FIRM

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.

Firmware Section Headers

OFFSET	SIZE	DESCRIPTION
0x000	4	Offset
0x004	4	Address
0x008	4	Size
0x00C	4	Firmware Type ('0'=ARM9/'1'=ARM11)
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).

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

• Hashes data from the region 0x10012000-0x10012090 using SHA256 via the SHA hardware.
• Initializes AES keyslot 0x11 keyX, keyY to the lower and higher portion of that hash, respectively.
• Decrypts an unknown 0x10-byte block with keyslot 0x11 using AES-ECB. Then the normalkey, keyX, and keyY, for keyslot 0x11 are cleared to zero. Then it uses the output block 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.
• Initialises KeyX for keyslots 0x18-0x20 with the output of decrypting a certain binary sequence with AES-ECB using keyslot 0x11. These are presumably New3DS-specific keys.
• The normalkey, keyX, and keyY, for keyslot 0x11 are cleared to zero.

It sets KeyY for keyslot 0x15 to arm9_bin_buf+16, the CTR to arm9_bin_buf+32. It then proceeds to decrypt the binary with AES-CTR. When done, it decrypts arm9_bin_buf+64 using an hardcoded keyY for keyslot 0x15 and makes sure it's all zeroes. If it is, it does some cleanup then it jumps to the entrypoint for the decrypted binary. Otherwise it will 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	16	?
0x040	16	Control block

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.

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.

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. It's unknown how exactly DS(i)-mode ARM7 code is run. 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.

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.

OFFSET	SIZE	DESCRIPTION
0x3C	0x4	CRC32, this is calculated starting at FIRM-params offset 0x0, 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.
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.
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	0x10	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.