This is because the pin configuration for a DDR3 RAM is different from that of DDR2 or DDR4, this is why you need to make sure that the RAM you buy must be compatible with your computer.

The Avalon Multi-port DDR2 Memory Controller IP Core is designed for maximizing the performance of an Altera Nios II processor in Avalon® multi-master streaming data systems. Advanced design features ...

The emerging DDR3 memory standard will extend the performance range of DDR memories considerably, while maintaining some amount of backwards compatibility with the existing DDR2 memory standard. It is ...