Socket AM3 breaks compatibility with AM2/AM2+ processors due to a subtle change in key placement. The socket has 941 pin contacts[1] 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.[6]
However, AM3 processors are backwards-compatible with Socket AM2+, contingent upon a BIOS update for the motherboard. Manufacturers including Asus,[7] Gigabyte,[8] 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