Boot Process

MCPX

Certain things are still missing, for example, getting the CPU to 32 bit protected mode and enabling caching.^[FIXME]

Xcodes

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

        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) {
        ++i;

        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;
    }
}

Notes

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

2BL

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

Self-copy

Now the 2BL copies itself from 0x00900000 to memory address 0x00400000.

Paging

Now a PDE is prepared at address 0x0000F000^[FIXME]:

Offset in PDE	Value	Notes

Once the PDE is set up, it 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.

esp is now also reloaded to point at the higher mapping. It will be set to 0x80400000 (absolute value, independent of previous esp value).

Memory cleanup

Writes 0xCC repeatedly from ^[FIXME] to ^[FIXME].

Loading the kernel

Kernel-copy

The Kernel is now copied into RAM.

Kernel decryption

The 2BL will copy the kernel decrytpion 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.

References

• Understanding the Xbox boot process
• Deconstructing the Boot ROM (I cannot express enough just how helpful this page is)