Boot0

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Analogous to the old Wii, the Wii U also has a first-stage bootloader dubbed boot0, which is placed inside 16K of Mask ROM in the Latte's ARM core Starbuck. Being 2x bigger than the Wii version, Wii U's boot0 contains a number of features that include the ability of loading a recovery second-stage image from a SD card.

What follows are general descriptions and pseudo-code that illustrates the several sub-stages of the Wii U's boot0.

Initialization

boot0 runs from address 0xFFFF0000 where the ARM exception vectors are located. At this point, all exception vectors point to themselves (deadlock) except for the reset vector. Following the execution into the reset vector, the following takes place:

// Clear 0x10 bytes (will be used later)
memset_range(0x0D417FE0, 0x0D417FF0, 0, 0x04);

// Copy boot0 into a mirror in SRAM
memcpy(0x0D4100A0, 0xFFFF00A0, 0x3680);

// Set LR to a deadlock
LR = 0xFFFF004C

// Set PC
PC = sub_D4100A0

// Deadlock
loc_D41004C();

In other words, boot0 copies it's own code (starts at 0xFFFF00A0) into a special memory region in SRAM (0x0D410000) and jumps there. This behavior could also be observed, to some extent, in the old Wii's bootloader.

Setup

Right after boot0 copies itself over to SRAM, it does the following:

// Invalidate ICache
MCR p15, 0, R0,c7,c5, 0

// Invalidate DCache
MCR p15, 0, R0,c7,c6, 0

// Read control register
MRC p15, 0, R0,c1,c0, 0

// Set replacement strategy to Round-Robin
r0 = r0 | 0x1000

// Write control register
MCR p15, 0, R0,c1,c0, 0

// Clear 0x351C bytes after boot0
memset_range(0x0D413720, 0x0D416C3C, 0, 0x04);

// Set stack pointer
SP = 0x0D414204 + 0x1000

// Set return address
LR = sub_D4100F8		// Jump to boot1

// Set PC
PC = sub_D410384		// boot0 main

Essentially, sets up it's own stack and jumps to boot0's main function.

Main

This is the bulk of the first-stage bootloader. During the main function's execution, boot0 will send different signals to debug ports via GPIO. These signals can be used to represent error codes or mark different execution sub-stages.

Start

boot0 reads and saves the value from register LT_TIMER.

// Get timer value
u32 time_now = *(u32 *)LT_TIMER;

// Set start time
*(u32 *)0x0D413768 = 0;

Stage 0x00

boot0 sets a flag in LT_BOOT0 and configures debug ports' GPIOs.

// Send debug mark
SendGPIODebugOut(0x00);

u32 boot0_val = *(u32 *)LT_BOOT0;

// Set something in LT_BOOT0
*(u32 *)LT_BOOT0 = boot0_val | 0xC0;

// Enable GPIO for debug ports
u32 gpio_enable_val = *(u32 *)LT_GPIO_ENABLE;
*(u32 *)LT_GPIO_ENABLE = gpio_enable_val | 0x00FF0000;

// Set direction to output
u32 gpio_dir_val = *(u32 *)LT_GPIO_DIR;
*(u32 *)LT_GPIO_DIR = gpio_dir_val | 0x00FF0000;

Stage 0x01

Set something for memory swap.

// Send debug mark
SendGPIODebugOut(0x01);

// Set memory swap
*(u32 *)LT_MEMIRR = 0x07;

Stage 0x02

boot0 initializes the AES engine.

// Send debug mark
SendGPIODebugOut(0x02);

*(u32 *)AES_CTRL = 0;
*(u32 *)AES_KEY = 0;
*(u32 *)AES_KEY = 0;
*(u32 *)AES_KEY = 0;
*(u32 *)AES_KEY = 0;
*(u32 *)AES_IV = 0;
*(u32 *)AES_IV = 0;
*(u32 *)AES_IV = 0;
*(u32 *)AES_IV = 0;
*(u32 *)AES_SRC = 0;
*(u32 *)AES_DEST = 0;

Stage 0x03

boot0 initializes the SHA-1 engine.

// Send debug mark
SendGPIODebugOut(0x03);

*(u32 *)SHA_CTRL = 0;
*(u32 *)SHA_H0 = 0;
*(u32 *)SHA_H1 = 0;
*(u32 *)SHA_H2 = 0;
*(u32 *)SHA_H3 = 0;
*(u32 *)SHA_H4 = 0;
*(u32 *)SHA_SRC = 0;

Stage 0x04

boot0 sets something in LT_COMPAT_AHB and enables OTP reading.

