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(This page is riddled with speculation and extrapolation of information from dodgy sources, so take it with a grain of salt.)
The Xbox 360 motherboard (or motherboard) has undergone a few revisions, as Microsoft has corrected mistakes or added/improved existing features.
- Known revisions include:
CPU, GPU, & Front side bus
According to these three sources 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 motherboard, 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. 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...). It might still be possible to probe the FSB with very expensive gear though.
It's not very likely that they made the mistake of sending sensitive information on an exposed bus again though.