NCCH/Extended Header

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This page documents the format of the NCCH Extended Header, or exheader for short.

The exheader has two sections:

  • The actual exheader data, containing System Control Info (SCI) and Access Control Info (ACI);
  • A signed copy of NCCH HDR public key, and exheader ACI. This version of the ACI is used as limitation to the actual ACI.

Main Structure[edit]

All values are little endian unless otherwise specified.

See also: [1]

Offset Size Description
0x0 0x200 SCI
0x200 0x200 ACI
0x400 0x100 AccessDesc signature (RSA-2048-SHA256)
0x500 0x100 NCCH HDR RSA-2048 public key
0x600 0x200 ACI (for limitation of first ACI)

The AccessDesc signature covers the NCCH HDR public key and second ACI. The AccessDesc public key is initialised by the boot ROM.

When loading the exheader, Process9 compares the exheader data with the data in the AccessDesc (note that not everything is compared here). When these don't match, an error is returned. The Process9 code handling this validation was updated with v6.0; the only change in this function seems to be the check for the "Ideal processor" field.

System Control Info[edit]

Offset Size Description
0x0 0x8 Application title (default is "CtrApp")
0x8 0x5 Reserved
0xD 0x1 Flag (bit 0: CompressExefsCode, bit 1: SDApplication)
0xE 0x2 Remaster version
0x10 0xC Text code set info
0x1C 0x4 Stack size
0x20 0xC Read-only code set info
0x2C 0x4 Reserved
0x30 0xC Data code set info
0x3C 0x4 BSS size
0x40 0x180 (48*8) Dependency module (program ID) list
0x1C0 0x40 SystemInfo

Most of these fields are used in LOADER:LoadProcess.

Code Set Info[edit]

Offset Size Description
0x0 4 Address
0x4 4 Physical region size (in page-multiples)
0x8 4 Size (in bytes)

System Info[edit]

Offset Size Description
0x0 0x8 SaveData Size
0x8 0x8 Jump ID
0x10 0x30 Reserved

Access Control Info[edit]

Offset Size Description
0x0 0x170 ARM11 local system capabilities
0x170 0x80 ARM11 kernel capabilities
0x1F0 0x10 ARM9 access control

ARM11 Local System Capabilities[edit]

Offset Size Description
0x0 0x8 Program ID
0x8 0x4 Core version (The Title ID low of the required FIRM)
0xC 0x2 Flag1 and Flag2 (both implemented starting from 8.0.0-18).
0xE 0x1 Flag0
0xF 0x1 Priority
0x10 0x20 (16*2) Resource limit descriptors. The first byte here controls the maximum allowed CpuTime.
0x30 0x20 Storage info
0x50 0x100 (32*8) Service access control
0x150 0x10 (2*8) Extended service access control, support for this was implemented with 9.3.0-X.
0x160 0xF Reserved
0x16F 0x1 Resource limit category. (0 = APPLICATION, 1 = SYS_APPLET, 2 = LIB_APPLET, 3 = OTHER (sysmodules running under the BASE memregion))


Bits Description
0-1 Ideal processor
2-3 Affinity mask
4-7 Old3DS system mode
Old3DS System Mode[edit]
Value Description
0 Prod (64MB of usable application memory)
1 Undefined (unusable)
2 Dev1 (96MB of usable application memory)
3 Dev2 (80MB of usable application memory)
4 Dev3 (72MB of usable application memory)
5 Dev4 (32MB of usable application memory)
6-7 Undefined Same as Prod?

In the exheader data, the ideal processor field is a bit-index, while in the AccessDesc the ideal processor field is a bitmask. When the bit specified by the exheader field is not set in the AccessDesc field, an error is returned.

if((1 << exheaderval) & accessdescval == 0) return error

During a FIRM-launch when a TitleInfo structure was specified, the field at offset 0x400 in the FIRM-launch parameters is set to the SystemMode of the specified title, however in some cases other values are written there. With 8.0.0-18 NS will now check the output of PTMSYSM command 0x040A0000, when the output is non-zero and a certain NS state field is value-zero, the following is executed otherwise this is skipped. With that check passed on 8.0.0-18, NS will then check (Flag2 & 0xF). When that is value2, the output value (used for the FIRM-launcher parameter field mentioned above) is set to value7. Otherwise, when that value is non-zero, the output value is set to 6.


Bits Description
0 EnableL2Cache (Unknown what this actually does, New3DS-only presumably)
1 cpuspeed_804MHz (Default "cpuspeed" when not set)
2-7 Unused

In order for the exheader to have any of the above new bits set, the AccessDesc must have the corresponding bit set, otherwise the invalid-exheader error is returned.

Homebrew which runs under a title which has the above cpuspeed flag set, runs much faster on New3DS. It's unknown how exactly the system handles these flags.

When launching titles / perhaps other things with APT, NS uses PTMSYSM:ConfigureNew3DSCPU with data which originally came from these flags; NS does this regardless of what the running 3DS system is. However, due to a bug(?) in NS the value sent to that command is always either 0x0 or 0x3. When calculating that value, the code only ever uses the cpuspeed field, not the cache field: code to actually load and check the value of the cache field appears to be missing.


