.
  • Initialises KeyX for keyslots 0x18..0x1F(0x19..0x1F with 9.6.0-X) with the output of decrypting a 0x10-byte block with AES-ECB using keyslot 0x11. This block was changed to a new one separate from keyslot 0x18, starting with 9.6.0-X. The last byte in this 0x10-byte input block is increased by 0x01 after initializing each keyslot. Before doing the crypto each time, the loader sets the normal-key for keyslot 0x11 to the plaintext normalkey from sector+0(+0x10 with 9.6.0-X). These are New3DS-specific keys.
  • 9.5.0-X(moved to above with 9.6.0-X): Sets the normal-key for keyslot 0x11 to the same one already decrypted on the stack. Decrypts the 0x10-byte block at arm9binhdr+0x60 with AES-ECB using keyslot 0x11, then sets the keyX for keyslot 0x16 to the output data.
  • 9.5.0-X: The normalkey, keyX, and keyY, for keyslot 0x11 are then cleared to zero.

When (u8*)0x10000000 bit 1 is set(which means this happens only when this loader runs again for firm-launch), the normalkey, keyX, and keyY, for keyslot 0x11 are cleared to zero.

It sets KeyY for keyslot 0x15(0x16 with 9.5.0-X) to arm9_bin_buf+16, the CTR to arm9_bin_buf+32 (both are unique for every version). It then proceeds to decrypt the binary with AES-CTR. When done, it sets the normal-key for the keyslot used for binary decryption to zeros. It then decrypts arm9_bin_buf+64 using an hardcoded keyY for keyslot 0x15(9.5.0-X/9.6.0-X also uses keyslot 0x15), sets the normal-key for this keyslot to zeros again, then makes sure the output block is all zeroes. If it is, it does some cleanup then it jumps to the entrypoint for the decrypted binary. Otherwise it will clear the keyX, keyY, and normal-key for each of the keyslots initialized by this loader (on 9.6.0-X+, on older versions this was bugged and cleared keys 0x00..0x07 instead of 0x18..0x1F), do cleanup(same cleanup as when the decrypted block is all-zero) then just loop forever.

Thus, the ARM9 binary has the following header:

OFFSET SIZE DESCRIPTION
0x000 16 Encrypted KeyX (same for all FIRM's)
0x010 16 KeyY
0x020 16 CTR
0x030 8 Size of encrypted binary, as ASCII text?
0x038 8 ?
0x040 16 Control block
0x050 16 Added with 9.5.0-X. Only used for hardware debugging: a nop instruction is executed with r0=0 and r1=<address of this data>.
0x060 16 Added with 9.5.0-X. Encrypted keyX for keyslot 0x16.

Originally the padding after the header before offset 0x800(start of actual ARM9-binary) was 0xFF bytes, with 9.5.0-X this was changed to 0x0.

For the New3DS NATIVE_FIRM arm9-section header, the only difference between the 8.1.0-0_New3DS version and the 9.0.0-20 version is that the keyY, CTR, and the block at 0x30 in the header were updated.

New3DS ARM9 binary loader versions

FIRM system version(s) Description
8.1.0-0_New3DS - 9.3.0-X Initial version.
9.5.0-X Added keyX initialization for keyslot 0x16(see above), and added code for clearing keyslot 0x11 immediately after the code finishes using keyslot 0x11. The keyslot used for arm9bin decryption was changed from 0x15 to 0x16. Added code for clearing keyslot 0x16 when control-block decryption fails. Added code for using arm9bin_hdr+0x50 with a nop instruction, at the very beginning of the main arm9-loader function. Added two new 0x10-blocks to the arm9bin-hdr.
9.6.0-X See above and here.

New3DS ARM9 kernel

The only actual code-difference for the Old3DS/New3DS ARM9-kernels' crt0, besides TWL AES / 0x10012000 related code, is that the New3DS ARM9-kernel writes 0x1 to REG_EXTMEMCNT9 in the crt0.

New3DS Process9

The following is all of the differences for Old3DS/New3DS Process9 with 9.3.0-X:

  • The FIRM-launch code called at the end of the New3DS proc9 main() has different mem-range checks.
  • In the New3DS proc9, the v6.0/v7.0 keyinit function at the very beginning(before the original code) had additional code added for setting CTRNAND keyslot 0x5, with keydata from .data. After setting the keyY, the keyY in .data is cleared.
  • In New3DS proc9, the functions for getting the gamecard crypto keyslots / NCCH keyslot can return New3DS keyslots when New3DS flags(NCSD/NCCH) are set.
  • The code/data for the binary near the end of arm9mem is slightly different, because of memory-region sizes.
  • The only difference in .data(besides the above code binary) is that the New3DS proc9 has an additional 0x10-byte block for the keyslot 0x5 keyY, see above.

NATIVE_FIRM and SAFE_MODE_FIRM

NATIVE_FIRM is the FIRM which is installed to the NAND firm partitions, which is loaded by bootrom. SAFE_MODE_FIRM and NATIVE_FIRM for the initial versions are exactly the same, except for the system core version fields. SAFE_MODE is used for running the System Updater.

