Boot Process

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Revision as of 09:03, 29 May 2017 by Eighthpence (talk | contribs) (Kernel decryption)
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Certain things are still missing, for example, getting the CPU to 32 bit protected mode and enabling caching.[FIXME]


The xcode interpreter is common through both versions of the MCPX ROM. The high level interpretation of the MCPX ROM might look like this:

void xcode_interpreter() {
    int run_xcodes = 1;
    uint32_t eip = 0xff000080; // Not really EIP. This is just a pointer to the next xcode
    uint32_t result, scratch = 0;
    while (run_xcodes) {
        opcode    = get_memory_byte(eip);
        operand_1 = get_memory_dword(eip+1);
        operand_2 = get_memory_dword(eip+5);

        if (opcode == 0x07) {
            opcode    = operand_1;
            operand_1 = operand_2;
            operand_2 = result;

        switch (opcode) {
            case 0x02:
                result = get_memory_dword(operand_1 & 0x0fffffff);
            case 0x03:
                set_memory_dword(operand_1) = operand_2;
            case 0x06:
                result = (result & operand_1) | operand_2;
            case 0x04:
                if (operand_1 == 0x80000880) {
                    operand_2 &= 0xfffffffd;
                outl(operand_1, 0xcf8);
                outl(operand_2, 0xcfc);
            case 0x05:
                outl(operand_1, 0xcf8);
                result = inl(0xcfc);
            case 0x08:
                if (result != operand_1) {
                    eip += operand_2;
            case 0x09:
                eip += operand_2;
            case 0x10:
                scratch = (scratch & operand_1) | operand_2;
                result = scratch;
            case 0x11:
                outb(operand_2, operand_1);
            case 0x12:
                result = inb(operand_1);
            case 0xee:
                run_xcodes = 0;

        eip += 9;

RC4 Decryption of the 2BL

Decryption of the 2BL seems to happen in 4 stages.

Stage 1

Initialising the working space. The MCPX ROM seems to just write 1, 2, 3, 4.... 253, 254, 255 in memory addresses 0x8f000 to 0x850FF. This might look something like:

void init_rc4() {
    uint32_t stack_pointer = 0x8f000;

    for (int iterator = 0; iterator <= 255; iterator++) {
        set_memory_byte(stack_pointer + iterator, iterator);

Stage 2

Preparing for decryption. This gets the key from memory location 0xFFFFFFA5 and starts preparing and environment for decryption of the 2BL.

void load_key() {
    uint32_t key_location = 0xffffffa5;
    uint32_t stack_pointer = 0x8f000;
    uint8_t i, j = 0;

    for (int iterator = 0; iterator <= 255; iterator++) {
        i = get_memory_byte(iterator + stack_pointer);
        j += i + get_memory_byte(key_location + (iterator % 16));
        set_memory_byte(iterator+stack_pointer, get_memory_byte(j+stack_pointer));
        set_memory_byte(j+stack_pointer, i);

Stage 3

Perform the decryption. The MCPX reads the 2BL from 0xFFFF9E00 and decrypts it to 0x00090000. It is 24KiB in size.

void decrypt() {
    uint32_t stack_pointer = 0x0008F000;
    uint32_t encrypted = 0xFFFF9E00;
    uint32_t decrypted = 0x00090000;

    uint8_t a, b, j, i = 0;

    i = get_memory_byte(stack_pointer + 0x100); // 0
    j = get_memory_byte(stack_pointer + 0x101); // 0

    for (int edi = 0; edi <= 0x6000; ++edi) {

        a = get_memory_byte(stack_pointer + i);
        j += a;
        b = get_memory_byte(stack_pointer + j);
        set_memory_byte(stack_pointer + i, b);
        set_memory_byte(stack_pointer + j, a);
        a += b;
        b = get_memory_byte(edi + encrypted);
        a = get_memory_byte(stack_pointer + a);
        b ^= a;
        set_memory_byte(edi + decrypted, b);

Stage 4

Verification. Finally the MCPX reads a string from the un-encrypted 2BL and compares it to a magic number. If it matches, all was successful, and we jump to the start of the 2BL to start decrypting the kernel.

void verify() {
    if (get_memory_dword(0x95FE4) == MAGIC_NUMBER) {
        eip = 0x900000;
    } else {
        // Else, things have gone wrong
        eip = 0xFFFFFF94;


The RC4 algorithm was included as part of MCPX 1.0 and seems to work fine with BIOS versions 3944, 4034, and 4134.


