Difference between revisions of "BIOS"

Revision as of 15:03, 14 May 2017

The BIOS (an acronym for Basic Input/Output System and also known as the BIOS ROM or Xbox ROM) is a firmware image that is mapped to the top 16MB of the CPU's physical address space (0xFF000000-0xFFFFFFFF). Like the standard PC BIOS, it is responsible for initializing the Xbox hardware and booting the system. Unlike the PC BIOS, however, the Xbox BIOS image also contains the kernel in a compressed and encrypted form.

On a standard Xbox, the BIOS image is stored on a 1MB non-volatile TSOP ROM chip and connected to the MCPX via the LPC bus. The image is actually 256KB, duplicated 4 times to fill the 1MB ROM chip. You can verify this by running:

$ split -n 4 xbox.bin 
$ md5sum xa*
542c62cb976a4993c8c5027dff9638ce  xaa
542c62cb976a4993c8c5027dff9638ce  xab
542c62cb976a4993c8c5027dff9638ce  xac
542c62cb976a4993c8c5027dff9638ce  xad

You'll notice it is the same file repeated 4 times. That explains how some BIOS chips are 1MB and some are 256KB. The next thing is that the BIOS is repeated from 0xFF000000 until it fills the rest of memory. In other words, that 256KB of data is repeated 64 times.

Components

The BIOS is split into different components. These are largely the same from BIOS to BIOS, but with some differences.

Unknown

From 0x00000000 - 0x000000079

Not sure what this does. Some people think it might be involved with initialising the MCPX ROM, but I don't know. The Reset Vector on the Pentium 3 would mean that this wasn't called before the MCPX ROM, but I'm guessing there are people out there who know a lot more about it than me.

xcodes

From 0x00000080 - Copyright string

These are the xcode operations run by the MCPX interpreter. The first couple of lines appear to be nonsense (they don't execute any functionality), but then the first actual codes that I have found are:

The xcodes in the BIOS versions 3944, 4034, 4134 all start with: 04 10 08 00 80 01 80 00 00

The xcodes in the BIOS versions 4817, 5101, 5530, 5713, 5838 all start with: 04 84 08 00 80 01 80 00 00

This leads me to believe that the first three BIOS versions that I have are compatible with the 1.0 MCPX, and the rest are compatible with the 1.1 MCPX.

Next, some people believed that there was another unknown section between the xcodes and the copyright string. As far as I can tell, that section was to allow the xcode instruction set to expand, as the 5838 instructions are considerably larger than those in the 3944 BIOS.

Copyright String

57 bytes long with the following start positions:

• 3394 - 0xcfa
• 4034 - 0xcfa
• 4134 - 0xcfa
• 4817 - 0xdb9
• 5101 - 0xe49
• 5530 - 0xe59
• 5713 - 0xe59
• 5838 - 0xdcc

Literally contains the text: "Copyright (c) Microsoft Corporation. All rights reserved.".

Kernel

Still need to analyse

Kernel Data Segment

Still need to analyse

2BL

Still need to analyse

Decoy bootloader

This is very similar to the MCPX ROM, only the xcode interpreter is different, and it doesn't include any decryption/hashing algorithms.

Known Retail BIOSs

• 3394
• 4034
• 4134
• 4817
• 5101
• 5530
• 5713
• 5838

See Also

BIOS Dumping

References

• Understanding the Xbox boot process