BCT (Boot Configuration Table) is a data structure present on Tegra based devices that supplies boot time configuration parameters.
The Switch's BCT is included in the firmware package titles (0100000000000819 and 010000000000081A) and is installed into eMMC storage's boot partition 0. A total of four BCT copies can be installed into the system: normal, normal backup, safe mode and safe mode backup.
By design, the BCT's data is only signed after offset 0x0510. Therefore, regions like customer_data can be freely modified without resigning. This is done by NS when injecting a new keyblob during a system update, for example.
During boot, the boot ROM parses the appropriate BCT from eMMC storage and stores a copy of it in IRAM at address 0x40000000.
Structure
NAND
Below is the BCT structure used by the Switch, which is a minimal variation of the Tegra 210 BCT format.
| Offset | Size | Field | Description |
|---|---|---|---|
| 0x0000 | 0x210 | bad_block_table | Table containing information on bad blocks
0x0000: num_entries (0x200) 0x0004: virtual_block_size (0x0F) 0x0005: block_size (0x0E) 0x0006: bad_blocks 0x0206: reserved |
| 0x0210 | 0x100 | bct_key | BCT RSA key modulus |
| 0x0310 | 0x30 | bct_signature | BCT object signature
0x0310: hash (empty) 0x0320: rsa_pss_signature |
| 0x0420 | 0x04 | sec_provisioning_key_num_insecure | Used for Factory Secure Provisioning. Always 0. |
| 0x0424 | 0x20 | sec_provisioning_key | Used for Factory Secure Provisioning. Always empty. |
| 0x0444 | 0xC4 | customer_data | Data block available for the customer. Used in key generation.
0x0444: padding_0x0C 0x0450: keyblob_0xB0 0x0500: padding_0x08 |
| 0x0508 | 0x04 | odm_data | Legacy field. Unused. |
| 0x050C | 0x04 | reserved0 | Legacy field. Unused. |
| 0x0510 | 0x10 | random_aes_block | Always empty. |
| 0x0520 | 0x10 | unique_chip_id | Always empty. |
| 0x0530 | 0x04 | boot_data_version | Set to 0x00210001 (BOOTDATA_VERSION_T210). |
| 0x0534 | 0x04 | block_size_log2 | Always 0x0E. |
| 0x0538 | 0x04 | page_size_log2 | Always 0x09. |
| 0x053C | 0x04 | partition_size | Always 0x01000000. |
| 0x0540 | 0x04 | num_param_sets | Number of device parameter sets. Always 0x01. |
| 0x0544 | 0x04 | dev_type | Device type. Set to 0x04 (dev_type_sdmmc). |
| 0x0548 | 0x40 | dev_params | Device parameters
0x0548: sdmmc_clock_divider (0x09 == 24MHz) 0x054C: sdmmc_data_width (0x02 == sdmmc_data_width_8bit) |
| 0x0588 | 0x04 | num_sdram_sets | Number of SDRAM parameter sets. Set to 0? |
| 0x058C | 0x768 | sdram_params0 | |
| 0x0CF4 | 0x768 | sdram_params1 | |
| 0x145C | 0x768 | sdram_params2 | |
| 0x1BC4 | 0x768 | sdram_params3 | |
| 0x232C | 0x04 | num_bootloaders | Number of bootloaders installed. Always 0x02 (maximum is 0x04). |
| 0x2330 | 0x12C | bootloader0_info | Configuration parameters for bootloader 0 (normal).
0x2330: version (variable) 0x2334: start_block (0x00000040) 0x2338: start_page (0x00000000) 0x233C: length (variable) 0x2340: load_addr (0x40010000) 0x2344: entry_point (0x40010020) 0x2348: attribute (0x00000000) 0x234C: bootloader0_hash (empty) 0x235C: bootloader0_rsa_pss_signature |
| 0x245C | 0x12C | bootloader1_info | Configuration parameters for bootloader 1 (safe mode).
0x245C: version (variable) 0x2460: start_block (0x00000050) 0x2464: start_page (0x00000000) 0x2468: length (variable) 0x246C: load_addr (0x40010000) 0x2470: entry_point (0x40010020) 0x2474: attribute (0x00000000) 0x2478: bootloader1_hash (empty) 0x2488: bootloader1_rsa_pss_signature |
| 0x2588 | 0x258 | reserved1 | Reserved space for bootloaders 2 and 3 (currently unused). |
| 0x27E0 | 0x01 | enable_fail_back | Always 0. |
| 0x27E1 | 0x04 | secure_debug_control | Always 0. |
| 0x27E5 | 0x04 | sec_provisioning_key_num_secure | Used for Factory Secure Provisioning. Always 0. |
| 0x27E9 | 0x12 | reserved2 | Always starts with 0x80000000 (NVBOOT padding pattern). |
| 0x27FB | 0x05 | padding | Empty. Not part of BCT data. |
customer_data
This data block is ignored by the boot ROM, therefore is available for the programmer to use freely. The Switch uses 0xB0 bytes of this area, at offset 0x0450, to store the active keyblob. All remaining bytes are zero.
The first bootloader validates and decrypts this block for further key generation. The decrypted keyblob payload is as follows.
| Offset | Size | Description |
|---|---|---|
| 0x0 | 0x80 | Array of master static key encryption keys |
| 0x80 | 0x10 | Stage 1 key |
bootloader0_info
1.0.0 - 2.3.0
The version field is set to 0x01, meaning that the first keyblob is to be used.
3.0.0
The version field was changed to 0x02, meaning that the second keyblob is now used.
3.0.1
The version field was changed to 0x03, meaning that the third keyblob is now used.
IRAM
When copied to IRAM, the BCT has an additional header as follows.
| Offset | Size | Field | Description |
|---|---|---|---|
| 0x00 | 0x50 | bct_global_header | Contains pointers to actual BCT data.
0x4C: bct_data_addr (address of the actual BCT) |
| 0x50 | 0x18 | bootloader0_header | Unknown structure.
0x00: is_active (if set to 0x01, bootloader0 is used) |
| 0x68 | 0x18 | bootloader1_header | Unknown structure.
0x00: is_active (if set to 0x01, bootloader1 is used) |
| 0x80 | 0x18 | bootloader2_header | Unknown structure.
0x00: is_active (if set to 0x01, bootloader2 is used) |
| 0x98 | 0x18 | bootloader3_header | Unknown structure.
0x00: is_active (if set to 0x01, bootloader3 is used) |