Bit Description
0-3 New3DS system mode
4-7 Unused
New3DS System Mode[edit]
Value Description
0 Legacy (use Old3DS system mode)
1 Prod (124MB of usable application memory)
2 Dev1 (178MB of usable application memory)
3 Dev2 (124MB of usable application memory)
4-7 Undefined Same as Prod?

When in Legacy mode, the actual memory layout is the same as in New3DS Prod, except the available application memory as reported to the application is reduced to the Old3DS size.

The exheader value for the New3DS system mode value must be ≤ to the AccessDesc value, otherwise the invalid-exheader error is returned.

Storage Info[edit]

Used in FSReg:Register.

Offset Size Description
0x0 8 Extdata ID
0x8 8 System savedata IDs
0x10 8 Storage accessible unique IDs
0x18 7 Filesystem access info
0x1F 1 Other attributes

File System Access Info:

Bit and bitmask Description
0, 0x1 Category system application
1, 0x2 Category hardware check
2, 0x4 Category filesystem tool
3, 0x8 Debug
4, 0x10 TWL card backup
5, 0x20 TWL NAND data
6, 0x40 BOSS
7, 0x80 sdmc:/
8, 0x100 Core
9, 0x200 nand:/ro/ (Read Only)
10, 0x400 nand:/rw/
11, 0x800 nand:/ro/ (Write Access)
12, 0x1000 Category system settings
13, 0x2000 Cardboard
14, 0x4000 Export/Import IVS
15, 0x8000 sdmc:/ (Write-only)
16, 0x10000 Switch cleanup (Introduced in 3.0.0?)
17, 0x20000 Savedata move (Introduced in 5.0.0)
18, 0x40000 Shop (Introduced in 5.0.0)
19, 0x80000 Shell (Introduced in 5.0.0)
20, 0x100000 Category home menu (Introduced in 6.0.0)
21, 0x200000 Seed DB. Introduced in 9.6.0-X FIRM. Home Menu has this bit set starting with 9.6.0-X.

Other Attributes[edit]

Bit Description
0 Not use ROMFS
1 Use Extended savedata access.

When Bit1 is set, the "Extdata ID" and "Storage Accessable Unique IDs" regions are used to store a total of 6 "Accessible Save IDs". Introduced in 6.0.0.

Service Access Control[edit]

This is the list of services which the process is allowed to access, this is registered with the services manager. Each service listed in the exheader must be listed in the AccessDesc, otherwise the invalid exheader error is returned. The order of the services for exheader/AccessDesc doesn't matter. The AccessDesc can list services which are not in the exheader, but normally the service-access-control data for exheader/AccessDesc are exactly the same.

This list is submitted to SRVPM:RegisterProcess.

ARM11 Kernel Capabilities[edit]

The kernel capability descriptors are passed to svcCreateProcess.

Offset Size Description
0x0 0x70 (28*4) Descriptors
0x70 0x10 Reserved

There are different descriptor types, determined by the number of leading ones in the binary value representation of bits 20-31. The different types are laid out as follows:

Pattern of bits 20-31 Type Fields
0b1110xxxxxxxx Interrupt info
0b11110xxxxxxx System call mask Bits 24-26: System call mask table index; Bits 0-23: mask
0b1111110xxxxx Kernel release version Bits 8-15: Major version; Bits 0-7: Minor version
0b11111110xxxx Handle table size Bits 0-18: size
0b111111110xxx Kernel flags See below
0b11111111100x Map IO/static address range Describes a memory mapping like the 0b111111111110 descriptor, but an entire range rather than a single page is mapped. Another 0b11111111100x descriptor must follow this one to denote the (exclusive) end of the address range to map. Bit20 on the first descriptor: map read-only (otherwise RW), bit20 on the second descriptor: map static (cacheable, otherwise IO if the bit is not set)
0b111111111110 Map IO memory page Bits 0-19: page index to map (virtual address >> 12; the physical address is determined per-page according to Memory layout); Bit 20: Map read-only (otherwise read-write)

ARM11 Kernel Flags[edit]

Bit Description
0 Allow debug
1 Force debug
2 Allow non-alphanum
3 Shared page writing
4 Privilege priority
5 Allow main() args
6 Shared device memory
7 Runnable on sleep
8-11 Memory type (1: application, 2: system, 3: base)
12 Special memory
13 Process has access to CPU core 2 (New3DS only)

ARM9 Access Control[edit]

Offset Size Description
0x0 15 Descriptors
0xF 1 ARM9 Descriptor Version. Originally this value had to be ≥ 2. Starting with 9.3.0-X this value has to be either value 2 or value 3.


Bit Description
0 Mount nand:/
1 Mount nand:/ro/ (Write Access)
2 Mount twln:/
3 Mount wnand:/
4 Mount card SPI
5 Use SDIF3
6 Create seed
7 Use card SPI
8 SD application (Not checked)
9 Mount sdmc:/ (Write Access)