Hacking the Xbox

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Hacking the Xbox: An Introduction to Reverse Engineering
The term “dynamic” is applied to RAM that has to be constantly
refreshed in order to preserve the integrity of data. For example, the
RAM used in the Xbox must have every location read out and written
back about thirty times a second. The performance penalty is not as bad
as it sounds, as special hardware is built into modern DRAM chips that
help optimize the process.
The “synchronous” prefix means that inside the DRAM, the procedure
for data access is broken down into a series of steps. Each of these steps
are independent and can occur in parallel, so that multiple data requests
can be in-flight simultaneously. An external timing signal, known as a
clock, is used to synchronize the movement of data access requests
through the various steps inside the DRAM. As a result, data access
requests flow through each step like water through a pipe, and this
technique is also known as pipelining. Synchronous DRAMs have higher
bandwidth throughput than their predecessors, because pipelining allows
multiple requests to be processed at once. However, the time required
from when an access is first issued to an SDRAM to when the data finally
appears on the output —the access latency — is not improved by
pipelining.
The term “Double Data Rate” refers to the way synchronous data is
transferred relative to the synchronizing clock. A clock waveform
consists of a repeating pattern of high and low signals. In traditional
systems, data is only transferred on the low-to-high transition of a clock
waveform. In a DDR system, data is transferred on both the low-to-high
and the high-to-low transitions. Thus, for the same clock frequency, twice
the amount of data can be transferred. The performance mnemonic
quoted by DDR SDRAM vendors, such as DDR266, refers to the
transfer rate, so the actual clock speed is one-half the performance
mnemonic, or 133 MHz in this case.
ROM
Every computer needs to have some kind of persistent or non-volatile
memory for storing the start-up, or boot, program. The DDR SDRAM
discussed above does not work for this application because all data in a
DDR SDRAM is lost when the power is removed. Current versions of
the Xbox use a FLASH ROM instead to store data that has to persist
even when the power is turned off. ROM stands for Read-Only Memory,
and FLASH refers to a specific style of storage element that is electronically
reprogrammable. FLASH style memories are convenient in PCs
because they can be reprogrammed by the end user to fix mistakes in the
boot code. However, in the Xbox, FLASH ROM programming by the
end user is purposely disabled. The write signal required for programming
is disconnected by leaving out the jumper located on the back of
the Xbox motherboard at component location R7R4 (see the sidebar
titled “Enabling FLASH ROM Programming Hardware” for more
information). In the case of the Xbox, the reprogrammability of FLASH
is primarily leveraged as a convenience for Microsoft during development
and production. It is quite likely that in a few months, the Xbox will