Hacking the Xbox

Chapter 6 - The Best Xbox Game: Security Hacking 97
Furthermore, the jam table opcodes seem to be corrupted. This phenomenon
was corroborated by other hackers working on the problem,
thus ruling out a code translation error. Clearly, there is more to the
Xbox than meets the eye.
Theories and rumors started to emerge to explain this strange behavior.
Some of the popular theories included:
• Address and/or data line scrambling. Somewhere, the address or
data lines were being inverted or permuted with some 1:1 mapping
function. The scrambling function could be programmed into the
chipset as part of the initialization procedure, so that the initial
boot block would read like plaintext while the rest of the data
would be scrambled.
• Secondary crypto processor. Another processor on the
Xbox was actually handling the Xbox’s initialization, and the boot
code in the ROM is bogus.
• Boot code contained in the processor. The processor is
actually initialized by a chunk of code sitting on the processor die,
and the boot code in the ROM is bogus.
• Boot code contained in the chipset. The processor
functions identically to a standard Pentium, but the chipset contains
boot code that overrides the bogus code inside the ROM.
For almost all of these theories, the only way to prove or disprove them is
to perform experiments on the hardware. For example, in order to ensure
that the SMC (System Management Controller, an 8-bit self-contained
processor that is always on when the Xbox is plugged in) played no role in
the secure boot sequence of the machine, hackers captured traces of the
waveforms on all of the SMC’s pins and analyzed them against the expected
sequence of events if the SMC were to play a crucial role in machine
initialization.
A crucial observation from a fellow hacker was that the Xbox booted
perfectly even when the reset vector code at 0xFFFF.FFF0 was changed.
One would expect that if the first instruction executed by the processor at
0xFFFF.FFF0 were corrupted, then the machine would crash. Rather, the
machine operated flawlessly. This observation was verified by a set of
experiments where various parts of the FLASH ROM were intentionally
corrupted. The results were that corrupting surprisingly large regions of the
FLASH ROM had no effect on the booting of the Xbox. In particular, the
entire boot initialization sequence from 0xFFFF.FE00 to 0xFFFF.FFFF
could be nulled out and the Xbox would boot just fine.
This finding alone strongly supported the theory of a bogus boot block in
FLASH ROM. The question remained, however, about where the real boot
code was stored. There were three options: in a secondary crypto-processor,
in the processor, and in the chipset. The secondary crypto-processor theory
was discounted on the basis that there were no chips on the motherboard
that were powerful enough or active enough at boot time to play the role