BCT

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.

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	0xCC	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.
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	0x04	enable_fail_back	Always 0.
0x27E4	0x04	secure_debug_control	Always 0.
0x27E8	0x04	sec_provisioning_key_num_secure	Always 0.
0x27EC	0x04	unk0	Always 0x08?
0x27F0	0x10	reserved2	Empty.

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 2 key

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)