// Send debug mark
SendGPIODebugOut(0x04);

// Set something in AHB compat
u32 ahb_val = *(u32 *)LT_COMPAT_AHB;
*(u32 *)LT_COMPAT_AHB = (ahb_val & 0xFFFFF3FF) | 0xC00;

// Enable OTP reading
SetOTPReadCommand();

Stages 0x05, 0x06 and 0x07

These three stages are all merged together and the only signal sent to the debug ports is effectively 0x05. boot0 asserts some resets and enables EXI.

// Send debug mark
SendGPIODebugOut(0x05);

// Assert RSTB_IOPI
u32 resets = *(u32 *)LT_RESETS;
*(u32 *)LT_RESETS = resets | 0x80000;

// Assert unknown reset
u32 resets = *(u32 *)LT_RESETS;
*(u32 *)LT_RESETS = resets | 0x40000;

// Enable EXI
u32 exi_ctrl = *(u32 *)LT_EXICTRL;
*(u32 *)LT_EXICTRL = exi_ctrl | 0x01;

Stage 0x08

During this stage boot0 reads all it needs from the OTP.

// Send debug mark
SendGPIODebugOut(0x08);

// Read security level flag from OTP
ReadOTP(0x20, 0x0D413760, 0x04);

// Read IOStrength flags from OTP
ReadOTP(0x21, 0x0D41375C, 0x04);

// Read SEEPROM pulse length from OTP
ReadOTP(0x22, 0x0D413758, 0x04);

// Read SEEPROM key from OTP
ReadOTP(0x28, 0x0D41376C, 0x10);

// Read boot1 key from OTP
ReadOTP(0xE8, 0x0D41377C, 0x10);

Stage 0x09

boot0 analyses the IOStrength flags read in the previous stage and sets the strength of various devices.

Stage 0x0A

boot0 sets the SEEPROM's pulse length and configures the SEEPROM GPIOs.

Stage 0x0B

boot0 analyses the OTP security level flag and decides which keys to use.

// Send debug mark
SendGPIODebugOut(0x0B);

u32 sec_lvl_flag = *(u32 *)0x0D413760;

// Forcefully throw error
if (sec_lvl_flag == 0x40000000)
   throw_error();

// Factory mode
if (sec_lvl_flag == 0x00000000)
    set_empty_aes_keys();
else
{
    // Disable boot1 AES key access
    u32 otpprot_val = *(u32 *)LT_OTPPROT;
    *(u32 *)LT_OTPPROT = otpprot_val & 0xDFFFFFFF;
    
    // This is a devkit unit
    if ((sec_lvl_flag & 0x18000000) == 0x08000000)
        set_debug_aes_keys();
    else if ((sec_lvl_flag & 0x10000000) == 0x10000000)  // This is a retail unit
        set_retail_aes_keys();
    else
        throw_error();
}

Stage 0x0C

boot0 generates a CRC32 table in it's stack.

Stage 0x0D

boot0 uses the SEEPROM key to decrypt SEEPROM data related to boot1.

// Send debug mark
SendGPIODebugOut(0x0D);

// Set SEEPROM AES key
AES_Set_Key(aes_seeprom_key);
	
// Read and decrypt from SEEPROM
SEEPROM_Read(0x1C, 0x0D41378C, 0x01);

// Read and decrypt from SEEPROM
SEEPROM_Read(0x1D, 0x0D41379C, 0x01);

// Read and decrypt from SEEPROM
SEEPROM_Read(0x1E, 0x0D4137AC, 0x01);

Stage 0x0E

boot0 validates the data decrypted from the SEEPROM at offset 0x1C0 in the previous stage using CRC32. This data is used to set miscellaneous settings during boot0's execution.

Stage 0x0F

boot0 validates the data decrypted from the SEEPROM at offsets 0x1D0 and 0x1E0 in the previous stage using CRC32. This data is used to determine boot1's version and sector inside the NAND. This stage is skipped in factory mode.

Stages 0x10, 0x11 and 0x12

boot0 configures an unknown feature for Starbuck. These stages are optional and only execute if the 2-byte flag read from the SEEPROM at offset 0x1C0 translates to a negative value. The three stages are merged together and the only signal sent to the debug ports is effectively 0x10.