An overview of NATIVE_FIRM versions along with their contentID is given in Configuration Memory.

TWL_FIRM and AGB_FIRM

TWL_FIRM handles DS(i) backwards compatibility, while AGB_FIRM handles running GBA VC titles. The ARM9 FIRM section for TWL_FIRM and AGB_FIRM are exactly the same(for TWL_FIRM and AGB_FIRM versions which were updated with the same system-update).

TWL_FIRM

The 3DS-mode ARM9 core seems to switch into DSi-mode(for running DSi-mode ARM9 code) by writing to a PDN register(this changes the memory layout to DSi-mode / etc, therefore this register poke *must* be executed from ITCM). This is the final 3DS-mode register poke before the ARM9 switches into DSi-mode. DS(i)-mode ARM7 code is run on the internal ARM7 core, which is started up during TWL_FIRM boot. Trying to read from the exception-vector region(address 0x0) under this DSi-mode ARM7 seems to only return 0x00/0xFF data. Also note that this DSi-mode ARM7 runs code(stored in TWL_FIRM) which pokes some DSi-mode registers that on the DSi were used for disabling access to the DSi bootROMs, however these registers do not affect the 3DS DSi-mode ARM9/ARM7 "bootrom" region(exceptionvector region + 0x8000) at all.

For shutting down the system, TWL_FIRM writes u8 value 8 to I2C MCU register 0x20. For returning to 3DS-mode, TWL_FIRM writes value 4 to that MCU register to trigger a hardware system reboot.

The TWL_FIRM ARM11-process includes a TWL bootloader, see here and here for details.

TWL_FIRM verifies all TWL RSA padding with the following. This is different from the DSi "BIOS" code.

  • The first byte must be 0x0.
  • The second byte must be 0x1 or 0x2.
  • Executes a while(<value of byte at current pos in RSA message>). When the second_byte in the message is 0x1, the byte at curpos must be 0xFF(otherwise the non-zero value of the byte at curpos doesn't matter). This loop must find a zero byte before offset 0x7F in the message otherwise an error is returned.
  • Returns an address for msg_curpos+1.

totalhashdatasize = rsasig_bytesize - above position in the message for the hashdata. The actual "totalhashdatasize" in the RSA message must be <= <expected hashdata_size>(0x74 for bootloader). The TWL_FIRM code copies the RSA "hashdata" to the output buffer, using the actual size of the RSA "hashdata".

FIRM Launch Parameters

The FIRM-launch parameters structure is located at FCRAM+0, size 0x1000-bytes. The ARM11-kernel copies this structure elsewhere, then clears the 0x1000-bytes at FCRAM+0. It will not handle an existing structure at FCRAM+0 if CFG_BOOTENV is zero. The ARM9 kernel writes some values about the boot environment to AXI WRAM during init to enable this.

OFFSET SIZE DESCRIPTION
0x400 0x4 Flags
0x410 0xC This is used for overriding the FIRM_* fields in Configuration_Memory, when the flag listed below is set, in the following order(basically just data-copy from here to 0x1FF80060): "FIRM_?", FIRM_VERSIONREVISION, FIRM_VERSIONMINOR, FIRM_VERSIONMAJOR, FIRM_SYSCOREVER, and FIRM_CTRSDKVERSION.
0x438 0x4 The kernel checks this field for value 0xFFFF, if it matches the kernel uses the rest of these parameter fields, otherwise FIRM-launch parameters fields are ignored by the kernel.
0x43C 0x4 CRC32, this is calculated starting at FIRM-params offset 0x400, with size 0x140(with this field cleared to zero during calculation). When invalid the kernel clears the entire buffer used for storing the FIRM-params, therefore no actual FIRM-params are handled after that.
0x440 0x10 Titleinfo structure, used by NS during NS startup, to launch the specified title when the below flag is set.
0x450 0x10 Titleinfo structure. This might be used for returning to the specified title, once the above launched title terminates?
0x460 0x4 Bit0: 0 = titleinfo structure isn't set, 1 = titleinfo structure is set.
0x480 0x20 This can be set via buf1 for APT:SendDeliverArg/APT:StartApplication.
0x4A0 0x10 This can be set by NSS:SetFIRMParams4A0.
0x4B0 0x14 SHA1-HMAC of the banner for TWL/NTR titles. This can be set by NSS:SetFIRMParams4B0.
0x500 0x40 This is used by APT:LoadSysMenuArg and APT:StoreSysMenuArg.

Flags from offset 0x400:

OFFSET SIZE DESCRIPTION
0x0 0x1 This can be used for overriding the default FCRAM memory-regions allocation sizes(APPLICATION, SYSTEM, and BASE). The values for this is the same as Configmem-APPMEMTYPE. Values 0-1 are handled the same way by the kernel. However for NS, 0=titleinfo structure for launching a title isn't set, while non-zero=titleinfo structure is set.
0x1 0x3 Setting bit0 here enables overriding the FIRM_* fields in Configuration_Memory.
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