Certain parts are still missing

MTRR Setup

First, the cache is disabled.[FIXME] Then, the MTRR (Memory Type Range Register) will be setup (using wrmsr) in the following way:

MTRR (ecx) High value (edx) Low value (eax) Notes
0x200 0x00000000 0x00000006
0x201 0x0000000F 0xFC000800 (For 64 MiB RAM BIOS)
0xF8000800 (For 128 MiB RAM BIOS)
0x202 0x00000000 0xFFF80005
0x203 0x0000000F 0xFFF80800
0x204 0x00000000 0x00000000 Clear all unused MTRR
0x20F 0x00000000 0x00000000
0x2FF 0x00000000 0x00000800

Once the MTRR have been written, the cache is enabled.[FIXME]

Register setup

Now the 2BL will set up the segment registers[FIXME] and stack:

Register Value Notes
ds 0x0010 Data segment[citation needed]
es 0x0010
ss 0x0010
esp 0x00400000
fs 0x0000
gs 0x0000


Now the 2BL copies itself (24 kiB) from 0x00900000 to memory address 0x00400000.


Now a PDE is prepared at address 0x0000F000:

Offset in PDE Value Notes
0x000 0x000000E3 Identity maps the first 256MiB of RAM: 0x00000000 and 0x80000000 will both map to physical page 0

0xE3: Flags:
* 0x80: 4 MiB page
* 0x40: Marked as previously written (Dirty)
* 0x20: Marked as previously accessed
* 0x02: Read/Write
* 0x01: Present
0x800 0x000000E3
0x004 0x004000E3
0x804 0x004000E3
0x8FC 0x0FC000E3
0x0FC 0x0FC000E3
0x900 0x00000000 Unmapping the rest of the pages
0x100 0x00000000
0xFFC 0x00000000
0x7FC 0x00000000
0xC00 0x0000F063 Maps the PDE (4 kiB page) to address 0xC0000000

0x63: Flags:
* 0x40: Marked as previously written (Dirty)
* 0x20: Marked as previously accessed
* 0x02: Read/Write
* 0x01: Present
0xFFC 0xFFC000E3 Identity maps the upper portion of the Flash (4 MiB page) to address 0xFFC00000

0xE3: Flags:
* 0x80: 4 MiB page
* 0x40: Marked as previously written (Dirty)
* 0x20: Marked as previously accessed
* 0x02: Read/Write
* 0x01: Present
0xFD0 0xFD0000FB Maps 16 MiB for the GPU control registers

0xFB: Flags:
* 0x80: 4 MiB page
* 0x40: Marked as previously written (Dirty)
* 0x20: Marked as previously accessed
* 0x10: Cache disabled
* 0x08: Write-Through caching
* 0x02: Read/Write
* 0x01: Present
0xFD4 0xFD4000FB
0xFD8 0xFD8000FB
0xFDC 0xFDC000FB

After setting up the PDE, the PAT is set up using wrmsr: [FIXME]

CR4 is touched [FIXME]

CR3 is touched [FIXME]

Now paging is activated by enabling the PG and WP bits in CR0. Additionally, the same or instruction is used to enable the NE bit in cr0.

2BL main

esp is now also reloaded to point at the relocated address. It will be set to 0x80400000 (absolute value, independent of previous esp value). The 2BL will now call into the relocated 2BL code somewhere near 0x00400000.

Disabling of the MCPX ROM

SMC handling

The SMC has a watchdog functionality which must be turned off. This is done by querying the SMC registers 0x1C - 0x1F. If all of them are 0x00 the 2BL will shutdown the system[FIXME]. If this is not the case, the bootloader calculates the watchdog challenge response and sends it to SMC registers 0x20 and 0x21.

Additionally, the 2BL will set SMC register 0x01 to 0 (which resets the cursor position for reading the SMC revision information).

Weird stuff 0[FIXME]

Memory cleanup

The 2BL fills memory with 0xCC from 0x80090000 to 0x80095FFF. These are the 24 kiB where the 2BL was stored previously.

Weird stuff 1[FIXME]

Weird stuff 2[FIXME]

Weird stuff 3[FIXME]

Loading the kernel


The Kernel is now copied into RAM.

Kernel decryption

The 2BL will copy the kernel decryption key (16 bytes) from offset 32 of an array of 3 keys:

Offset Use
0 EEPROM key
16 Certificate key
32 Kernel key

The Kernel is then decrypted in-place using RC4.

Kernel decompression

The Kernel is decompressed directly to 0x80010000 where it will reside until a full system shutdown.

Running the kernel

The xboxkrnl.exe header at 0x8001000 is checked[FIXME]. If it is invalid, [FIXME]. If it is valid, the kernel entry point is looked up from the PE optional header. The hardcoded image base of 0x8001000 is added to the entry point. The entry-point is now being called. Argumnts are passed on the stack, from right to left. The first argument is a commandline string loaded from memory address 0x80400000. It is an empty string for retail BIOS[FIXME]. A pointer to the previously mentioned array of 3 keys is passed as the second argument.


Startup animation

Kernel Re-initialization