Socket AM3 breaks compatibility with AM2/AM2+ processors due to a subtle
change in key placement. The socket has 941 pin contacts while current
AM3 processors have only 938 pins. Tom's Hardware removed the two
obstructing key pins from an AM2+ Phenom II in order to fit it into an AM3
socket. The processor did not work in the AM3 socket, but still worked in
an AM2+ socket, suggesting that compatibility issues run deeper than
merely the key pins.
However, AM3 processors are backwards-compatible with Socket AM2+,
contingent upon a BIOS update for the motherboard. Manufacturers including
Asus, Gigabyte, and others have labeled existing AM2/AM2+ boards as
being "AM3 Ready" or similar, indicating that BIOS support is provided for
the specified boards. This allows existing AM2/AM2+ systems to upgrade the
CPU without having to upgrade any other components.
After i read this. I consulted with multiple IBM employees to see what they thought. The most we could come up with was that the pins taken out were grounding pins or memory hyperthreading pins for the new DDR3.
We also discussed weather this could be due to different BIOS instructions to the chip. If this is true hypothetically if you flashed a AM2 bios on a AM3 board you could get the AM2 chip (with obstructing pins cut) to work on the AM3 board.
That sounds like too much work and extra money. Also, wouldn't it sorat kill the whole point of having a IMC? I mean a IMC allows for better bandwidth. Having it go through the PCI or even a PCI-E slot would probably cut some performance.