Mainboard
From Free60 Project
Photos
http://arstechnica.com/articles/paedia/cpu/xbox360-1.ars
http://www.darkmoon.org/xbox360_photos.htm
Details
The main board is a four layer PCB (sources: [1] (http://www.ccw.com.cn/notebook/pczx/sy/htm2005/20051028_16OMI.htm), [2] (http://www.anandtech.com/video/showdoc.aspx?i=2414)), implying that the top and bottom layers (the ones we can see and probe) contain most of the signals. This is a good thing :)
CPU - GPU Front side bus
According to these (http://www.ccw.com.cn/notebook/pczx/sy/htm2005/20051028_16OMI.htm) three (http://www.cdrinfo.com/images/uploaded/Xbox_Architecture_large.gif) sources (http://en.wikipedia.org/wiki/Xbox_360#Hardware_specifications) the FSB is a serial bus with 16 differential pairs in each direction, and each pair runs at 5.4Gbit/s. This agrees with Microsoft's published bandwidth specification of 21.6 Gbyte/sec (10.8 Gbyte/sec each way, 16 bits/2 bytes at a time, 5.4 Gbit/sec per pair).
Looking at photos of the main board, we can see 40 pairs of carefully routed tracks between the CPU and GPU. 32 pairs (16 each way) + 8 clock/handshake/parity? pairs = 40 pairs.
This very high frequency interface will make probing and snooping on the bus extremely difficult, which is unfortunate because the Xbox1 security was compromised in part by a FSB bus snoop done by bunnie (http://www.xenatera.com/bunnie/proj/anatak/xboxmod.html). At this frequency, the (relatively) simple task of attaching probe wires to the FSB will change the impedance seen on the bus by the FSB PHYs and cause the bus to fail to work correctly (look at eye diagram (http://www.ccw.com.cn/notebook/pczx/sy/htm2005/20051028_16OMI.htm)...). It might still be possible to probe the FSB with very expensive gear though.
It's not very likely that they are making the mistake of sending critical information over an exposed bus again though.
(This page is riddled with speculation and extrapolation of information from dodgey sources, so take it with a grain of salt.)
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