Memory not recognised 128MB not 256MB

I have a PC with an NMC motherboard, and I have installed a 256MB DIMM module. However the DIMM module is only recognised as a 128MB. The manual for the motherboard mentions something called Registered SDRAM, but I have never heard of this type of RAM.

Can anyone help with a description of this type of memory or if I am doing anything wrong, which I do not believe I am.