// Send debug mark
SendGPIODebugOut(0x10);

// Allow IRQ 12 (AHBLT)
*(u32 *)LT_INTMR_AHBLT_ARM = 0x1000;
	
// Turn IOP2X on?
*(u32 *)LT_IOP2X = 0x03;
	
// Wait for interrupt
sub_D4132E8();
	
// Disable IRQ 12 (AHBLT)
*(u32 *)LT_INTMR_AHBLT_ARM = 0;

Stage 0x13

boot0 analyses the remaining data read from SEEPROM at offset 0x1C0 and uses it to configure the NAND_CONFIG and NAND_BANK registers. It then initializes the NAND engine.

Stage 0x14

This stage only executes if NAND engine's initialization was successful. boot0 checks if it's start time is valid (must be 0, otherwise boot0 throws an error).

Stages 0x15, 0x16 and 0x17

These stages only execute if NAND engine's initialization was successful. Only signal 0x15 is sent to the debug ports. boot0 flushes AHB memory, reads boot1's ancast header from NAND and checks the boot1's image size by looking into the respective field inside the header. If the size is valid (must not exceed 0xF800, so it doesn't overflow boot1's memory region), boot0 then proceeds to read the full boot1's image from NAND into address 0x0D400000: Finally, boot0 calculates how long this operation took and stores this value for the IOS-MCP to read later on.

Stage 0x18

This stage only executes if NAND engine's initialization was successful. boot0 checks if boot1 is encrypted or not (boot1 is not encrypted in factory mode).

Stage 0x19

This stage only executes if NAND engine's initialization was successful. boot0 verifies boot1's hash (SHA-1) and signature (RSA).

Stage 0x1A

This stage only executes if NAND engine's initialization was successful. boot0 decrypts boot1 (using the AES engine) in place.

Stage 0x1B

boot0 reads a flag from SEEPROM to determine how long it should wait before attempting to initialize the SD card host.

Stage 0x1C

boot0 initializes EXI.

// Send debug mark
SendGPIODebugOut(0x1C);

// Assert RSTB_IOEXI
u32 resets = *(u32 *)LT_RESETS;
*(u32 *)LT_RESETS = resets | 0x10000;

// Setup EXI
sub_D410BB8();

Stage 0x1D

boot0 uses EXI to capture events coming from surface mounted components (SMC). If a special button combo (power + eject) is being held, boot0 will attempt to load a recovery boot1 image from a SD card.

Stage 0x1E

This stage only executes if EXI told boot0 to load an image from the SD card. boot0 starts by configuring the SDC0S0Power GPIO. It then initializes and configures the SD host controller, flushes AHB memory and loads the recovery image's ancast header from the SD card. It checks the recovery image's size by looking at the size field in it's header (must not exceed 0xF800, so it doesn't overflow boot1's memory region) and then reads in the full image into memory address 0x0D400000 (replacing what was read from the NAND).

Stage 0x1F

This stage only executes if EXI told boot0 to load an image from the SD card. boot0 checks if the recovery image is encrypted or not (it is not encrypted in factory mode).

Stage 0x20

This stage only executes if EXI told boot0 to load an image from the SD card. boot0 verifies the recovery image's hash (SHA-1) and signature (RSA).

Stage 0x21

This stage only executes if EXI told boot0 to load an image from the SD card. boot0 decrypts the recovery image (using the AES engine) in place.

Stages 0x22, 0x23 and 0x24

These stages do not exist, but code leftovers indicate they could have been related to loading a recovery image via the 802.11 Wireless host.

Stage 0x25

boot0 clears boot1 and SEEPROM keys from memory, calculates and stores how long it took to run and returns.

// Send debug mark
SendGPIODebugOut(0x25);

// Clear boot1 key
memset(0x0D41377C, 0, 0x10);

// Clear SEEPROM key
memset(0x0D41376C, 0, 0x10);

// boot1/recovery image start address
r0 = 0x0D400200

// Store boot0's boot time
*(u32 *)0x0D417FE0 = time_now - initial_time;

return;

Loading boot1

After boot0's main function returns, execution falls into the pointer that was set in the LR register.

// Jump to boot1
sub_D4100F8(addr)
{
   r1 = addr

   // Read control register
   MRC p15, 0, R0,c1,c0, 0

   // Set replacement strategy to normal
   r0 = r0 & ~(0x1000)

   // Write control register
   MCR p15, 0, R0,c1,c0, 0

   PC = addr
}

Since boot0 finishes by setting r0 to 0x0D400200, returning from boot0 is equivalent to call sub_D4100F8(0x0D400200).

Error codes